Availability

Another related heuristic that Tversky and Kahneman (1974, p. 1127) have analyzed is the availability heuristic. This is a rule of thumb where we ‘assess the frequency of a class or the probability of an event by the ease with which instances or occurrences can be brought to mind". A simple example of this is from a paper in 1973: 'Are there more words in the English language that have the letter R as the first letter or as the third letter?' People find it easier to call to mind words beginning with R than with R as the third letter and so the ease with which they recall words biases their judgement.

It seems common sense to base our judgements on the information that comes most readily to mind and hence stands out. That information is, therefore, salient and available to use in making decisions or judgements. Unfortunately, in this complicated old world of ours, we may not have access to all the relevant information about a topic. You might read about a celebrity involved in some scandal in a newspaper but fail to read the subsequent apology in a small paragraph inside the paper a few days later essentially saying the story was untrue. As a result, what you think you know about the celebrity will be wrong (see Chapter 8). At the turn of the century, there were ‘greatest" lists such as the greatest films of the 20th century. These tended to be ones from the second half of the century as they were relatively recent and hence the ones most readily recalled. To give another example, if what comes to mind are the images on TV of a child with autism who had an measles, mumps and rulella (MMR) vaccine shortly before and whose parents believe there is a causal link between the two, that may be the information that comes to mind when the MMR vaccine is later discussed.

Another example is given by Gigerenzer (2014). After the 9/11 terrorist attacks on the Twin Towers in New York and the Pentagon, people became reluctant to fly to distant destinations and chose instead to drive. When people tried to assess how safe it was to fly, the plane crashes were what readily came to mind and influenced their decisions. He estimated that approximately

1600 extra people lost their lives in road traffic accidents due to their decision to avoid flying.

Anchoring and adjustment

Another important heuristic identified by Tversky and Kahneman (1974) was the anchoring and adjustment heuristic. It refers to a phenomenon whereby, when people are asked to judge a number such as a date, a time, or a distance, and a number is presented beforehand, they tend to provide an estimate that is influenced by this previously presented number. Tversky and Kahneman gave a number of simple examples of the heuristic at work. One involved asking participants to estimate the percentage of African countries in the UN. The experiment used a rigged wheel of fortune that generated a number such as 10 or 65 and the participants were asked to say whether the percentage was higher or lower than that number. Those who were asked to compare it to 10 produced an estimate of 25% whereas those who were given 65 as an anchor gave 45% as an estimate. In another example, one group was asked to guess quickly the product of the numbers 1 to 8, i.e., Ix2x3x4x5x6x7x8, Another was asked to guess the product of8x?x6x5x4x3x2x 1. They are, of course the same, but the low numbers at the start of the first sequence produced a median estimate of 512 whereas those who saw the sequence beginning with 8 produces 2,250 as a median estimate. (The correct answer is 40,320.) In these instances, participants were making an insufficient adjustment to generate their answers.

There have been many replications of the anchoring and adjustment effect and it is used by salespeople trying to sell double glazing, to give one example. ‘The normal price for your windows is $5200 (the anchor) but we can offer a reduction to $4700. And if you choose to buy today, I can get you a price of $4200 although I'd have to clear that with the manager’. The final price is quite a drop from the original anchor and hence seems like a relative bargain.

What has been unclear from the original studies of the heuristic is how the adjustment is computed by the participant. Epley and

Gilovich (2006) conducted experiments to identify the origins of the adjustment and why they were insufficient. They found evidence that people stop adjusting when they reach an ‘implicit range of plausible values', a form of satisficing. They also found evidence that adjustment is effortful and hence people stop thinking further when a plausible estimate is reached. If people are encouraged to seek more appropriate estimates, then this weakens the magnitude of the anchoring bias.