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In ordinary language, an average is a single number taken as representative of a list of numbers, usually the sum of the numbers divided by how many numbers are in the list (the
arithmetic mean In mathematics and statistics, the arithmetic mean ( ) or arithmetic average, or just the ''mean'' or the ''average'' (when the context is clear), is the sum of a collection of numbers divided by the count of numbers in the collection. The colle ...
). For example, the average of the numbers 2, 3, 4, 7, and 9 (summing to 25) is 5. Depending on the context, an average might be another
statistic A statistic (singular) or sample statistic is any quantity computed from values in a sample which is considered for a statistical purpose. Statistical purposes include estimating a population parameter, describing a sample, or evaluating a hypo ...
such as the
median In statistics and probability theory, the median is the value separating the higher half from the lower half of a data sample, a population, or a probability distribution. For a data set, it may be thought of as "the middle" value. The basic fe ...
, or
mode Mode ( la, modus meaning "manner, tune, measure, due measure, rhythm, melody") may refer to: Arts and entertainment * '' MO''D''E (magazine)'', a defunct U.S. women's fashion magazine * ''Mode'' magazine, a fictional fashion magazine which is ...
. For example, the average
personal income In economics, personal income refers to an individual's total earnings from wages, investment enterprises, and other ventures. It is the sum of all the incomes received by all the individuals or household during a given period. Personal income is ...
is often given as the median—the number below which are 50% of personal incomes and above which are 50% of personal incomes—because the
mean There are several kinds of mean in mathematics, especially in statistics. Each mean serves to summarize a given group of data, often to better understand the overall value (magnitude and sign) of a given data set. For a data set, the ''arithme ...
would be higher by including personal incomes from a few
billionaire A billionaire is a person with a net worth of at least one billion (1,000,000,000, i.e., a thousand million) units of a given currency, usually of a major currency such as the United States dollar, euro, or pound sterling. The American busin ...
s. For this reason, it is recommended to avoid using the word "average" when discussing measures of
central tendency In statistics, a central tendency (or measure of central tendency) is a central or typical value for a probability distribution.Weisberg H.F (1992) ''Central Tendency and Variability'', Sage University Paper Series on Quantitative Applications ...
.


General properties

If all numbers in a list are the same number, then their average is also equal to this number. This property is shared by each of the many types of average. Another universal property is
monotonicity In mathematics, a monotonic function (or monotone function) is a function between ordered sets that preserves or reverses the given order. This concept first arose in calculus, and was later generalized to the more abstract setting of orde ...
: if two lists of numbers ''A'' and ''B'' have the same length, and each entry of list ''A'' is at least as large as the corresponding entry on list ''B'', then the average of list ''A'' is at least that of list ''B''. Also, all averages satisfy linear homogeneity: if all numbers of a list are multiplied by the same positive number, then its average changes by the same factor. In some types of average, the items in the list are assigned different weights before the average is determined. These include the
weighted arithmetic mean The weighted arithmetic mean is similar to an ordinary arithmetic mean (the most common type of average), except that instead of each of the data points contributing equally to the final average, some data points contribute more than others. The ...
, the
weighted geometric mean In statistics, the weighted geometric mean is a generalization of the geometric mean using the weighted arithmetic mean. Given a sample x=(x_1,x_2\dots,x_n) and weights w=(w_1, w_2,\dots,w_n), it is calculated as: : \bar = \left(\prod_^n x_i^\ri ...
and the
weighted median In statistics, a weighted median of a sample is the 50% weighted percentile. It was first proposed by F. Y. Edgeworth in 1888. Like the median, it is useful as an estimator of central tendency, robust against outliers. It allows for non-unifor ...
. Also, for some types of
moving average In statistics, a moving average (rolling average or running average) is a calculation to analyze data points by creating a series of averages of different subsets of the full data set. It is also called a moving mean (MM) or rolling mean and is ...
, the weight of an item depends on its position in the list. Most types of average, however, satisfy
permutation In mathematics, a permutation of a set is, loosely speaking, an arrangement of its members into a sequence or linear order, or if the set is already ordered, a rearrangement of its elements. The word "permutation" also refers to the act or proc ...
-insensitivity: all items count equally in determining their average value and their positions in the list are irrelevant; the average of (1, 2, 3, 4, 6) is the same as that of (3, 2, 6, 4, 1).


