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Experimental psychology is the work done by those who apply experimental methods to psychological study and the underlying processes. Experimental psychologists employ human participants and animal subjects to study a great many topics, including (among others)
sensation Sensation (psychology) refers to the processing of the senses by the sensory system. Sensation or sensations may also refer to: In arts and entertainment In literature *Sensation (fiction), a fiction writing mode *Sensation novel, a British ...
,
perception Perception () is the organization, identification, and interpretation of sensory information in order to represent and understand the presented information or environment. All perception involves signals that go through the nervous syste ...
,
memory Memory is the faculty of the mind by which data or information is encoded, stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If past events could not be remembe ...
,
cognition Cognition is the "mental action or process of acquiring knowledge and understanding through thought, experience, and the senses". It encompasses all aspects of intellectual functions and processes such as: perception, attention, thought, ...
,
learning Learning is the process of acquiring new understanding, knowledge, behaviors, skills, value (personal and cultural), values, Attitude (psychology), attitudes, and preferences. The ability to learn is possessed by humans, non-human animals, and ...
,
motivation Motivation is an mental state, internal state that propels individuals to engage in goal-directed behavior. It is often understood as a force that explains why people or animals initiate, continue, or terminate a certain behavior at a particul ...
,
emotion Emotions are physical and mental states brought on by neurophysiology, neurophysiological changes, variously associated with thoughts, feelings, behavior, behavioral responses, and a degree of pleasure or suffering, displeasure. There is ...
; developmental processes,
social psychology Social psychology is the methodical study of how thoughts, feelings, and behaviors are influenced by the actual, imagined, or implied presence of others. Although studying many of the same substantive topics as its counterpart in the field ...
, and the neural substrates of all of these.


History


Early experimental psychology


Wilhelm Wundt

Experimental psychology emerged as a modern academic discipline in the 19th century when
Wilhelm Wundt Wilhelm Maximilian Wundt (; ; 16 August 1832 – 31 August 1920) was a German physiologist, philosopher, and professor, one of the fathers of modern psychology. Wundt, who distinguished psychology as a science from philosophy and biology, was t ...
introduced a mathematical and experimental approach to the field. Wundt founded the first
psychology Psychology is the scientific study of mind and behavior. Its subject matter includes the behavior of humans and nonhumans, both consciousness, conscious and Unconscious mind, unconscious phenomena, and mental processes such as thoughts, feel ...
laboratory in
Leipzig, Germany Leipzig (, ; ; Upper Saxon: ; ) is the most populous city in the States of Germany, German state of Saxony. The city has a population of 628,718 inhabitants as of 2023. It is the List of cities in Germany by population, eighth-largest city in Ge ...
. Other experimental psychologists, including
Hermann Ebbinghaus Hermann Ebbinghaus (24 January 1850 – 26 February 1909) was a German psychologist who pioneered the experimental study of memory. Ebbinghaus discovered the forgetting curve and the spacing effect. He was the first person to describe the learnin ...
and
Edward Titchener Edward Bradford Titchener (11 January 1867 – 3 August 1927) was an English psychologist who studied under Wilhelm Wundt for several years. Titchener is best known for creating his version of psychology that described the structure of the mind: ...
, included
introspection Introspection is the examination of one's own conscious thoughts and feelings. In psychology, the process of introspection relies on the observation of one's mental state, while in a spiritual context it may refer to the examination of one's s ...
in their experimental methods.


Charles Bell

Charles Bell Sir Charles Bell (12 November 177428 April 1842) was a Scottish surgeon, anatomist, physiologist, neurologist, artist, and philosophical theologian. He is noted for discovering the difference between sensory nerves and motor nerves in the ...
was a British
physiologist Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a subdiscipline of biology, physiology focuses on how organisms, organ systems, individual organs, cells, and biomolecules carry out chemical and ...
whose main contribution to the medical and scientific community was his research on the
nervous system In biology, the nervous system is the complex system, highly complex part of an animal that coordinates its behavior, actions and sense, sensory information by transmitting action potential, signals to and from different parts of its body. Th ...
. He wrote a
pamphlet A pamphlet is an unbound book (that is, without a Hardcover, hard cover or Bookbinding, binding). Pamphlets may consist of a single sheet of paper that is printed on both sides and folded in half, in thirds, or in fourths, called a ''leaflet'' ...
summarizing his research on
rabbits Rabbits are small mammals in the family Leporidae (which also includes the hares), which is in the order Lagomorpha (which also includes pikas). They are familiar throughout the world as a small herbivore, a prey animal, a domesticated form ...
. His research concluded that sensory nerves enter at the posterior (dorsal) roots of the spinal cord, and motor nerves emerge from the anterior (ventral) roots of the spinal cord. Eleven years later, a French
physiologist Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a subdiscipline of biology, physiology focuses on how organisms, organ systems, individual organs, cells, and biomolecules carry out chemical and ...
, Francois Magendie, published the same findings without being aware of Bell's research. As a result of Bell not publishing his research, this discovery was called the
Bell–Magendie law In anatomy and neurophysiology, this is the finding that the anterior spinal nerve roots contain only motor fibers and posterior roots only sensory fibers and that nerve impulses are conducted in only one direction in each case. The nature and th ...
to honor both individuals. Bell's discovery disproved the belief that nerves transmitted either vibrations or spirits.


Ernst Heinrich Weber

Ernst Heinrich Weber Ernst Heinrich Weber (; ; 24 June 1795 – 26 January 1878) was a German physician who is considered one of the founders of experimental psychology. Ernst Weber was born into an academic background, with his father serving as a professor at t ...
, a German physician, is credited as one of experimental psychology's founders. Weber's main interests were the sense of touch and kinesthesis. His most memorable contribution to the field of experimental psychology is the suggestion that judgments of sensory differences are relative and not absolute. This relativity is expressed in "Weber's Law," which suggests that the
just-noticeable difference In the branch of experimental psychology focused on sense, sensation, and perception, which is called psychophysics, a just-noticeable difference or JND is the amount something must be changed in order for a difference to be noticeable, detectabl ...
or jnd is a constant proportion of the ongoing stimulus level. Weber's Law is stated as an equation: : \frac = k, where I\! is the original intensity of stimulation, \Delta I\! is the addition to it required for the difference to be perceived (the jnd), and ''k'' is a constant. Thus, for k to remain constant, \Delta I\! must rise as I increases. Weber's law is considered to be the first quantitative law in the history of psychology.


