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{{see also|Confirmation bias|Cherry picking (fallacy)}}

'''Anecdotal''' '''evidence''' is [[evidence]] from [[anecdote]]s, i.e., evidence collected in a casual or informal manner and relying heavily or entirely on personal testimony. 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]] as some anecdotal evidence can be both empirical and verifiable, e.g. in the use of [[case studies]] in medicine. Other anecdotal evidence, however, does not qualify as scientific evidence, 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 picking (fallacy)|cherry-picked]] or otherwise [[Sampling (statistics)|non-representative]] samples of typical cases.<ref name=weiten>[https://books.google.com/books?id=sILajOhJpOsC&lpg=PP1&pg=PT121#v=onepage&q&f=false p. 75 of Psychology: Themes and Variations] by Wayne Weiten</ref><ref>[https://books.google.com/books?id=eNsVUGTMcDoC p. 25 in Research in Psychology: Methods and Design], by C. James Goodwin.</ref> Similarly, psychologists have found that due to [[cognitive bias]] people are more likely to remember notable or unusual examples rather than typical examples.<ref>{{cite journal |last=Gibson |first=Rhonda |last2=Zillman |first2=Dolf |year=1994 |title=Exaggerated Versus Representative Exemplification in News Reports: Perception of Issues and Personal Consequences |journal=Communication Research |volume=21 |issue=5 |pages=603–624 |doi=10.1177/009365094021005003 }}</ref> Thus, even when accurate, anecdotal evidence is not necessarily representative of a typical experience. Accurate determination of whether an anecdote is typical requires [[statistical]] evidence.<ref>Schwarz J, Barrett S. ''Some Notes on the Nature of Evidence''.[http://www.quackwatch.com/01QuackeryRelatedTopics/science.html Link.] Retrieved 26 August 2012.</ref> Misuse of anecdotal evidence is an [[informal fallacy]] 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. Compare with [[hasty generalization]].

The term is sometimes used in a legal context to describe certain kinds of [[testimony]] which are uncorroborated by objective, independent evidence such as notarized documentation, photographs, audio-visual recordings, etc.

When used in [[advertising]] or promotion of a product, service, or idea, anecdotal reports are often called a [[testimonial]], which are highly regulated<ref>[https://www.ftc.gov/sites/default/files/attachments/press-releases/ftc-publishes-final-guides-governing-endorsements-testimonials/091005revisedendorsementguides.pdf Guides Concerning the Use of Endorsements and Testimonials in Advertising]</ref> or banned in some{{which|date=December 2015}} jurisdictions.{{Citation needed|date=July 2008}}

==Introduction==

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"<ref name="yourdictionary">[http://www.yourdictionary.com/ahd/a/a0294100.html YourDictionary.com] {{webarchive |url=https://web.archive.org/web/20070312062259/http://www.yourdictionary.com/ahd/a/a0294100.html |date=March 12, 2007 }}</ref>
*"information passed along by word-of-mouth but not documented scientifically"{{Citation needed|date=May 2013}}

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]], and is subjected to formal [[peer review]].<ref name="Jenicek">{{cite book |last=Jenicek |first=M. |title=Clinical Case Reporting in Evidence-Based Medicine |location=Oxford |publisher=Butterworth–Heinemann |year=1999 |pages=117 |isbn=0-7506-4592-X }}</ref> Although such evidence is not seen as conclusive, it is sometimes regarded as an invitation to more rigorous scientific study of the phenomenon in question.<ref name="Vandenbroucke">{{cite journal |last=Vandenbroucke |first=J. P. |year=2001 |title=In Defense of Case Reports and Case Series |journal=Annals of Internal Medicine |volume=134 |issue=4 |pages=330–334 |url=http://www.annals.org/content/134/4/330.short |pmid=11182844 |doi=10.7326/0003-4819-134-4-200102200-00017}}</ref> For instance, one study found that 35 of 47 anecdotal reports of drug side-effects were later sustained as "clearly correct."<ref name="Venning">{{cite journal |last=Venning |first=G. R. |title=Validity of anecdotal reports of suspected adverse drug reactions: the problem of false alarms |journal=Br Med J (Clin Res Ed) |year=1982 |volume=284 |issue=6311 |pages=249–52 |pmid=0006799125 |doi=10.1136/bmj.284.6311.249 |pmc=1495801}}</ref>

Anecdotal evidence is considered the least certain type of scientific information.<ref name="isbn0-12-588560-1">{{cite book |author=Riffenburgh, R. H. |title=Statistics in medicine |publisher=Academic Press |location=Boston |year=1999 |pages=196 |isbn=0-12-588560-1 |oclc= |doi= |accessdate=}}</ref> Researchers may use anecdotal evidence for suggesting new [[hypothesis|hypotheses]], but never as validating evidence.

==Faulty logic==

Anecdotal evidence is often unscientific or [[pseudoscience|pseudoscientific]] because various forms of [[cognitive bias]] 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 [[falsifiability|falsifiable]]. This phenomenon can also happen to large groups of people through [[subjective validation]].

