Also called the fallacy of insufficient statistics or sample, the hasty generalization fallacy occurs when someone assumes something is true about a large group based on an extremely small sample size. Fallacies, as flaws in logical reasoning in an argument, are seen in both speech and writing. The hasty generalization fallacy, however, is frequently — and often unintentionally — used in everything from formal arguments to casual conversation. Often, it occurs as a result of prejudice or lazy reasoning.
In a hasty generalization fallacy, the writer or speaker makes a claim that because something is true about a sample of a larger group, it is true about the group as a whole. For example, some might say "I have dated three redheads, and they all had tempers. Therefore, all redheads have tempers." This is a hasty generalization because three is not a large enough sample size to accurately determine the temper of all redheads.
Hasty generalization is a fallacy of an informal argument. Informal arguments deal with the content of the argument versus the structure. This means the actual structure of the hasty generalization fallacy is logically sound. In other words, if the information presented by the generalization is reasonable and accurate, a fallacy has not occurred.
For example, a researcher surveying 600 students on a campus with a total population of 1,000 discovers that 85 percent of those students surveyed normally went to parties on Friday nights. Based on this sample size, stating that the majority of college students at that university spend Friday nights at parties would be a valid conclusion. If, however, the researcher only surveyed ten people and reached the same conclusion, that researcher would be guilty of the hasty generalization fallacy. Even if the conclusion was correct, the sample the researcher collected to support the claim is too small and is, therefore, not credible.
Appropriate sample sizes are variable depending on the size of the total population in question. Sample sizes may be small and still be valid if the population in question is small. For example, although surveying ten people in the university example resulted in a insufficient sample size, and thus the hasty generalization fallacy, surveying ten people in a club with only twenty members would generally be a sufficient sample size.
Although the hasty generalization fallacy is seen in formal written and spoken arguments, it is also often used in casual conversation as well. Stemming from prejudice or the desire to place groups into quick categories, hasty generalizations can often lead to untrue and unfair assumptions about large groups of people. From the man who decides that no woman can drive because of the woman who cut him off to the woman who decides that all foreigners are thieves because a foreigner stole her purse, hasty generalizations slip into every day thinking, often without the person responsible realizing the fallacy at all.