Pythagorean means

The
arithmetic mean In mathematics and statistics, the arithmetic mean ( ) or arithmetic average, or just the ''mean'' or the ''average'' (when the context is clear), is the sum of a collection of numbers divided by the count of numbers in the collection. The colle ...
, the
geometric mean In mathematics, the geometric mean is a mean or average which indicates a central tendency of a set of numbers by using the product of their values (as opposed to the arithmetic mean which uses their sum). The geometric mean is defined as the ...
and the
harmonic mean In mathematics, the harmonic mean is one of several kinds of average, and in particular, one of the Pythagorean means. It is sometimes appropriate for situations when the average rate is desired. The harmonic mean can be expressed as the recipro ...
are known collectively as the ''Pythagorean means''.


Statistical location

The
mode Mode ( la, modus meaning "manner, tune, measure, due measure, rhythm, melody") may refer to: Arts and entertainment * '' MO''D''E (magazine)'', a defunct U.S. women's fashion magazine * ''Mode'' magazine, a fictional fashion magazine which is ...
, the
median In statistics and probability theory, the median is the value separating the higher half from the lower half of a data sample, a population, or a probability distribution. For a data set, it may be thought of as "the middle" value. The basic fe ...
, and the
mid-range In statistics, the mid-range or mid-extreme is a measure of central tendency of a sample defined as the arithmetic mean of the maximum and minimum values of the data set: :M=\frac. The mid-range is closely related to the range, a measure of ...
are often used in addition to the
mean There are several kinds of mean in mathematics, especially in statistics. Each mean serves to summarize a given group of data, often to better understand the overall value (magnitude and sign) of a given data set. For a data set, the ''arithme ...
as estimates of
central tendency In statistics, a central tendency (or measure of central tendency) is a central or typical value for a probability distribution.Weisberg H.F (1992) ''Central Tendency and Variability'', Sage University Paper Series on Quantitative Applications ...
in
descriptive statistics A descriptive statistic (in the count noun sense) is a summary statistic that quantitatively describes or summarizes features from a collection of information, while descriptive statistics (in the mass noun sense) is the process of using and an ...
. These can all be seen as minimizing variation by some measure; see .


Mode

The most frequently occurring number in a list is called the mode. For example, the mode of the list (1, 2, 2, 3, 3, 3, 4) is 3. It may happen that there are two or more numbers which occur equally often and more often than any other number. In this case there is no agreed definition of mode. Some authors say they are all modes and some say there is no mode.


Median

The median is the middle number of the group when they are ranked in order. (If there are an even number of numbers, the mean of the middle two is taken.) Thus to find the median, order the list according to its elements' magnitude and then repeatedly remove the pair consisting of the highest and lowest values until either one or two values are left. If exactly one value is left, it is the median; if two values, the median is the arithmetic mean of these two. This method takes the list 1, 7, 3, 13 and orders it to read 1, 3, 7, 13. Then the 1 and 13 are removed to obtain the list 3, 7. Since there are two elements in this remaining list, the median is their arithmetic mean, (3 + 7)/2 = 5.


Mid-range

The mid-range is the arithmetic mean of the highest and lowest values of a set.


Summary of types

The
table of mathematical symbols A mathematical symbol is a figure or a combination of figures that is used to represent a mathematical object, an action on mathematical objects, a relation between mathematical objects, or for structuring the other symbols that occur in a formula ...
explains the symbols used below.


Miscellaneous types

Other more sophisticated averages are:
trimean In statistics the trimean (TM), or Tukey's trimean, is a measure of a probability distribution's location defined as a weighted average of the distribution's median and its two quartiles: : TM= \frac This is equivalent to the average of the med ...
, trimedian, and
normalized mean Normalization or normalisation refers to a process that makes something more normal or regular. Most commonly it refers to: * Normalization (sociology) or social normalization, the process through which ideas and behaviors that may fall outside of ...
, with their generalizations. One can create one's own average metric using the generalized ''f''-mean: : y = f^\left(\frac\left (x_1) + f(x_2) + \cdots + f(x_n)\rightright) where ''f'' is any invertible function. The harmonic mean is an example of this using ''f''(''x'') = 1/''x'', and the geometric mean is another, using ''f''(''x'') = log ''x''. However, this method for generating means is not general enough to capture all averages. A more general method for defining an average takes any function ''g''(''x''1, ''x''2, ..., ''x''''n'') of a list of arguments that is
continuous Continuity or continuous may refer to: Mathematics * Continuity (mathematics), the opposing concept to discreteness; common examples include ** Continuous probability distribution or random variable in probability and statistics ** Continuous ...
, strictly increasing in each argument, and symmetric (invariant under
permutation In mathematics, a permutation of a set is, loosely speaking, an arrangement of its members into a sequence or linear order, or if the set is already ordered, a rearrangement of its elements. The word "permutation" also refers to the act or proc ...
of the arguments). The average ''y'' is then the value that, when replacing each member of the list, results in the same function value: . This most general definition still captures the important property of all averages that the average of a list of identical elements is that element itself. The function provides the arithmetic mean. The function (where the list elements are positive numbers) provides the geometric mean. The function (where the list elements are positive numbers) provides the harmonic mean.