Anecdotal evidence is also frequently misinterpreted via the [[availability heuristic]], 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 [[fallacy|fallacious]] reasoning such as the ''[[Post hoc ergo propter hoc]]'' 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]]. For instance, if an anecdote illustrates a desired conclusion rather than a logical conclusion, it is considered a [[faulty generalization|faulty]] or [[hasty generalization]].<ref name="Thompson">Thompson B. [http://www.cuyamaca.edu/bruce.thompson/Fallacies/anecdotal.asp Fallacies.] {{webarchive |url=https://web.archive.org/web/20060420154531/http://www.cuyamaca.edu/bruce.thompson/Fallacies/anecdotal.asp |date=April 20, 2006 }}</ref> For example, here is anecdotal evidence presented as proof of a desired conclusion:

<blockquote>There's abundant proof that drinking water cures cancer. Just last week I read about a girl who was dying of cancer. After drinking water she was cured.</blockquote>

Anecdotes like this do not prove anything.<ref>{{cite web |url=http://www.infidels.org/news/atheism/logic.html |title=The Atheism Web: Logic & Fallacies |website=infidels.org |archiveurl=https://web.archive.org/web/20030602105657/http://www.infidels.org/news/atheism/logic.html |archivedate=2003-06-02}}</ref> 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 40 a day until he died at 90" and "my sister never went near anyone who 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."<ref>{{cite journal |first=Scott H. |last=Sicherer |title=Food allergy: When and how to perform oral food challenges |journal=Pediatric Allergy & Immunology |year=1999 |volume=10 |issue=4 |pages=226–234 |doi=10.1034/j.1399-3038.1999.00040.x }}</ref>

More generally, a statistical correlation between things does not in itself prove that one causes the other (a [[causal]] 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]]s:<ref name="Lee">Lee D (2005). [http://www.medicinenet.com/script/main/art.asp?articlekey=55090&page=2 Evaluating Medications and Supplement Products.] via MedicineNet</ref> it is well-established that a patient's (or doctor's) expectation can genuinely change the outcome of treatment. Only [[double-blind]] [[random sample|randomized]] [[placebo]]-controlled [[clinical trial]]s can confirm a [[hypothesis]] 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==
{{refimprove section|date=March 2016}}
[[Witness]] [[testimony]] is a common form of [[evidence (law)|evidence]] 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]] as part of a [[class action lawsuit]]. 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."<ref name="Altman">{{cite journal |last=Altman |first=D. G. |last2=Bland |first2=M. |title=Absence of evidence is not evidence of absence |journal=British Medical Journal |year=1995 |volume=311 |issue=7003 |pages=485 |doi=10.1136/bmj.311.7003.485 }}</ref>

==See also==
{{wiktionary|anecdotal evidence}}
* [[Anecdotal value]]
* [[Confirmation bias]]
* [[Empirical evidence]]
* [[Fallacy]]
* [[Faulty generalization]]
* [[Hasty generalization]]
* [[Post hoc ergo propter hoc]]
* [[Presumption of guilt]]
* [[Scientific method]]

==References==
{{reflist|30em}}

== Further reading ==
*[http://www.med.govt.nz/ri/insolvency/review/consultation/lawcomreview/lawcomreview-04.html Overall Assessment of Approach and Analysis of the Law Commission Report] Ministry of Economic Development, New Zealand
*[http://www.sprengerlang.com/legal/terms Legal Terms & Definitions] Sprenger & Lang, Attorneys
*[http://www.msnbc.msn.com/id/5269131/ Judge certifies Wal-Mart class action lawsuit] MSBN, June 22, 2004
*[https://web.archive.org/web/20060113020414/http://phoenix.gov/CITYGOV/disparty.html Second Generation Disparity Study Final Report] City of Phoenix study
*[https://web.archive.org/web/20060216235414/http://www.santarosa.edu/~dpeterso/permanenthtml/propaganda/prop_anecdotal.htm "Anecdotal Evidence"] from a course in [[Critical thinking]] at [[Santa Rosa Junior College]]. No longer active {{as of|2013|01|17|lc=y}}
*Lindsay, Don. [http://www.don-lindsay-archive.org/skeptic/anecdotal.html Anecdotal Evidence], via don-lindsay-archive.org
*Carroll, Robert Todd. [http://skepdic.com/testimon.html Anecdotal (testimonial) evidence], from the [[Skeptic's Dictionary]].
*Thompson, Bruce. [https://web.archive.org/web/20120731034427/http://courses.csusm.edu/fallacies/anecdotal.htm Anecdotal Evidence], describing its use as a fallacious argument.

{{Informal fallacy}}

[[Category:Informal fallacies]]
[[Category:Philosophy of science]]
[[Category:Skepticism]]
[[Category:Evidence]]
[[Category:Testimony]]
[[Category:Inductive fallacies]]
[[Category:Diversionary tactics]]
[[Category:Misuse of statistics]]

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