An anecdotal evidence (or anecdata^[1]) is a piece of evidence based on descriptions and reports of individual, personal experiences, or observations,^[2][3] collected in a non-systematic manner.^[4]

The word anecdotal constitutes a variety of forms of evidence. This word refers to personal experiences, self-reported claims,^[3] or eyewitness accounts of others,^[5] including those from fictional sources, making it a broad category that can lead to confusion due to its varied interpretations.

Anecdotal evidence can be true or false but is not usually subjected to the methodology of scholarly method, the scientific method, or the rules of legal, historical, academic, or intellectual rigor, meaning that there are little or no safeguards against fabrication or inaccuracy.^[2] However, the use of anecdotal reports in advertising or promotion of a product, service, or idea may be considered a testimonial, which is highly regulated in some jurisdictions.^[6]

The persuasiveness of anecdotal evidence compared to that of statistical evidence has been a subject of debate; some studies have argued for the presence a generalized tendency to overvalue anecdotal evidence, whereas others have emphasized the types of argument as a prerequisite or rejected the conclusion altogether.^{[7][8][9][10][11]}

In science, definitions of anecdotal evidence include:

• "casual observations/indications rather than rigorous or scientific analysis"^[12]
• "information passed along by word-of-mouth but not documented scientifically"^[13]
• "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."^[14]
• "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"^[15]

Anecdotal evidence 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, for instance, in that of folklore or in the case of intentionally fictional anecdotes. Where only one or a few anecdotes are presented, there is a chance that they may be unreliable due to cherry-picked or otherwise non-representative samples of typical cases.^[16][17] Similarly, psychologists have found that due to cognitive bias people are more likely to remember notable or unusual examples rather than typical examples.^[18] 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.^[19] Misuse of anecdotal evidence in the form of argument from anecdote is an informal fallacy^[20] 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.

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.^[21] 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.^[22] For instance, one study found that 35 of 47 anecdotal reports of drug side-effects were later sustained as "clearly correct."^[23]

Anecdotal evidence is considered the least certain type of scientific information.^[24] Researchers may use anecdotal evidence for suggesting new hypotheses, but never as validating evidence.^[25][26]

If an anecdote illustrates a desired conclusion rather than a logical conclusion, it is considered a faulty or hasty generalization.^[27]

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."^[28]

In medicine, anecdotal evidence is also subject to placebo effects.^[29]

In the legal sphere, anecdotal evidence, if it passes certain legal requirements and is admitted as testimony, is a common form of evidence used in a court of law. Often this form of anecdotal evidence is the only evidence presented at trial.^[30] Scientific evidence in a court of law is called physical evidence, but this is much rarer. Anecdotal evidence, with a few safeguards, represents the bulk of evidence in court.

The legal rigors applied to testimony for it to be considered evidence is that it must be given under oath, that the person is only testifying to their own words and actions, and that someone intentionally lying under oath is subject to perjury. However, these rigors do not make testimony in a court of law equal to scientific evidence as there are far less legal rigors. Testimony about another person's experiences or words is called hearsay and is usually not admissible, though there are certain exceptions. However, any hearsay that is not objected to or thrown out by a judge is considered evidence for a jury. This means that trials contain quite a bit of anecdotal evidence, which is considered as relevant evidence by a jury. Eyewitness testimony (which is a form of anecdotal evidence) is considered the most compelling form of evidence by a jury.^[31]