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Extension neglect[a] is a type of cognitive bias that occurs when the mind tends to ignore the size of the set during an evaluation in which the size of the set is logically relevant. For instance, when reading an article about a scientific study, extension neglect occurs when the reader ignores the number of people involved in the study (sample size) and concludes the article is important because of its results. In reality, if the sample size is too small, the results of the study are suspect, because the larger the sample size the more likely any non-typical subjects' answers tend to average out. A study based on only a few people may draw very invalid conclusions because only one person has exceptionally high or low scores, and there are not enough people there to correct this via averaging out. But often, the sample size is not prominently displayed in science articles, and the reader in this case might still believe the article's conclusion due to extension neglect (ignoring the small sample size and therefore the large possibility of a wrong conclusion.)
Forms of extension neglect include:
- base rate neglect
- insensitivity to sample size
- scope neglect
- duration neglect
- the peak-end rule
- the conjunction fallacy
- the less-is-better effect
The extension effect is "neither universal nor absolute". If attention is drawn to set size in an easily interpretable way, an additive extension effect is reported, according to which the valuation of a set is a function of the valuation of a prototypical member of the set added to set size, rather than multiplied.
- The concept of extensionality is used throughout Kahneman and Tversky's research as synonymous of extent (range, size) of the set. It should not be confused with Arrow's concept of extensionality, which in Kahneman and Tversky's work is called invariance.
- Kahneman, Daniel; Ritov, Ilana; Schkade, David (1999). "Economic Preferences or Attitude Expressions? An Analysis of Dollar Responses to Public Issues" (PDF). Journal of Risk and Uncertainty. 19 (1/3): 203–235. doi:10.1023/A:1007835629236.
- Kahneman, Daniel (2000). "Evaluation by moments, past and future". In Kahneman, Daniel; Tversky, Amos (eds.). Choices, Values and Frames (PDF). Cambridge University Press. p. 708. ISBN 978-0521627498.