Informal logical fallacy
An argument from anecdote is an informal logical fallacy, when an anecdote is used to draw an improper logical conclusion. The fallacy can take many forms, such as cherry picking, hasty generalization, proof by assertion, and so on.[1]
The fallacy does not mean that every single instance of sense data or testimony must be considered a fallacy, only that anecdotal evidence, when improperly used in logic, results in a fallacy. Since anecdotal evidence can result in different kinds of logical fallacies, it is important to understand when this fallacy is being used and how it is being used.
The most common form of the fallacy is the use of anecdotes to create a fallacy of Hasty Generalization. Language surrounding the fallacy must indicate a logical conclusion, and includes absolute statements such as "every", "all", and so forth.
However, other forms of the fallacy exist. For instance, a person citing a myth or made-up story as evidence is engaging in proof by assertion. This is because, if the anecdote is fictional, it is not logically part of the argument. All that is left is the assertion that the argument is true, and it is thus the proof by assertion fallacy.
The Cherry-Picking fallacy can occur within the Anecdotal fallacy if an example is used but it is not representative of the average occurrence of such a thing.
Post Hoc Ergo Propter Hoc is another fallacy that is often paired with anecdotes.
Examples
"I was stung by a bee, so I know that therefore, all bees sting and all bees are aggressive."
This is an example of an anecdotal fallacy and hasty generalization, because a singular personal experience is being made to generalize to all bees.
This can be contrasted with another statement which is not fallacious:
"I was stung by a bee, some bees sting."
This statement is not fallacious because no logical claims were made about the experience (no use of terms like "therefore" and "all), and because the conclusion drawn from the experience was limited in scope and breadth to that particular experience.
An example of anecdotal evidence within the cherry-picking fallacy would be
I knocked a hornet's nest down once, but I wasn't stung. Therefore, hornets don't sting.
While it is possible to knock down a hornet's nest and not be stung, most people who knock down a hornet's nest will be stung. Here, the person draws the logical conclusion that, because of this singular experience (which is not representative of the average experience), hornets do not sting.
An example of anecdotal evidence within a proof by assertion fallacy would be as follows:
"I was reading a novel where it said that bees don't sting, therefore bees do not sting."
Since the anecdote here cited is admittedly fictional, it cannot be used as evidence. Since it cannot be used as evidence, there is no evidence and all that is left is just an assertion, thus proof by assertion. This can also be applied to anecdotal evidence with no attributable source, such as urban legends, myths, folk sayings and folklore.
References
