Psychology and biases
The availability heuristic
We judge how likely something is by how easily examples come to mind, so vivid and recent things look more common than they are.
Amos Tversky and Daniel Kahneman, Availability: A Heuristic for Judging Frequency and Probability, Cognitive Psychology (1973), and the 1974 Science paper.
After a plane crash in the news, the flight feels more dangerous than the drive to the airport. After one outage caused by a bad deploy, every deploy looks risky and the disk that is quietly filling up is forgotten. From the inside this feels like judgement informed by experience. Tversky and Kahneman described it as a heuristic: asked how frequent or probable something is, people answer a different and easier question, which is how readily instances come to mind. The 1974 paper gives a clean case. Asked whether more English words start with r or have r as their third letter, most people say the first, because searching memory by first letter is easy; in fact the third position is the more common.
It works as a rule because frequent things usually are easier to recall. It fails when something other than frequency sets the ease of recall: recency, emotional force, personal involvement, or how much coverage a thing gets. Rare, dramatic deaths are overestimated and common quiet ones underestimated, and the gap follows the news. The consequence can be measured. Gigerenzer studied what Americans did after the attacks of 11 September 2001: air travel fell, interstate road travel rose, and for twelve months fatal road crashes ran above the baseline of previous years every month. His estimate is that around 1,500 extra people died on the roads trying to avoid the fate of the passengers on the four flights.
That figure is a statistical inference from traffic trends, not a count of named victims, and the exact number depends on the baseline chosen; the direction is harder to argue with. The heuristic is also not something to switch off. In domains where your experience is a fair sample, what comes to mind is decent evidence, and base rates may not exist. The failure is specific: a judgement about frequency made from a sample that was curated by someone else's editorial choices or by your own worst day. The remedy is to ask where the examples in your head came from and, when it matters, to replace them with a count.
Example
After the attacks of 11 September 2001, Gerd Gigerenzer tracked how Americans changed their travel. Air travel fell, interstate highway travel rose, and in each of the following twelve months the number of fatal highway crashes exceeded the baseline from previous years. In a 2006 paper in Risk Analysis he estimated that about 1,500 Americans died on the road in the attempt to avoid the fate of the passengers killed on the four flights.
Where it applies
When prioritising risks, list them from logs and incident records rather than from memory, because memory keeps the dramatic ones. When evaluating a model, the failure cases you remember are the ones that embarrassed you, not the ones that are common; sample the traffic.
Think about it
Which risk have you spent the most time on this month: the one that most often causes harm, or the one you most recently saw?
Applied it to something real? Log it.
Test yourselfWhy do most people think more English words begin with r than have r as their third letter, when the reverse is true?
They judge frequency by how easily examples come to mind, and memory is searched by first letter far more easily than by third.
Related models
- Probabilistic thinkingThink in odds rather than certainties: estimate how likely each outcome is, and judge a decision by those odds rather than by the single result that happened.
- AnchoringAny number you see before estimating pulls your estimate towards it, even when you know the number is random.
- Confirmation biasPeople look for, notice and remember evidence that fits what they already believe, and rarely test what would prove them wrong.
- Base ratesHow common something is to begin with matters as much as the evidence in front of you, and people routinely leave it out.