Estimating frequencies and probabilities is tricky. Imagine we want to judge the likelihood that someone we just met – let’s call him Steve – is left-handed. One way we might answer this question is by asking ourselves an easier one: “How easily do instances of left-handed people come to mind?” If you can easily come up with examples of left-handed people – perhaps a sibling or a few friends – you are likely to overestimate the likelihood that Steve is left-handed. This is the availability heuristic – when we judge probabilities based on how easily examples come to mind.

In one study on the availability heuristic, people were asked to estimate if the letter “r” was more likely to appear in the first or third position in a word. Despite the fact that “r” is twice as likely to appear in the third position in the English language, more than two-thirds of the people tested believed that it was more likely to appear in the first position. The reason is simple: it is easier to think of words that begin with “r” than words where it is the third letter.

One area where the availability heuristic really matters is in how we assess risk. In the months after the events of September 11, 2001, the memory of the attacks loomed large in the minds of the public. In response, many Americans stopped flying and drove instead. Unfortunately, the corresponding increase in miles driven is estimated to have caused the deaths of sixteen hundred Americans – over half the number of lives lost in the attacks themselves.

There’s no “one weird trick” to get around our tendency to make use of the availability heuristic. When possible, try to make decisions based on what you know about “base rates” – that is, the average probability of a given event – rather than what seems likely given the examples you can remember.

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