anecdotal evidence
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Anecdotal evidence is evidence based only on personal observation, collected in a casual or non-systematic manner. The term is sometimes used in a legal context to describe certain kinds of
testimony In law and in religion, testimony is a solemn attestation as to the truth of a matter. Etymology The words "testimony" and "testify" both derive from the Latin word ''testis'', referring to the notion of a disinterested Third-party source, thir ...
which are uncorroborated by objective, independent evidence such as notarized documentation, photographs, audio-visual recordings, etc. When used in
advertising Advertising is the practice and techniques employed to bring attention to a product or service. Advertising aims to put a product or service in the spotlight in hopes of drawing it attention from consumers. It is typically used to promote a ...
or promotion of a product, service, or idea, anecdotal reports are often called a
testimonial In promotion (marketing), promotion and advertising, a testimonial or show consists of a person's written or spoken statement extolling the virtue of a product (business), product. The term "testimonial" most commonly applies to the sales pitc ...
, which are highly regulated in some jurisdictions. When compared to other types of evidence, anecdotal evidence is generally regarded as limited in value due to a number of potential weaknesses, but may be considered within the scope of
scientific method The scientific method is an Empirical evidence, empirical method for acquiring knowledge that has characterized the development of science since at least the 17th century (with notable practitioners in previous centuries; see the article hist ...
as some anecdotal evidence can be both empirical and verifiable, e.g. in the use of
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 Medicine is the science and Praxis (process), practice of caring for a patient, managing the di ...
in medicine. Other anecdotal evidence, however, does not qualify as
scientific evidence Scientific evidence is evidence that serves to either support or counter a scientific theory or hypothesis, although scientists also use evidence in other ways, such as when applying theories to practical problems. "Discussions about empirical ev ...
, because its nature prevents it from being investigated by the scientific method. Where only one or a few anecdotes are presented, there is a larger chance that they may be unreliable due to cherry-picked or otherwise non-representative samples of typical cases. Similarly, psychologists have found that due to
cognitive bias A cognitive bias is a systematic pattern of deviation from norm (philosophy), norm or rationality in judgment. Individuals create their own "subjective reality" from their perception of the input. An individual's construction of reality, not the ...
people are more likely to remember notable or unusual examples rather than typical examples. Thus, even when accurate, anecdotal evidence is not necessarily representative of a typical experience. Accurate determination of whether an anecdote is typical requires
statistical Statistics (from German language, German: ''wikt:Statistik#German, Statistik'', "description of a State (polity), state, a country") is the discipline that concerns the collection, organization, analysis, interpretation, and presentation of ...
evidence. Misuse of anecdotal evidence in the form of argument from anecdote is an
informal fallacy Informal fallacies are a type of incorrect argument in natural language. The source of the error is not just due to the ''form'' of the argument, as is the case for formal fallacies, but can also be due to their ''content'' and ''context''. Fall ...
and is sometimes referred to as the "person who" fallacy ("I know a person who..."; "I know of a case where..." etc.) which places undue weight on experiences of close peers which may not be typical. In all forms of anecdotal evidence its reliability by objective independent assessment may be in doubt. This is a consequence of the informal way the information is gathered, documented, presented, or any combination of the three. The term is often used to describe evidence for which there is an absence of documentation, leaving verification dependent on the credibility of the party presenting the evidence.


