Bias 7: The Law of Small Number

A study carried out by an eminent institution in a poor, tropical country reveals that 20% of the inhabitants of a small rural village have a psychological disease whose average prevalence in the country is 1%. This region is very dry so their inhabitants lack vitamin C. Moreover, the level of endogamy is particularly high in this village. What do you think are the most probable reasons for the psychological disease?

Actually, you cannot say much as the given percentage of 20% may be due to the small sample size compared to the rest of the population. Moreover, you can wonder if the method of evaluating a psychological disease in a poor country is reliable, however, we tend to draw fast conclusions from this study for several reasons:

  1. Authority argument: the newspaper is ‘’eminent’’ so we tend to trust the data it provides
  2. Hypercausality and story-telling: we tend to make correlations for everything whereas most events that occur in our daily life are random and uncorrelated. Our intuition or heuristics like story-telling and tend to automatically build a coherent story from random events.
  3. The laziness of system 1: we prefer trust than doubt.


Thinking fast and slow, D.Kahneman


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