Denominator neglect
(or ratio bias)
Description
Tendency for people to pay too much attention to numerators and inadequate attention to denominators. This results in a tendency to judge a low probability event as more likely when presented as a large-numbered ratio (e.g., 10/100) than as a smaller-numbered but equivalent ratio (e.g., 1/10).
Reyna, V. F., & Brainerd, C. J. (2008). Numeracy, ratio bias, and denominator neglect in judgments of risk and probability. Learning and Individual Differences, 18(1), 89–107.
Task
The task is a marble game that is modeled on a task introduced by Kirkpatrick and Epstein (1992).
Items (1)
Assume that you are presented with two trays of black and white marbles: a large tray that contains 100 marbles and a small tray that contains 10 marbles. The marbles are spread in a single layer on each tray. You must draw out one marble (without peeking, of course) from either tray. If you draw a black marble, you win $2. Consider a condition in which the small tray contains 1 black marble and 9 white marbles, and the large tray contains 8 black marbles and 92 white marbles.
From which tray would you prefer to select a marble in a real situation?
Scoring
Participants who chose the large tray are coded as biased.
Sources
Toplak, M. E., West, R. F., & Stanovich, K. E. (2011). The Cognitive Reflection Test as a predictor of performance on heuristics-and-biases tasks. Memory & cognition, 39(7), 1275–1289.
Kirkpatrick, L. A., & Epstein, S. (1992). Cognitive-experiential self-theory and subjective probability: Further evidence for two conceptual systems. Journal of Personality and Social Psychology, 63(4), 534–544.