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The representativeness heuristic 255
Test your judgment: answer to question 4
Q4. To what extent was your estimate influenced by the low-reliability
personality sketch and the extent to which this appeared to be
representative of someone with a criminal record? The base rate
would suggest that the probability should be close to 8%.
2. Expecting sequences of events to appear random
When a sequence of events is generated by a random process people
expect the sequence to represent the characteristics of randomness. For
example, if a coin is about to be thrown six times they would tend
to expect the sequence H-H-H-T-T-T to be less likely to occur than
the sequence T-H-H-T-H-T which appears to be more representative
of a random process (of course, both sequences are equally likely).
The belief is that even short sequences of events should manifest the
essential characteristics of the random process that is generating them.
Runs, where the same result is repeated, are expected to be short and
a frequent alternation between the events is expected. In fact, short
random sequences will often contain several repetitions of the same
event. For example, of the 64 possible, and equally likely, sequences
which can be generated by six throws of a fair coin, 38 have runs of the
same result coming up on three or more consecutive throws.
Biases like this can lead to errors in forecasts based on judgment. For
example, Eggleton 4 asked subjects to produce forecasts of artificially
generated monthly production costs. The subjects perceived the series
where there was a systematic alternation between high and low costs to
be random, while they thought that the truly random series contained
systematic patterns. Indeed, reading systematic patterns in random
events, and hence over-reacting to the latest sales or cost figure, is a
common problem in judgmental forecasting. 5,6
Test your judgment: answer to question 5
Q5. Did you choose C, because you thought it appeared to be more
representative of randomness? In fact, all of the sequences are
equally likely to occur.