Gustav Fechner

Fechner published in 1860 what is considered to be the first work of experimental psychology, "Elemente der Psychophysik."Fraisse, P, Piaget, J, & Reuchlin, M. (1963). Experimental psychology: its scope and method. 1. History and method. New York: Basic Books. Some historians date the beginning of experimental psychology to the publication of "Elemente."
Ernst Heinrich Weber Ernst Heinrich Weber (; ; 24 June 1795 – 26 January 1878) was a German physician who is considered one of the founders of experimental psychology. Ernst Weber was born into an academic background, with his father serving as a professor at t ...
was not a psychologist, but it was Fechner who realized the importance of Weber's research to psychology. Weber's law and Fechner's law was published in Fechner's work, "Elemente der Psychophysik," and Fechner, a student of Weber named his first law in honor of his mentor. Fechner was profoundly interested in establishing a scientific study of the mind-body relationship, which became known as
psychophysics Psychophysics is the field of psychology which quantitatively investigates the relationship between physical stimulus (physiology), stimuli and the sensation (psychology), sensations and perceptions they produce. Psychophysics has been described ...
. Much of Fechner's research focused on the measurement of psychophysical thresholds and just-noticeable differences. He invented the psychophysical method of limits, the method of constant stimuli, and the method of adjustment, which are still in use.


Oswald Külpe

Oswald Külpe is the main founder of the Würzburg School in Germany. He was a pupil of
Wilhelm Wundt Wilhelm Maximilian Wundt (; ; 16 August 1832 – 31 August 1920) was a German physiologist, philosopher, and professor, one of the fathers of modern psychology. Wundt, who distinguished psychology as a science from philosophy and biology, was t ...
for about twelve years. Unlike Wundt, Külpe believed experiments were possible to test higher mental processes. In 1883 he wrote Grundriss der Psychologie, which had strictly scientific facts and no mention of thought.Fraisse, P, Piaget, J, & Reuchlin, M. (1963). Experimental psychology: its scope and method. 1. History and method. New York: Basic Books. The lack of thought in his book is odd because the Würzburg School put a lot of emphasis on mental set and imageless thought.


Würzburg School

The work of the Würzburg School was a milestone in the development of experimental psychology. The School was founded by a group of psychologists led by Oswald Külpe, and it provided an alternative to the
structuralism Structuralism is an intellectual current and methodological approach, primarily in the social sciences, that interprets elements of human culture by way of their relationship to a broader system. It works to uncover the structural patterns t ...
of Edward Titchener and Wilhelm Wundt. Those in the School focused mainly on mental operations such as mental set (''Einstellung'') and imageless thought. Mental set affects perception and problem solving without the awareness of the individual; it can be triggered by instructions or by experience. Similarly, according to Külpe, imageless thought consists of pure mental acts that do not involve mental images. William Bryan, an American student, working in Külpe's laboratory, provided an example of mental set. Bryan presented subjects with cards that had nonsense syllables written on them in various colors. The subjects were told to attend to the syllables, and in consequence, they did not remember the colors of the nonsense syllables. Such results made people question the validity of introspection as a research tool, leading to a decline in voluntarism and structuralism. The work of the Würzburg School later influenced many
Gestalt psychologists Gestalt psychology, gestaltism, or configurationism is a List of psychological schools, school of psychology and a theory of perception that emphasises the processing of entire patterns and configurations, and not merely individual components. ...
, including
Max Wertheimer Max Wertheimer (; April 15, 1880 – October 12, 1943) was a psychologist who was one of the three founders of Gestalt psychology, along with Kurt Koffka and Wolfgang Köhler. He is known for his book ''Productive Thinking'' and for conceiving the ...
.


George Trumbull Ladd

George Trumbull Ladd introduced experimental psychology into the United States and founded
Yale University Yale University is a Private university, private Ivy League research university in New Haven, Connecticut, United States. Founded in 1701, Yale is the List of Colonial Colleges, third-oldest institution of higher education in the United Stat ...
's psychological laboratory during his time there (from 1881 to 1905). In 1887, Ladd published ''Elements of Physiological Psychology'', the first American textbook that extensively discussed experimental psychology. Between Ladd's founding of the Yale Laboratory and his textbook, the center of experimental psychology in the US shifted to
Johns Hopkins University The Johns Hopkins University (often abbreviated as Johns Hopkins, Hopkins, or JHU) is a private university, private research university in Baltimore, Maryland, United States. Founded in 1876 based on the European research institution model, J ...
, where George Hall and
Charles Sanders Peirce Charles Sanders Peirce ( ; September 10, 1839 – April 19, 1914) was an American scientist, mathematician, logician, and philosopher who is sometimes known as "the father of pragmatism". According to philosopher Paul Weiss (philosopher), Paul ...
were extending and qualifying Wundt's work.