Average percentage return and CAGR

A type of average used in finance is the average percentage return. It is an example of a geometric mean. When the returns are annual, it is called the Compound Annual Growth Rate (CAGR). For example, if we are considering a period of two years, and the investment return in the first year is −10% and the return in the second year is +60%, then the average percentage return or CAGR, ''R'', can be obtained by solving the equation: . The value of ''R'' that makes this equation true is 0.2, or 20%. This means that the total return over the 2-year period is the same as if there had been 20% growth each year. The order of the years makes no difference – the average percentage returns of +60% and −10% is the same result as that for −10% and +60%. This method can be generalized to examples in which the periods are not equal. For example, consider a period of a half of a year for which the return is −23% and a period of two and a half years for which the return is +13%. The average percentage return for the combined period is the single year return, ''R'', that is the solution of the following equation: , giving an average return ''R'' of 0.0600 or 6.00%.


Moving average

Given a
time series In mathematics, a time series is a series of data points indexed (or listed or graphed) in time order. Most commonly, a time series is a sequence taken at successive equally spaced points in time. Thus it is a sequence of discrete-time data. Exa ...
, such as daily stock market prices or yearly temperatures, people often want to create a smoother series. This helps to show underlying trends or perhaps periodic behavior. An easy way to do this is the ''moving average'': one chooses a number ''n'' and creates a new series by taking the arithmetic mean of the first ''n'' values, then moving forward one place by dropping the oldest value and introducing a new value at the other end of the list, and so on. This is the simplest form of moving average. More complicated forms involve using a
weighted average The weighted arithmetic mean is similar to an ordinary arithmetic mean (the most common type of average), except that instead of each of the data points contributing equally to the final average, some data points contribute more than others. The ...
. The weighting can be used to enhance or suppress various periodic behavior and there is very extensive analysis of what weightings to use in the literature on
filtering Filter, filtering or filters may refer to: Science and technology Computing * Filter (higher-order function), in functional programming * Filter (software), a computer program to process a data stream * Filter (video), a software component tha ...
. In
digital signal processing Digital signal processing (DSP) is the use of digital processing, such as by computers or more specialized digital signal processors, to perform a wide variety of signal processing operations. The digital signals processed in this manner are ...
the term "moving average" is used even when the sum of the weights is not 1.0 (so the output series is a scaled version of the averages). The reason for this is that the analyst is usually interested only in the trend or the periodic behavior.


History


Origin

The first recorded time that the
arithmetic mean In mathematics and statistics, the arithmetic mean ( ) or arithmetic average, or just the ''mean'' or the ''average'' (when the context is clear), is the sum of a collection of numbers divided by the count of numbers in the collection. The colle ...
was extended from 2 to n cases for the use of
estimation Estimation (or estimating) is the process of finding an estimate or approximation, which is a value that is usable for some purpose even if input data may be incomplete, uncertain, or unstable. The value is nonetheless usable because it is der ...
was in the sixteenth century. From the late sixteenth century onwards, it gradually became a common method to use for reducing errors of measurement in various areas.Eisenhart, Churchill. "The development of the concept of the best mean of a set of measurements from antiquity to the present day." Unpublished presidential address, American Statistical Association, 131st Annual Meeting, Fort Collins, Colorado. 1971.
/ref> At the time, astronomers wanted to know a real value from noisy measurement, such as the position of a planet or the diameter of the moon. Using the mean of several measured values, scientists assumed that the errors add up to a relatively small number when compared to the total of all measured values. The method of taking the mean for reducing observation errors was indeed mainly developed in astronomy.
/ref> A possible precursor to the arithmetic mean is the
mid-range In statistics, the mid-range or mid-extreme is a measure of central tendency of a sample defined as the arithmetic mean of the maximum and minimum values of the data set: :M=\frac. The mid-range is closely related to the range, a measure of ...
(the mean of the two extreme values), used for example in Arabian astronomy of the ninth to eleventh centuries, but also in metallurgy and navigation. However, there are various older vague references to the use of the arithmetic mean (which are not as clear, but might reasonably have to do with our modern definition of the mean). In a text from the 4th century, it was written that (text in square brackets is a possible missing text that might clarify the meaning): : In the first place, we must set out in a row the sequence of numbers from the monad up to nine: 1, 2, 3, 4, 5, 6, 7, 8, 9. Then we must add up the amount of all of them together, and since the row contains nine terms, we must look for the ninth part of the total to see if it is already naturally present among the numbers in the row; and we will find that the property of being neninth
f the sum F, or f, is the sixth letter in the Latin alphabet, used in the modern English alphabet, the alphabets of other western European languages and others worldwide. Its name in English is ''ef'' (pronounced ), and the plural is ''efs''. Hist ...
only belongs to the
rithmetic Arithmetic () is an elementary part of mathematics that consists of the study of the properties of the traditional operations on numbers— addition, subtraction, multiplication, division, exponentiation, and extraction of roots. In the 19th ...
mean itself... Even older potential references exist. There are records that from about 700 BC, merchants and shippers agreed that damage to the cargo and ship (their "contribution" in case of damage by the sea) should be shared equally among themselves. This might have been calculated using the average, although there seem to be no direct record of the calculation.


