From The Art and Popular Culture Encyclopedia
The expression anecdotal evidence has two quite distinct meanings.
(2) Evidence, which may itself be true and verifiable, used to deduce a conclusion which does not follow from it, usually by generalizing from an insufficient amount of evidence. For example "my grandfather smoked like a chimney and died healthy in a car crash at the age of 99" does not disprove the proposition that "smoking markedly increases the probability of cancer and heart disease at a relatively early age". In this case, the evidence may itself be true, but does not warrant the conclusion.
In both cases the conclusion is unreliable; it might happen not to be untrue, but it doesn't follow from the "evidence".
Evidence can be anecdotal in both senses: "Goat yogurt prolongs life: I heard that a man in a mountain village who ate only yogurt lived to 120."
The term is often used in contrast to scientific evidence, such as evidence-based medicine, which are types of formal accounts. Some anecdotal evidence does not qualify as scientific evidence because its nature prevents it from being investigated using the scientific method. Misuse of anecdotal evidence is a logical fallacy and is sometimes informally referred to as the "person who" fallacy ("I know a person who..."; "I know of a case where..." etc. Compare with hasty generalization). Anecdotal evidence is not necessarily representative of a "typical" experience; statistical evidence can more accurately determine how typical something is.
When used in advertising or promotion of a product, service, or idea, anecdotal reports are often called a testimonial, which are banned in some jurisdictions. The term is also sometimes used in a legal context to describe certain kinds of testimony. Psychologists have found that people are more likely to remember notable examples than typical examples.