Charles Sanders Peirce

With his student
Joseph Jastrow Joseph Jastrow (January 30, 1863 – January 8, 1944) was a Polish-born American psychologist renowned for his contributions to experimental psychology, design of experiments, and psychophysics. He also worked on the phenomena of optical illu ...
, Charles S. Peirce randomly assigned volunteers to a blinded, repeated-measures design to evaluate their ability to discriminate weights. Peirce's experiment inspired other researchers in psychology and education, which developed a research tradition of randomized experiments in laboratories and specialized textbooks in the 1800s. The Peirce-Jastrow experiments were conducted as part of Peirce's pragmatic program to understand
human perception Perception () is the organization, identification, and interpretation of sensory information in order to represent and understand the presented information or environment. All perception involves signals that go through the nervous syste ...
; other studies considered perception of light, etc. While Peirce was making advances in experimental psychology and
psychophysics Psychophysics is the field of psychology which quantitatively investigates the relationship between physical stimulus (physiology), stimuli and the sensation (psychology), sensations and perceptions they produce. Psychophysics has been described ...
, he was also developing a theory of
statistical inference Statistical inference is the process of using data analysis to infer properties of an underlying probability distribution.Upton, G., Cook, I. (2008) ''Oxford Dictionary of Statistics'', OUP. . Inferential statistical analysis infers properties of ...
, which was published in " Illustrations of the Logic of Science" (1877–78) and " A Theory of Probable Inference" (1883); both publications that emphasized the importance of randomization-based inference in statistics. To Peirce and to experimental psychology belongs the honor of having invented
randomized experiment In scientific method, science, randomized experiments are the experiments that allow the greatest reliability and validity of statistical estimates of treatment effects. Randomization-based inference is especially important in experimental design ...
s decades before the innovations of
Jerzy Neyman Jerzy Spława-Neyman (April 16, 1894 – August 5, 1981; ) was a Polish mathematician and statistician who first introduced the modern concept of a confidence interval into statistical hypothesis testing and, with Egon Pearson, revised Ronald Fis ...
and
Ronald Fisher Sir Ronald Aylmer Fisher (17 February 1890 – 29 July 1962) was a British polymath who was active as a mathematician, statistician, biologist, geneticist, and academic. For his work in statistics, he has been described as "a genius who a ...
in agriculture. Peirce's pragmaticist philosophy also included an extensive theory of mental representations and cognition, which he studied under the name of
semiotics Semiotics ( ) is the systematic study of sign processes and the communication of meaning. In semiotics, a sign is defined as anything that communicates intentional and unintentional meaning or feelings to the sign's interpreter. Semiosis is a ...
. Peirce's student
Joseph Jastrow Joseph Jastrow (January 30, 1863 – January 8, 1944) was a Polish-born American psychologist renowned for his contributions to experimental psychology, design of experiments, and psychophysics. He also worked on the phenomena of optical illu ...
continued to conduct randomized experiments throughout his distinguished career in experimental psychology, much of which would later be recognized as
cognitive psychology Cognitive psychology is the scientific study of human mental processes such as attention, language use, memory, perception, problem solving, creativity, and reasoning. Cognitive psychology originated in the 1960s in a break from behaviorism, whi ...
. There has been a resurgence of interest in Peirce's work in cognitive psychology. Another student of Peirce,
John Dewey John Dewey (; October 20, 1859 – June 1, 1952) was an American philosopher, psychologist, and Education reform, educational reformer. He was one of the most prominent American scholars in the first half of the twentieth century. The overridi ...
, conducted experiments on human
cognition Cognition is the "mental action or process of acquiring knowledge and understanding through thought, experience, and the senses". It encompasses all aspects of intellectual functions and processes such as: perception, attention, thought, ...
, particularly in schools, as part of his "experimental logic" and "public philosophy."


20th century

In the middle of the 20th century,
behaviorism Behaviorism is a systematic approach to understand the behavior of humans and other animals. It assumes that behavior is either a reflex elicited by the pairing of certain antecedent stimuli in the environment, or a consequence of that indivi ...
became a dominant paradigm within psychology, especially in the
United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...
. This led to some neglect of mental phenomena within experimental psychology. In
Europe Europe is a continent located entirely in the Northern Hemisphere and mostly in the Eastern Hemisphere. It is bordered by the Arctic Ocean to the north, the Atlantic Ocean to the west, the Mediterranean Sea to the south, and Asia to the east ...
, this was less the case, as European psychology was influenced by psychologists such as Sir Frederic Bartlett,
Kenneth Craik Kenneth James William Craik (; 1914 – 1945) was a Scottish philosopher and psychologist. A pioneer of cybernetics, he hypothesized that a human behaves basically as a servomechanism that controlled at discrete points in time. He influenced W ...
, W.E. Hick, and Donald Broadbent, who focused on topics such as
thinking In their most common sense, the terms thought and thinking refer to cognitive processes that can happen independently of sensory stimulation. Their most paradigmatic forms are judging, reasoning, concept formation, problem solving, and delibe ...
,
memory Memory is the faculty of the mind by which data or information is encoded, stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If past events could not be remembe ...
, and
attention Attention or focus, is the concentration of awareness on some phenomenon to the exclusion of other stimuli. It is the selective concentration on discrete information, either subjectively or objectively. William James (1890) wrote that "Atte ...
. This laid the foundations for the subsequent development of cognitive psychology. In the latter half of the 20th century, the phrase "experimental psychology" had shifted in meaning due to the expansion of psychology as a discipline and the growth in its sub-disciplines. Experimental psychologists use a range of methods and do not confine themselves to a strictly experimental approach, partly because developments in the
philosophy of science Philosophy of science is the branch of philosophy concerned with the foundations, methods, and implications of science. Amongst its central questions are the difference between science and non-science, the reliability of scientific theories, ...
have affected the exclusive prestige of experimentation. In contrast, experimental methods are now widely used in fields such as developmental and
social psychology Social psychology is the methodical study of how thoughts, feelings, and behaviors are influenced by the actual, imagined, or implied presence of others. Although studying many of the same substantive topics as its counterpart in the field ...
, which were not previously part of experimental psychology. The phrase continues in use in the titles of a number of well-established, high prestige
learned societies A learned society ( ; also scholarly, intellectual, or academic society) is an organization that exists to promote an academic discipline, profession, or a group of related disciplines such as the arts and sciences. Membership may be open to al ...
and
scientific journals Science is a systematic discipline that builds and organises knowledge in the form of testable hypotheses and predictions about the universe. Modern science is typically divided into twoor threemajor branches: the natural sciences, which stu ...
, as well as some
university A university () is an educational institution, institution of tertiary education and research which awards academic degrees in several Discipline (academia), academic disciplines. ''University'' is derived from the Latin phrase , which roughly ...
courses of study in psychology.