Etymology

The root is found in Arabic as عوار ''ʿawār'', a defect, or anything defective or damaged, including partially spoiled merchandise; and عواري ''ʿawārī'' (also عوارة ''ʿawāra'') = "of or relating to ''ʿawār'', a state of partial damage". Within the Western languages the word's history begins in medieval sea-commerce on the Mediterranean. 12th and 13th century Genoa Latin ''avaria'' meant "damage, loss and non-normal expenses arising in connection with a merchant sea voyage"; and the same meaning for ''avaria'' is in Marseille in 1210, Barcelona in 1258 and Florence in the late 13th. 15th-century French ''avarie'' had the same meaning, and it begot English "averay" (1491) and English "average" (1502) with the same meaning. Today, Italian ''avaria'', Catalan ''avaria'' and French ''avarie'' still have the primary meaning of "damage". The huge transformation of the meaning in English began with the practice in later medieval and early modern Western merchant-marine law contracts under which if the ship met a bad storm and some of the goods had to be thrown overboard to make the ship lighter and safer, then all merchants whose goods were on the ship were to suffer proportionately (and not whoever's goods were thrown overboard); and more generally there was to be proportionate distribution of any ''avaria''. From there the word was adopted by British insurers, creditors, and merchants for talking about their losses as being spread across their whole portfolio of assets and having a mean proportion. Today's meaning developed out of that, and started in the mid-18th century, and started in English.The Arabic origin of ''avaria'' was first reported by Reinhart Dozy in the 19th century. Dozy's original summary is in his 1869 boo
''Glossaire''
Summary information about the word's early records in Italian-Latin, Italian, Catalan, and French is a
''avarie'' @ CNRTL.fr
. The seaport of Genoa is the location of the earliest-known record in European languages, year 1157. A set of medieval Latin records of ''avaria'' at Genoa is in the downloadable lexico
''Vocabolario Ligure''
by Sergio Aprosio, year 2001, ''avaria'' in Volume 1 pages 115-116. Many more records in medieval Latin at Genoa are a
StoriaPatriaGenova.it
usually in the plurals ''avariis'' and ''avarias''. At the port of Marseille in the 1st half of the 13th century notarized commercial contracts have dozens of instances of Latin ''avariis'' (ablative plural of ''avaria''), as published i
Blancard year 1884
Some information about the English word over the centuries is a
NED (year 1888)
See also the definition of English "average" in English dictionaries published in the early 18th century, i.e., in the time period just before the big transformation of the meaning
Kersey-Phillips' dictionary (1706)Blount's dictionary (1707 edition)Hatton's dictionary (1712)Bailey's dictionary (1726)Martin's dictionary (1749)
Some complexities surrounding the English word's history are discussed i
Hensleigh Wedgwood year 1882 page 11
an
Walter Skeat year 1888 page 781
Today there is consensus that: (#1) today's English "average" descends from medieval Italian ''avaria'', Catalan ''avaria'', and (#2) among the Latins the word ''avaria'' started in the 12th century and it started as a term of Mediterranean sea-commerce, and (#3) there is no root for ''avaria'' to be found in Latin, and (#4) a substantial number of Arabic words entered Italian, Catalan and Provençal in the 12th and 13th centuries starting as terms of Mediterranean sea-commerce, and (#5) the Arabic ''ʿawār , ʿawārī'' is phonetically a good match for ''avaria'', as conversion of w to v was regular in Latin and Italian, and ''-ia'' is a suffix in Italian, and the Western word's earliest records are in Italian-speaking locales (writing in Latin). And most commentators agree that (#6) the Arabic ''ʿawār , ʿawārī'' = "damage , relating to damage" is semantically a good match for ''avaria'' = "damage or damage expenses". A minority of commentators have been dubious about this on the grounds that the early records of Italian-Latin ''avaria'' have, in some cases, a meaning of "an expense" in a more general sense

