Another opportunity to err in estimating probability arises by assigning odds or frequency by the ease with which instances can be brought to mind. The more recent and frequent, the more something seems common. This is the availability heuristic, which is useful and statistically unsound.
In one experiment, participants heard a list of well-known people of both sexes, and subsequently asked whether the list had more men or women. Different lists were read to different groups. Some lists had more famous men than women. Others vice versa. Regardless of numbers, people consistently thought that a list had more women or men depending on how heavily loaded the list was with famous personalities of a certain gender.
At least partly because of the availability heuristic, people feel less safe in a plane than in a car, even though auto travel is statistically much riskier.
A related heuristic – imaginability – employs the ease with which an object or circumstance can be imagined.
The risk of an adventure is assessed by imagining contingencies which could not coped with. If many difficulties are vividly portrayed, an expedition becomes dangerous, even as the ease of imagining has no correlation to the probability of occurrence. Conversely, the risk of an undertaking is easily underestimated by failure to anticipate trouble, or if the dangers are difficult to conceive.
Commonly encountered instances are more easily recalled than those that are less frequent, likelier occurrences easier to imagine than unlikely ones, and our mental connection between events is strengthened when the events frequently co-occur. The availability heuristic works for estimating the numerosity of a class, the likelihood of an event, or the frequency of co-occurrences – all by the ease with which the relevant mental operations of retrieval, construction, or association can be performed. Alas, this trusted estimation procedure is prone to systematic errors.