Institutional review board (IRB)

In 1974, the
National Research Act The National Research Act is an American law enacted by the 93rd United States Congress and signed into law by President Richard Nixon on July 12, 1974. The law was passed following a series of congressional hearings on human-subjects research, d ...
established the existence of the
institutional review board An institutional review board (IRB), also known as an independent ethics committee (IEC), ethical review board (ERB), or research ethics board (REB), is a committee at an institution that applies research ethics by reviewing the methods proposed ...
in the United States following several controversial experiments. Institutional Review Boards (IRBs) play an important role in monitoring the conduct of psychological experiments. Their presence is required by law at institutions such as universities where psychological research occurs. Their purpose is to make sure that experiments do not violate ethical codes or legal requirements; thus they protect human subjects from physical or psychological harm and assure the humane treatment of animal subjects. An IRB must review the procedure to be used in each experiment before that experiment may begin. The IRB also assures that human participants give informed consent in advance; that is, the participants are told the general nature of the experiment and what will be required of them. There are three types of review that may be undertaken by an IRB - exempt, expedited, and full review. More information is available on the main IRB page.


Methodology

Sound methodology is essential to the study of complex behavioral and mental processes, and this implies, especially, the careful definition and control of experimental variables. The research methodologies employed in experimental psychology utilize techniques in research to seek to uncover new knowledge or validate existing claims. Typically, this entails a number of stages, including selecting a sample, gathering data from this sample, and evaluating this data. From assumptions made by researchers when undertaking a project, to the scales used, the research design, and data analysis, proper methodology in experimental psychology is made up of several critical stages.


Assumptions


Empiricism

Perhaps the most basic assumption of science is that factual statements about the world must ultimately be based on observations of the world. This notion of
empiricism In philosophy, empiricism is an epistemological view which holds that true knowledge or justification comes only or primarily from sensory experience and empirical evidence. It is one of several competing views within epistemology, along ...
requires that hypotheses and theories be tested against observations of the natural world rather than on a priori reasoning, intuition, or revelation.


Testability

Closely related to empiricism is the idea that, to be useful, a scientific law or theory must be testable with available research methods. If a theory cannot be tested in any conceivable way then many scientists consider the theory to be meaningless. Testability implies
falsifiability Falsifiability (or refutability) is a deductive standard of evaluation of scientific theories and hypotheses, introduced by the Philosophy of science, philosopher of science Karl Popper in his book ''The Logic of Scientific Discovery'' (1934). ...
, which is the idea that some set of observations could prove the theory to be incorrect. Testability has been emphasized in psychology because influential or well-known theories like those of Freud have been difficult to test.


Determinism

Experimental psychologists, like most scientists, accept the notion of
determinism Determinism is the Metaphysics, metaphysical view that all events within the universe (or multiverse) can occur only in one possible way. Deterministic theories throughout the history of philosophy have developed from diverse and sometimes ov ...
. This is the assumption that any state of an object or event is determined by prior states. In other words, behavioral or mental phenomena are typically stated in terms of cause and effect. If a phenomenon is sufficiently general and widely confirmed, it may be called a "law"; psychological theories serve to organize and integrate laws.


Parsimony

Another guiding idea of science is parsimony, the search for simplicity. For example, most scientists agree that if two theories handle a set of empirical observations equally well, we should prefer the simpler or more parsimonious of the two. A notable early argument for parsimony was stated by the medieval English philosopher William of Occam, and for this reason, the principle of parsimony is often referred to as
Occam's razor In philosophy, Occam's razor (also spelled Ockham's razor or Ocham's razor; ) is the problem-solving principle that recommends searching for explanations constructed with the smallest possible set of elements. It is also known as the principle o ...
.


Operational definition

Some well-known behaviorists such as Edward C. Tolman and
Clark Hull Clark Leonard Hull (May 24, 1884 – May 10, 1952) was an Americans, American psychologist who sought to explain learning and motivation by scientific laws of behavior. Hull is known for his debates with Edward C. Tolman. He is also known for his ...
popularized the idea of operationism, or operational definition. Operational definition implies that a concept be defined in terms of concrete, observable procedures. Experimental psychologists attempt to define currently unobservable phenomena, such as mental events, by connecting them to observations by chains of reasoning.


Reliability and Validity


Reliability

Reliability measures the consistency or repeatability of an observation. For example, one way to assess reliability is the "test-retest" method, done by measuring a group of participants at one time and then testing them a second time to see if the results are consistent. Because the first test itself may alter the results of a second test, other methods are often used. For example, in the "split-half" measure, a group of participants is divided at random into two comparable sub-groups, and reliability is measured by comparing the test results from these groups, It is important to note that a reliable measure need not yield a valid conclusion.


Validity

Validity measures the relative accuracy or correctness of conclusions drawn from a study. To determine the validity of a measurement quantitatively, it must be compared with a criterion. For example, to determine the validity of a test of academic ability, that test might be given to a group of students and the results correlated with the grade-point averages of the individuals in that group. As this example suggests, there is often controversy in the selection of appropriate criteria for a given measure. In addition, a conclusion can only be valid to the extent that the observations upon which it is based are reliable. Several types of validity have been distinguished, as follows:


= Internal validity

=
Internal validity Internal validity is the extent to which a piece of evidence supports a claim about cause and effect, within the context of a particular study. It is one of the most important properties of scientific studies and is an important concept in reason ...
refers to the extent to which a set of research findings provides compelling information about causality. High internal validity implies that the experimental design of a study excludes extraneous influences, such that one can confidently conclude that variations in the independent variable caused any observed changes in the dependent variable.