The majority view is that the meaning of "an expense" was an expansion from "damage and damage expense", and the chronological order of the meanings in the records supports this view, and the broad meaning "an expense" was never the most commonly used meaning. On the basis of the above points, the inferential step is made that the Latinate word came or probably came from the Arabic word.
Marine damage is either ''particular average'', which is borne only by the owner of the damaged property, or
general average The law of general average is a principle of maritime law whereby all stakeholders in a sea venture proportionately share any losses resulting from a voluntary sacrifice of part of the ship or cargo to save the whole in an emergency. For inst ...
, where the owner can claim a proportional contribution from all the parties to the marine venture. The type of calculations used in adjusting general average gave rise to the use of "average" to mean "arithmetic mean". A second English usage, documented as early as 1674 and sometimes spelled "averish", is as the residue and second growth of field crops, which were considered suited to consumption by
draught animal A working animal is an animal, usually domesticated, that is kept by humans and trained to perform tasks instead of being slaughtered to harvest animal products. Some are used for their physical strength (e.g. oxen and draft horses) or for t ...
s ("avers"). There is earlier (from at least the 11th century), unrelated use of the word. It appears to be an old legal term for a tenant's day labour obligation to a sheriff, probably anglicised from "avera" found in the English
Domesday Book Domesday Book () – the Middle English spelling of "Doomsday Book" – is a manuscript record of the "Great Survey" of much of England and parts of Wales completed in 1086 by order of King William I, known as William the Conqueror. The manusc ...
(1085). The Oxford English Dictionary, however, says that derivations from German ''hafen'' haven, and Arabic ''ʿawâr'' loss, damage, have been "quite disposed of" and the word has a Romance origin."average, n.2". OED Online. September 2019. Oxford University Press. https://www.oed.com/view/Entry/13681 (accessed September 05, 2019).


Averages as a rhetorical tool

Due to the aforementioned colloquial nature of the term "average", the term can be used to obfuscate the true meaning of data and suggest varying answers to questions based on the averaging method (most frequently arithmetic mean, median, or mode) used. In his article "Framed for Lying: Statistics as In/Artistic Proof",
University of Pittsburgh The University of Pittsburgh (Pitt) is a public state-related research university in Pittsburgh, Pennsylvania. The university is composed of 17 undergraduate and graduate schools and colleges at its urban Pittsburgh campus, home to the universit ...
faculty member Daniel Libertz comments that statistical information is frequently dismissed from rhetorical arguments for this reason. However, due to their persuasive power, averages and other statistical values should not be discarded completely, but instead used and interpreted with caution. Libertz invites us to engage critically not only with statistical information such as averages, but also with the language used to describe the data and its uses, saying: "If statistics rely on interpretation, rhetors should invite their audience to interpret rather than insist on an interpretation." In many cases, data and specific calculations are provided to help facilitate this audience-based interpretation.


See also

*
Average absolute deviation The average absolute deviation (AAD) of a data set is the average of the absolute deviations from a central point. It is a summary statistic of statistical dispersion or variability. In the general form, the central point can be a mean, median, m ...
*
Law of averages The law of averages is the commonly held belief that a particular outcome or event will, over certain periods of time, occur at a frequency that is similar to its probability. Depending on context or application it can be considered a valid common ...
*
Expected value In probability theory, the expected value (also called expectation, expectancy, mathematical expectation, mean, average, or first moment) is a generalization of the weighted average. Informally, the expected value is the arithmetic mean of a l ...
*
Central limit theorem In probability theory, the central limit theorem (CLT) establishes that, in many situations, when independent random variables are summed up, their properly normalized sum tends toward a normal distribution even if the original variables themselv ...
*
Population mean In statistics, a population is a set of similar items or events which is of interest for some question or experiment. A statistical population can be a group of existing objects (e.g. the set of all stars within the Milky Way galaxy) or a hypothe ...
*
Sample mean The sample mean (or "empirical mean") and the sample covariance are statistics computed from a Sample (statistics), sample of data on one or more random variables. The sample mean is the average value (or mean, mean value) of a sample (statistic ...


References


External links

{{Wiktionary
Median as a weighted arithmetic mean of all Sample Observations
Summary statistics Means Arithmetic functions