Individual differences in base rate neglect: A fuzzy processing preference index

•The FPPI assesses individual differences in base rate neglect and base rate respect.•The FPPI is reliable, consistently producing Cronbach's Alpha > 0.90 in 5 studies.•Content validity suggested by solid correlation with the Process Dissociation Procedure.•FPPI scores reliably predict conjunction fallacies in joint probability estimation.

Little is known about individual differences in integrating numeric base-rates and qualitative text in making probability judgments. Fuzzy-Trace Theory predicts a preference for fuzzy processing. We conducted six studies to develop the FPPI, a reliable and valid instrument assessing individual differences in this fuzzy processing preference. It consists of 19 probability estimation items plus 4 “M-scale” items that distinguish simple pattern matching from “base rate respect.” Cronbach's Alpha was consistently above 0.90. Validity is suggested by significant correlations between FPPI scores and three other measurers: “Rule Based” Process Dissociation Procedure scores; the number of conjunction fallacies in joint probability estimation; and logic index scores on syllogistic reasoning. Replicating norms collected in a university study with a web-based study produced negligible differences in FPPI scores, indicating robustness. The predicted relationships between individual differences in base rate respect and both conjunction fallacies and syllogistic reasoning were partially replicated in two web-based studies.