= External validity

=
External Validity External validity is the validity of applying the conclusions of a scientific study outside the context of that study. In other words, it is the extent to which the results of a study can generalize or transport to other situations, people, stimul ...
refers to the extent to which the outcome of an experiment can be generalized to apply to other situations than those of the experiment - for example, to other people, other physical or social environments, or even other cultures.


= Construct validity

=
Construct validity Construct validity concerns how well a set of indicators represent or reflect a concept that is not directly measurable. ''Construct validation'' is the accumulation of evidence to support the interpretation of what a measure reflects.Polit DF Bec ...
refers to the extent to which the independent and dependent variables in a study represent the abstract hypothetical variables of interest. In other words, it has to do with whether the manipulated and/or measured variables in a study accurately reflect the variables the researcher hoped to manipulate. Construct validity also reflects the quality of one's operational definitions. If a researcher has done a good job of converting the abstract to the observable, construct validity is high.


= Conceptual validity

= Conceptual validity refers to how well specific research maps onto the broader theory that it was designed to test. Conceptual and construct validity have a lot in common, but conceptual validity relates a study to broad theoretical issues whereas construct validity has more to do with specific manipulations and measures.


Scales of measurement

Measurement can be defined as "the assignment of numerals to objects or events according to rules."Torgerson, W. S. (1962) Theory and Methods of Scaling. New York: WileyStevens, S. S. (1951) Mathematics, Measurement and Psychophysics in S. S. Stevens (Ed) Handbook of Experimental Psychology. New York: Wiley Almost all psychological experiments involve some sort of measurement, if only to determine the reliability and validity of results, and of course measurement is essential if results are to be relevant to quantitative theories. The rule for assigning numbers to a property of an object or event is called a "scale". Following are the basic scales used in psychological measurement.


Nominal measurement

In a nominal scale, numbers are used simply as labels – a letter or name would do as well. Examples are the numbers on the shirts of football or baseball players. The labels are more useful if the same label can be given to more than one thing, meaning that the things are equal in some way, and can be classified together.


Ordinal measurement

An ordinal scale arises from the ordering or ranking objects, so that A is greater than B, B is greater than C, and so on. Many psychological experiments yield numbers of this sort; for example, a participant might be able to rank odors such that A is more pleasant than B, and B is more pleasant than C, but these rankings ("1, 2, 3 ...") would not tell by how much each odor differed from another. Some statistics can be computed from ordinal measures – for example,
median The median of a set of numbers is the value separating the higher half from the lower half of a Sample (statistics), data sample, a statistical population, population, or a probability distribution. For a data set, it may be thought of as the “ ...
, percentile, and order correlation – but others, such as
standard deviation In statistics, the standard deviation is a measure of the amount of variation of the values of a variable about its Expected value, mean. A low standard Deviation (statistics), deviation indicates that the values tend to be close to the mean ( ...
, cannot properly be used.


Interval measurement

An interval scale is constructed by determining the equality of differences between the things measured. That is, numbers form an interval scale when the differences between the numbers correspond to differences between the properties measured. For instance, one can say that the difference between 5 and 10 degrees on a Fahrenheit thermometer equals the difference between 25 and 30, but it is meaningless to say that something with a temperature of 20 degrees Fahrenheit is "twice as hot" as something with a temperature of 10 degrees. (Such ratios are meaningful on an absolute temperature scale such as the Kelvin scale. See next section.) "Standard scores" on an achievement test are said to be measurements on an interval scale, but this is difficult to prove.


Ratio measurement

A ratio scale is constructed by determining the equality of ratios. For example, if, on a balance instrument, object A balances two identical objects B, then one can say that A is twice as heavy as B and can give them appropriate numbers, for example "A weighs 2 grams" and "B weighs 1 gram". A key idea is that such ratios remain the same regardless of the scale units used; for example, the ratio of A to B remains the same whether grams or ounces are used. Length, resistance, and Kelvin temperature are other things that can be measured on ratio scales. Some psychological properties such as the loudness of a sound can be measured on a ratio scale.


Research design


One-way designs

The simplest experimental design is a one-way design, in which there is only one independent variable. The simplest kind of one-way design involves just two-groups, each of which receives one value of the independent variable. A two-group design typically consists of an experimental group (a group that receives treatment) and a
control group In the design of experiments, hypotheses are applied to experimental units in a treatment group. In comparative experiments, members of a control group receive a standard treatment, a placebo, or no treatment at all. There may be more than one tr ...
(a group that does not receive treatment). The one-way design may be expanded to a one-way, multiple groups design. Here a single independent variable takes on three or more levels. This type of design is particularly useful because it can help to outline a functional relationship between the independent and dependent variables.


Factorial designs

One-way designs are limited in that they allow researchers to look at only one independent variable at a time, whereas many phenomena of interest are dependent on multiple variables. Because of this, R.A Fisher popularized the use of factorial designs. Factorial designs contain two or more independent variables that are completely "crossed," which means that every level each independent variable appears in combination with every level of all other independent variables. Factorial designs carry labels that specify the number of independent variables and the number of levels of each independent variable there are in the design. For example, a 2x3 factorial design has two independent variables (because there are two numbers in the description), the first variable having two levels and the second having three.


Main effects and interactions

The effects of
independent variables A variable is considered dependent if it depends on (or is hypothesized to depend on) an independent variable. Dependent variables are studied under the supposition or demand that they depend, by some law or rule (e.g., by a mathematical function ...
in factorial studies, taken singly, are referred to as main effects. This refers to the overall effect of an independent variable, averaging across all levels of the other independent variables. A main effect is the only effect detectable in a one-way design. Often more important than main effects are "interactions", which occur when the effect of one independent variable on a dependent variable depends on the level of a second independent variable. For example, the ability to catch a ball (dependent variable) might depend on the interaction of visual acuity (independent variable #1) and the size of the ball being caught (independent variable #2). A person with good eyesight might catch a small ball most easily, and person with very poor eyesight might do better with a large ball, so the two variables can be said to interact.