Scientific context

In science, definitions of anecdotal evidence include: * "casual observations or indications rather than rigorous or scientific analysis" * "information passed along by word-of-mouth but not documented scientifically" *"evidence that comes from an individual experience. This may be the experience of a person with an illness or the experience of a practitioner based on one or more patients outside a formal research study." *"the report of an experience by one or more persons that is not objectively documented or an experience or outcome that occurred outside of a controlled environment" Anecdotal evidence can have varying degrees of formality. For instance, in medicine, published anecdotal evidence by a trained observer (a doctor) is called a
case report In medicine Medicine is the science and Praxis (process), practice of caring for a patient, managing the diagnosis, prognosis, Preventive medicine, prevention, therapy, treatment, Palliative care, palliation of their injury or disease, and Heal ...
, and is subjected to formal
peer review Peer review is the evaluation of work by one or more people with similar competencies as the producers of the work ( peers). It functions as a form of self-regulation by qualified members of a profession within the relevant field. Peer revie ...
. Although such evidence is not seen as conclusive, researchers may sometimes regard it as an invitation to more rigorous scientific study of the phenomenon in question. For instance, one study found that 35 of 47 anecdotal reports of drug side-effects were later sustained as "clearly correct." Anecdotal evidence is considered the least certain type of scientific information. Researchers may use anecdotal evidence for suggesting new
hypotheses A hypothesis (plural hypotheses) is a proposed explanation for a phenomenon A phenomenon (plural, : phenomena) is an observable event. The term came into its modern Philosophy, philosophical usage through Immanuel Kant, who contrasted i ...
, but never as validating evidence. Anecdotal evidence is often unscientific or
pseudoscientific Pseudoscience consists of statements, beliefs, or practices that claim to be both scientific and factual but are incompatible with the scientific method. Pseudoscience is often characterized by contradictory, exaggerated or falsifiability, unfa ...
because various forms of
cognitive bias A cognitive bias is a systematic pattern of deviation from norm (philosophy), norm or rationality in judgment. Individuals create their own "subjective reality" from their perception of the input. An individual's construction of reality, not the ...
may affect the collection or presentation of evidence. For instance, someone who claims to have had an encounter with a supernatural being or alien may present a very vivid story, but this is not
falsifiable Falsifiability is a standard of evaluation of scientific theories and hypotheses that was introduced by the philosopher of science Karl Popper in his book '' The Logic of Scientific Discovery'' (1934). He proposed it as the cornerstone of a ...
. This phenomenon can also happen to large groups of people through
subjective validation Subjective validation, sometimes called personal validation effect, is a cognitive bias A cognitive bias is a systematic pattern of deviation from norm (philosophy), norm or rationality in judgment. Individuals create their own "subjective real ...
. Anecdotal evidence is also frequently misinterpreted via the
availability heuristic The availability heuristic, also known as availability bias, is a mental shortcut that relies on immediate examples that come to a given person's mind when evaluating a specific topic, concept, method, or decision. This heuristics in judgment and d ...
, which leads to an overestimation of prevalence. Where a cause can be easily linked to an effect, people overestimate the likelihood of the cause having that effect (availability). In particular, vivid, emotionally charged anecdotes seem more plausible, and are given greater weight. A related issue is that it is usually impossible to assess for every piece of anecdotal evidence, the rate of people not reporting that anecdotal evidence in the population. A common way anecdotal evidence becomes unscientific is through
fallacious A fallacy is the use of Validity (logic), invalid or otherwise faulty reasoning, or "wrong moves," in the construction of an argument which may appear stronger than it really is if the fallacy is not spotted. The term in the Western intellectual ...
reasoning such as the ''
post hoc ergo propter hoc ''Post hoc ergo propter hoc'' (Latin: 'after this, therefore because of this') is an informal fallacy that states: "Since event Y ''followed'' event X, event Y must have been ''caused'' by event X." It is often shortened simply to ''post hoc fall ...
'' fallacy, the human tendency to assume that if one event happens after another, then the first must be the cause of the second. Another fallacy involves
inductive reasoning Inductive reasoning is a method of reasoning in which a general principle is derived from a body of observations. It consists of making broad generalizations based on specific observations. Inductive reasoning is distinct from Deductive reasonin ...
. For instance, if an anecdote illustrates a desired conclusion rather than a logical conclusion, it is considered a faulty or
hasty generalization A faulty generalization is an informal fallacy wherein a conclusion is drawn about all or many instances of a phenomenon on the basis of one or a few instances of that phenomenon. It is similar to a proof by example in mathematics. It is an exampl ...
.Thompson B
Fallacies.
For example, here is anecdotal evidence presented as proof of a desired conclusion: Anecdotes like this do not prove anything. In any case where some factor affects the probability of an outcome, rather than uniquely determining it, selected individual cases prove nothing; e.g. "my grandfather smoked two packs a day until he died at 90" and "my sister never smoked but died of lung cancer". Anecdotes often refer to the exception, rather than the rule: "Anecdotes are useless precisely because they may point to idiosyncratic responses." More generally, a statistical correlation between things does not in itself prove that one causes the other (a
causal Causality (also referred to as causation, or cause and effect) is influence by which one Event (relativity), event, process, state, or object (''a'' ''cause'') contributes to the production of another event, process, state, or object (an ''eff ...
link). A study found that television viewing was strongly correlated with sugar consumption, but this does not prove that viewing causes sugar intake (or vice versa). In medicine, anecdotal evidence is also subject to
placebo effect A placebo ( ) is a substance or treatment which is designed to have no therapeutic value. Common placebos include inert tablets (like sugar pills), inert injections (like Saline (medicine), saline), sham surgery, and other procedures. In general ...
s: it is well-established that a patient's (or doctor's) expectation can genuinely change the outcome of treatment. Only
double-blind In a blind or blinded experiment, information which may influence the participants of the experiment An experiment is a procedure carried out to support or refute a hypothesis, or determine the efficacy or likelihood of something previousl ...
randomized In common usage, randomness is the apparent or actual lack of pattern or predictability in events. A random sequence of events, symbols or steps often has no :wikt:order, order and does not follow an intelligible pattern or combination. Ind ...
placebo A placebo ( ) is a substance or treatment which is designed to have no therapeutic value. Common placebos include inert tablets (like sugar pills), inert injections (like Saline (medicine), saline), sham surgery, and other procedures. In general ...
-controlled
clinical trial Clinical trials are prospective biomedical or behavioral research studies on human subject research, human participants designed to answer specific questions about biomedical or behavioral interventions, including new treatments (such as novel v ...
s can confirm a
hypothesis A hypothesis (plural hypotheses) is a proposed explanation for a phenomenon. For a hypothesis to be a scientific hypothesis, the scientific method requires that one can testable, test it. Scientists generally base scientific hypotheses on prev ...
about the effectiveness of a treatment independently of expectations. By contrast, in science and logic, the "relative strength of an explanation" is based upon its ability to be: * tested or repeated * proven to be due to the stated cause, and * verifiable under neutral conditions in a manner that other researchers will agree has been performed competently, and can check for themselves


