From group diffusion to ratio bias: Effects of denominator and numerator salience on intuitive risk and likelihood judgments
The authors start from the group-diffusion effect, the tendency to judge your own risk as lower when more people share the same threat even though the probability is stated. They replicate it in health scenarios with US students, then find the same pattern for lottery odds, for winning and for losing. Stating how many people are expected to be affected removes the effect, and stating only that number reverses it, producing a ratio bias. They propose that both effects come from judging ratios: group diffusion appears when the denominator, the total exposed, is more salient than the numerator, and ratio bias appears when the numerator, the number affected, is more salient.
The authors start from the group-diffusion effect, the tendency to judge your own risk as lower when more people share the same threat even though the probability is stated. They replicate it in health scenarios with US students, then find the same pattern for lottery odds, for winning and for losing. Stating how many people are expected to be affected removes the effect, and stating only that number reverses it, producing a ratio bias. They propose that both effects come from judging ratios: group diffusion appears when the denominator, the total exposed, is more salient than the numerator, and ratio bias appears when the numerator, the number affected, is more salient.
Written from the abstract, Cambridge Core.
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Price, P. C., & Matthews, T. V. (2009). From group diffusion to ratio bias: Effects of denominator and numerator salience on intuitive risk and likelihood judgments. Judgment and Decision Making, 4(6), 436-446