Within- and between-subjects designs

Two basic approaches to
research design Research design refers to the overall strategy utilized to answer research questions. A research design typically outlines the theories and models underlying a project; the research question(s) of a project; a strategy for gathering data and info ...
are within-subject design and between-subjects design. In within-subjects or repeated measures designs, each participant serves in more than one or perhaps all of the conditions of a study. In between-subjects designs each participant serves in only one condition of an experiment. Within-subjects designs have significant advantages over between-subjects designs, especially when it comes to complex factorial designs that have many conditions. In particular, within-subjects designs eliminate person confounds, that is, they get rid of effects caused by differences among subjects that are irrelevant to the phenomenon under study. However, the within-subject design has the serious disadvantage of possible sequence effects. Because each participant serves in more than one condition, the passage of time or the performance of an earlier task may affect the performance of a later task. For example, a participant might learn something from the first task that affects the second.


Research in experimental psychology


Experiments

The use of experimental methods was perhaps the main characteristic by which psychology became distinguishable from philosophy in the late 19th century. Ever since then experiments have been an integral part of most psychological research. Following is a sample of some major areas that use experimental methods. In experiments, human participants often respond to visual, auditory or other stimuli, following instructions given by an experimenter; animals may be similarly "instructed" by rewarding appropriate responses. Since the 1990s, computers have commonly been used to automate stimulus presentation and behavioral measurement in the laboratory. Behavioral experiments with both humans and animals typically measure reaction time, choices among two or more alternatives, and/or response rate or strength; they may also record movements, facial expressions, or other behaviors. Experiments with humans may also obtain written responses before, during, and after experimental procedures.
Psychophysiological Psychophysiology (from Greek , ''psȳkhē'', "breath, life, soul"; , ''physis'', "nature, origin"; and , ''-logia'') is the branch of psychology that is concerned with the physiological bases of psychological processes. While psychophysiology w ...
experiments, on the other hand, measure brain or (mostly in animals) single-cell activation during the presentation of a stimulus using methods such as
fMRI Functional magnetic resonance imaging or functional MRI (fMRI) measures brain activity by detecting changes associated with blood flow. This technique relies on the fact that cerebral blood flow and neuronal activation are coupled. When an area o ...
, EEG,
PET A pet, or companion animal, is an animal kept primarily for a person's company or entertainment rather than as a working animal, livestock, or a laboratory animal. Popular pets are often considered to have attractive/ cute appearances, inte ...
or similar. Control of
extraneous variables A variable is considered dependent if it depends on (or is hypothesized to depend on) an independent variable. Dependent variables are studied under the supposition or demand that they depend, by some law or rule (e.g., by a mathematical function ...
, minimizing the potential for experimenter bias, counterbalancing the order of experimental tasks, adequate
sample size Sample size determination or estimation is the act of choosing the number of observations or replicates to include in a statistical sample. The sample size is an important feature of any empirical study in which the goal is to make inferences abo ...
, the use of operational definitions, emphasis on both the reliability and validity of results, and proper
statistical analysis Statistical inference is the process of using data analysis to infer properties of an underlying probability distribution.Upton, G., Cook, I. (2008) ''Oxford Dictionary of Statistics'', OUP. . Inferential statistical analysis infers properties of ...
are central to experimental methods in psychology. Because an understanding of these matters is important to the interpretation of data in almost all fields of psychology, undergraduate programs in psychology usually include mandatory courses in
research Research is creative and systematic work undertaken to increase the stock of knowledge. It involves the collection, organization, and analysis of evidence to increase understanding of a topic, characterized by a particular attentiveness to ...
methods and
statistics Statistics (from German language, German: ', "description of a State (polity), state, a country") is the discipline that concerns the collection, organization, analysis, interpretation, and presentation of data. In applying statistics to a s ...
. A crucial experiment is an experiment that is intended to test several hypotheses at the same time. Ideally, one hypothesis may be confirmed and all the others rejected. However, the data may also be consistent with several hypotheses, a result that calls for further research to narrow down the possibilities. A pilot study may be run before a major experiment, in order to try out different procedures, determine optimal values of the experimental variables, or uncover weaknesses in experimental design. The pilot study may not be an experiment as usually defined; it might, for example, consist simply of self-reports. In a field experiment, participants are observed in a naturalistic setting outside the laboratory. Field experiments differ from field ''studies'' in that some part of the environment (''field'') is manipulated in a controlled way (for example, researchers give different kinds of toys to two different groups of children in a nursery school). Control is typically more lax than it would be in a laboratory setting. Other methods of research such as
case studies A case study is an in-depth, detailed examination of a particular case (or cases) within a real-world context. For example, case studies in medicine may focus on an individual patient or ailment; case studies in business might cover a particular fi ...
, interviews, opinion polls and naturalistic observation, are often used by psychologists. These are not experimental methods, as they lack such aspects as well-defined, controlled variables, randomization, and isolation from unwanted variables.


Cognitive psychology

Some of the major topics studied by cognitive psychologists are
memory Memory is the faculty of the mind by which data or information is encoded, stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If past events could not be remembe ...
,
learning Learning is the process of acquiring new understanding, knowledge, behaviors, skills, value (personal and cultural), values, Attitude (psychology), attitudes, and preferences. The ability to learn is possessed by humans, non-human animals, and ...
,
problem solving Problem solving is the process of achieving a goal by overcoming obstacles, a frequent part of most activities. Problems in need of solutions range from simple personal tasks (e.g. how to turn on an appliance) to complex issues in business an ...
, and
attention Attention or focus, is the concentration of awareness on some phenomenon to the exclusion of other stimuli. It is the selective concentration on discrete information, either subjectively or objectively. William James (1890) wrote that "Atte ...
. Most cognitive experiments are done in a lab instead of a social setting; this is done mainly to provide maximum control of experimental variables and minimal interference from irrelevant events and other aspects of the situation. A great many experimental methods are used; frequently used methods are described on the main pages of the topics just listed. In addition to studying behavior, experimenters may use EEG or fMRI to help understand how the brain carries out cognitive processes, sometimes in conjunction with computational modelling. They may also study patients with focal brain damage or neurologic disease (see cognitive neuropsychology) or use brain stimulation techniques like
transcranial magnetic stimulation Transcranial magnetic stimulation (TMS) is a noninvasive neurostimulation technique in which a changing magnetic field is used to induce an electric current in a targeted area of the brain through electromagnetic induction. A device called a st ...
to help confirm the causal role and specific functional contribution of neural regions or modules.