Law

Witness In law, a witness is someone who has knowledge about a matter, whether they have sensed it or are testifying on another witnesses' behalf. In law a witness is someone who, either voluntarily or under compulsion, provides testimonial evidence, e ...
testimony In law and in religion, testimony is a solemn attestation as to the truth of a matter. Etymology The words "testimony" and "testify" both derive from the Latin word ''testis'', referring to the notion of a disinterested Third-party source, thir ...
is a common form of
evidence Evidence for a proposition is what supports this proposition. It is usually understood as an indication that the supported proposition is true. What role evidence plays and how it is conceived varies from field to field. In epistemology, evid ...
in law, and law has mechanisms to test witness evidence for reliability or credibility. Legal processes for the taking and assessment of evidence are formalized. Some witness testimony may be described as anecdotal evidence, such as individual stories of
harassment Harassment covers a wide range of behaviors of wikt:offensive, offensive nature. It is commonly understood as behavior that demeans, humiliates or embarrasses a person, and it is characteristically identified by its unlikelihood in terms of soc ...
as part of a
class action lawsuit A class action, also known as a class-action lawsuit, class suit, or representative action, is a type of lawsuit where one of the parties is a group of people who are represented collectively by a member or members of that group. The class action ...
. However, witness testimony can be tested and assessed for reliability. Examples of approaches to testing and assessment include the use of questioning to identify possible gaps or inconsistencies, evidence of corroborating witnesses, documents, video and forensic evidence. Where a court lacks suitable means to test and assess testimony of a particular witness, such as the absence of forms of corroboration or substantiation, it may afford that testimony limited or no "weight" when making a decision on the facts.


Scientific evidence as legal evidence

In certain situations, scientific evidence presented in court must also meet the legal requirements for evidence. For instance, in the United States, expert testimony of witnesses must meet the Daubert standard. This ruling holds that before evidence is presented to witnesses by experts, the methodology must be "generally accepted" among scientists. In some situations, anecdotal evidence may meet this threshold (such as certain case reports which corroborate or refute other evidence). Altman and Bland argue that the case report or statistical outlier cannot be dismissed as having no weight: "With rare and uncommonly occurring diseases, a nonsignificant finding in a randomized trial does not necessarily mean that there is no causal association between the agent in question and the disease."


See also

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References

{{Authority control Informal fallacies Philosophy of science Skepticism Evidence Testimony Inductive fallacies Diversionary tactics Misuse of statistics