A judgemental heuristic through which a person makes an estimate by starting from an initial value and then adjusting it to yield a final estimate, the adjustment typically being insufficient and the estimate therefore biased towards the initial value. The initial value may be suggested by the formulation of the problem, as in a classic experiment in which people were asked to estimate the number of African countries in the United Nations by first indicating whether a number suggested by the experimenter was too high or too low and then estimating the actual number; those who began from a suggested number of 10 gave a median estimate of 25 (too low), and those who began from 65 gave a median estimate of 45 (too high). Alternatively, the initial value may result from partial computation, as in another classic experiment in which high school students who were given five seconds to estimate the value of the product 1× 2 × 3 × 4 × 5 × 6 × 7 × 8 produced a median estimate of 512 (too low), whereas those asked to estimate the product 8 × 7 × 6 × 5 × 4 × 3 × 2 × 1 produced a median estimate of 2,250 (too high), apparently because members of both groups calculated the first few steps and then extrapolated without sufficient adjustment. When revealed preferences are determined empirically, the technique of asking decision makers whether they prefer various gambles to a fixed alternative tends to lead to anchoring and adjustment on the probability scale, whereas asking them how much they would be willing to stake on the gamble tends to lead to anchoring and adjustment on the money scale. The heuristic was first introduced in 1971 by the US psychologists Paul Slovic (born 1938) and Sarah C. Lichtenstein (born 1933). See also overconfidence effect, preference reversal (1).