Animal cognition

Animal cognition refers to the mental capacities of non-human animals, and research in this field often focuses on matters similar to those of interest to cognitive psychologists using human participants. Cognitive studies using animals can often control conditions more closely and use methods not open to research with humans. In addition, processes such as conditioning my appear in simpler form in animals, certain animals display unique capacities (such as echo location in bats) that clarify important cognitive functions, and animal studies often have important implications for the survival and evolution of species.


Sensation and perception

Experiments on sensation and perception have a very long history in experimental psychology (see History above). Experimenters typically manipulate stimuli affecting vision, hearing, touch, smell, taste and
proprioception Proprioception ( ) is the sense of self-movement, force, and body position. Proprioception is mediated by proprioceptors, a type of sensory receptor, located within muscles, tendons, and joints. Most animals possess multiple subtypes of propri ...
. Sensory measurement plays a large role in the field, covering many aspects of sensory performance - for example, minimum discriminable differences in brightness or the detection of odors; such measurement involves the use of instruments such as the oscillator, attenuator, stroboscope, and many others listed earlier in this article. Experiments also probe subtle phenomena such as visual illusions, or the emotions aroused by stimuli of different sorts.


Behavioral psychology

The behavioristic approach to psychology reached its peak of popularity in the mid-twentieth century but still underlies much experimental research and clinical application. Its founders include such figures as
Ivan Pavlov Ivan Petrovich Pavlov (, ; 27 February 1936) was a Russian and Soviet experimental neurologist and physiologist known for his discovery of classical conditioning through his experiments with dogs. Pavlov also conducted significant research on ...
,
John B. Watson John Broadus Watson (January 9, 1878 – September 25, 1958) was an American psychologist who popularized the scientific theory of behaviorism, establishing it as a List of psychological schools, psychological school.Cohn, Aaron S. 2014.Watson, J ...
, and B.F. Skinner. Pavlov's experimental study of the digestive system in dogs led to extensive experiments through which he established the basic principles of classical conditioning. Watson popularized the behaviorist approach to human behavior; his experiments with Little Albert are particularly well known. Skinner distinguished
operant conditioning Operant conditioning, also called instrumental conditioning, is a learning process in which voluntary behaviors are modified by association with the addition (or removal) of reward or aversive stimuli. The frequency or duration of the behavior ma ...
from
classical conditioning Classical conditioning (also respondent conditioning and Pavlovian conditioning) is a behavioral procedure in which a biologically potent Stimulus (physiology), stimulus (e.g. food, a puff of air on the eye, a potential rival) is paired with a n ...
and established the experimental analysis of behavior as a major component in the subsequent development of experimental psychology.


Social psychology

Social psychologists use both empirical research and experimental methods to study the effects of social interactions on individuals' thoughts, feelings, and behaviors. Social psychologists use experimental methods, both within and outside the laboratory, and there have been several key experiments done in the past few centuries. The Triplett experiment, one of the first social psychology experiments conducted in 1898 by
Norman Triplett Norman Triplett (October 1, 1861 – 1934) was a psychologist at Indiana University. He is best known for conducting one of the earliest experiments in social psychology, on the phenomenon of social facilitation.Christopher D. Green, Ludy T. Ben ...
, noticed that the presence of others had an effect on children's performance times. Other widely cited experiments in social psychology are projects like the Stanford prison experiment conducted by
Philip Zimbardo Philip George Zimbardo (; March 23, 1933 – October 14, 2024) was an American psychologist and a professor at Stanford University. He was an internationally known educator, researcher, author and media personality in psychology who authored mo ...
in 1971 and the Milgram obedience experiment by
Stanley Milgram Stanley Milgram (August 15, 1933 – December 20, 1984) was an American social psychologist known for his controversial Milgram experiment, experiments on obedience conducted in the 1960s during his professorship at Yale University, Yale.Blass, T ...
. In both experiments ordinary individuals were induced to engage in remarkably cruel behavior, suggesting that such behavior could be influenced by
social pressure Peer pressure is a direct or indirect influence on peers, i.e., members of social groups with similar interests and experiences, or social statuses. Members of a peer group are more likely to influence a person's beliefs, values, religion and beh ...
. Zimbardo's experiment noted the effect of
conformity Conformity or conformism is the act of matching attitudes, beliefs, and behaviors to social group, group norms, politics or being like-minded. Social norm, Norms are implicit, specific rules, guidance shared by a group of individuals, that guide t ...
to specific roles in society and the social world. Milgram's study looked at the role of authority in shaping behavior, even when it had adverse effects on another person. Because of possible negative effects on the participants, neither of these experiments could be legally performed in the United States today. These two experiments also took place during the era prior to the existence of IRBs, and most likely played a role in the establishment of such boards


Experimental instruments

Instruments used in experimental psychology evolved along with technical advances and with the shifting demands of experiments. The earliest instruments, such as the Hipp Chronoscope and the kymograph, were originally used for other purposes. The list below exemplifies some of the different instruments used over the years. These are only a few core instruments used in current research.


Hipp chronoscope/chronograph

This instrument, invented by Matthäus Hipp around 1850, uses a vibrating reed to tick off time in 1000ths of a second. Originally designed for experiments in physics, it was later adapted to study the speed of bullets. After then being introduced to physiology, it was finally used in psychology to measure reaction time and the duration of mental processes.


