What is the availability heuristic?

Daniel Kahneman, answered from the texts and cited to the page.

Daniel Kahneman answers

The availability heuristic is the tendency to estimate the frequency of a class or the probability of an event by assessing how easily relevant instances or associations come to mind.¹ Amos and I proposed it as one of a small number of heuristics people use to reduce difficult probability judgments to simpler ones — in this case, to an assessment of associative or retrieval ease rather than to any direct count of actual frequencies.²

The logic behind it is not arbitrary. Life-long experience does teach us that instances of large classes are recalled more readily than instances of small ones, and that likely events are easier to imagine than unlikely ones.³ So using ease of retrieval as a proxy for frequency is not an irrational strategy — it is ecologically valid much of the time.

The trouble is that availability is also affected by factors that have nothing to do with actual frequency: salience, recency, vividness, personal experience.⁴ When those factors drive the ease of retrieval, the heuristic produces systematic bias. A dramatic plane crash will temporarily raise your estimate of aviation risk, not because the base rate changed but because the category became more cognitively available.⁵

A judicial error that happens to you personally will shake your faith in the justice system far more than a statistically equivalent error you read about in a newspaper.⁶ In both cases, the mind is answering a question about ease of imagination rather than the question that was actually asked — and the substitution goes unnoticed. One thing we learned after the original formulation deserves emphasis: you do not actually need to retrieve specific instances to get an impression of ease.

You know almost immediately, without generating examples, that more words can be constructed from TAPCERHOB than from XUZONLCJM.⁷ The sense of fluency precedes the retrieval; it is that sense, not the instances themselves, which drives the judgment.

Sources

1. A person is said to employ the availability heuristic whenever he estimates frequency or probability by the ease with which instances or associations could be brought to mind.

   Judgment under Uncertainty, the Kahneman and Tversky Chapters, pp. 106–107

2. We propose that when faced with the difficult task of judging probability or frequency, people employ a limited number of heuristics which reduce these judgments to simpler ones.

   Judgment under Uncertainty, the Kahneman and Tversky Chapters, p. 106

3. Life-long experience has taught us that instances of large classes are recalled better and faster than instances of less frequent classes, that likely occurrences are easier to imagine than unlikely ones, and that associative connections are strengthened when two events frequently co-occur.

   Judgment under Uncertainty, the Kahneman and Tversky Chapters, p. 106

4. availability is also affected by various factors which are unrelated to actual frequency. If the availability heuristic is applied, then such factors will affect the perceived frequency of classes and the subjective probability of events.

   Judgment under Uncertainty, the Kahneman and Tversky Chapters, pp. 106–107

5. A dramatic event temporarily increases the availability of its category. A plane crash that attracts media coverage will temporarily alter your feelings about the safety of flying.

   Thinking, Fast and Slow

6. A judicial error that affects you will undermine your faith in the justice system more than a similar incident you read about in a newspaper.

   Thinking, Fast and Slow

7. You knew almost immediately, without generating any instances, that one set offers far more possibilities than the other, probably by a factor of 10 or more.

   Thinking, Fast and Slow