"Anecdotal Evidence"

The expression anecdotal evidence has two distinct meanings.

(1) Evidence in the form of an anecdote or hearsay is called anecdotal if there is doubt about its veracity; the evidence itself is considered untrustworthy.

(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 may not be untrue, but it doesn't follow from the "evidence".

Common misuse: "I know a person who..."; "I know of a case where..." etc.

When used in advertising anecdotal reports are called “testimonials” and are subject to local or federal laws.

A common way anecdotal evidence becomes unscientific is through fallacious reasoning such as 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 or hasty generalization.

For example, here is anecdotal evidence presented as proof of a desired conclusion:

"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."

Anecdotes like this do not prove anything.