Stereoscope

The first stereoscope was invented by Wheatstone in 1838. It presents two slightly different images, one to each eye, at the same time. Typically the images are photographs of the same object taken from camera positions that mimic the position and separation of the eyes in the head. When one looks through the stereoscope the photos fuse into a single image that conveys a powerful sense of depth and solidity.


Kymograph

Developed by Carl Ludwig in the 19th century, the kymograph is a revolving drum on which a moving stylus tracks the size of some measurement as a function of time. The kymograph is similar to the polygraph, which has a strip of paper moving under one or more pens. The kymograph was originally used to measure blood pressure and it later was used to measure muscle contractions and speech sounds. In psychology, it was often used to record response times.


Photokymographs

This device is a photographic recorder. It used mirrors and light to record the photos. Inside a small box with a slit for light there are two drive rollers with film connecting the two. The light enters through the slit to record on the film. Some photokymographs have a lens so an appropriate speed for the film can be reached.


Galvanometer

The galvanometer is an early instrument used to measure the strength of an electric current. Hermann von Helmholtz used it to detect the electrical signals generated by nerve impulses, and thus to measure the time taken by impulses to travel between two points on a nerve.


Audiometer

This apparatus was designed to produce several fixed frequencies at different levels of intensity. It could either deliver the tone to a subject's ear or transmit sound oscillations to the skull. An experimenter would generally use an audiometer to find the auditory threshold of a subject. The data received from an audiometer is called an audiogram.


Colorimeters

These determine the color composition by measuring its tricolor characteristics or matching of a color sample. This type of device would be used in visual experiments.


Algesiometers and algometers

Both of these are mechanical stimulations of pain. They have a sharp needle-like stimulus point so it does not give the sensation of pressure. Experimenters use these when doing an experiment on analgesia.


Olfactometer

An olfactometer is any device that is used to measure the sense of smell. The most basic type in early studies was placing a subject in a room containing a specific measured amount of an odorous substance. More intricate devices involve some form of sniffing device, such as the neck of a bottle. The most common olfactometer found in psychology laboratories at one point was the Zwaardemker olfactometer. It had two glass nasal tubes projecting through a screen. One end would be inserted into a stimulus chamber, the other end is inserted directly into the nostrils.


Mazes

Probably one of the oldest instruments for studying memory would be the maze. The common goal is to get from point A to point B, however the mazes can vary in size and complexity. Two types of mazes commonly used with rats are the radial arm maze and the Morris water maze. The radial arm maze consists of multiple arms radiating from a central point. Each arm has a small piece of food at the end. The Morris water maze is meant to test spatial learning. It uses a large round pool of water that is made opaque. The rat must swim around until it finds the escape platform that is hidden from view just below the surface of the water.


Electroencephalograph (EEG)

The EEG is an instrument that can reflect the summed electrical activity of neural cell assemblies in the brain. It was originally used as an attempt to improve medical diagnoses. Later it became a key instrument to psychologists in examining brain activity and it remains a key instrument used in the field today.


Functional magnetic resonance imaging (fMRI)

The fMRI is an instrument that can detect changes in blood oxygen levels over time. The increase in blood oxygen levels shows where brain activity occurs. These are rather bulky and expensive instruments which are generally found in hospitals. They are most commonly used for cognitive experiments.


Positron emission tomography (PET)

PET is also used to look at brain activity. It can detect drugs binding neurotransmitter receptors in the brain. A down side to PET is that it requires radioisotopes to be injected into the body so the brain activity can be mapped out. The radioisotopes decay quickly so they do not accumulate in the body.


Eye tracking

Eye trackers are used to measure where someone is looking or how their eyes are moving relative to the head. Eye trackers are used in the study of visual perception and—because people typically direct their attention to the place they are looking—also to provide directly observable measures of attention.


Criticism


Frankfurt school

One school opposed to experimental psychology has been associated with the Frankfurt School, which calls its ideas "
Critical Theory Critical theory is a social, historical, and political school of thought and philosophical perspective which centers on analyzing and challenging systemic power relations in society, arguing that knowledge, truth, and social structures are ...
." Critical psychologists claim that experimental psychology approaches humans as entities independent of the cultural, economic, and historical context in which they exist. These contexts of human mental processes and behavior are neglected, according to critical psychologists, like
Herbert Marcuse Herbert Marcuse ( ; ; July 19, 1898 – July 29, 1979) was a German–American philosopher, social critic, and Political philosophy, political theorist, associated with the Frankfurt School of critical theory. Born in Berlin, Marcuse studied at ...
. In so doing, experimental psychologists paint an inaccurate portrait of human nature while lending tacit support to the prevailing social order, according to critical theorists like
Theodor Adorno Theodor is a masculine given name. It is a German form of Theodore. It is also a variant of Teodor. List of people with the given name Theodor * Theodor Adorno, (1903–1969), German philosopher * Theodor Aman, Romanian painter * Theodor Blue ...
and
Jürgen Habermas Jürgen Habermas ( , ; ; born 18 June 1929) is a German philosopher and social theorist in the tradition of critical theory and pragmatism. His work addresses communicative rationality and the public sphere. Associated with the Frankfurt S ...
(in their essays in ''The Positivist Debate in German Sociology'').


See also

* Empirical psychology * List of psychological laboratories * Outline of psychology *
Psychonomic Society The Psychonomic Society is an international scientific society of over 4,500 scientists in the field of experimental psychology. The mission of the Psychonomic Society is to foster the science of cognition through the advancement and communicatio ...
* Sackett self-selection circus *
Society of Experimental Psychologists The Society of Experimental Psychologists (SEP), originally called the Society of Experimentalists, is an academic society for experimental psychologists. It was founded by Edward Bradford Titchener in 1904 to be an ongoing workshop in which mem ...


Notes


References

* * {{Authority control Mathematical and quantitative methods (economics)