Background: Understanding information about medical screening tests often requires estimating positive predictive values (i.e., posterior probabilities), which is a notoriously difficult task. Previous studies have shown that representation of information in terms of natural frequencies (i.e., counts of occurrences that preserve base rates) facilitates judgments of positive predictive values. The objective of this study was to investigate whether natural frequencies facilitate accurate estimates in elderly people and whether performance depends on numeracy skills. Elderly people are more often than younger people required to use such information to make informed choices regarding medical procedures (e.g., screenings).
Method: This was an experimental study in which information about 2 medical screening tests was presented either as conditional probabilities or natural frequencies. Participants were 47 older adults (62-77 years of age; average numeracy score 8.6) and 115 younger adults (18-35 years of age; average numeracy score 10.3).
Results: When the screening information was presented in terms of conditional probabilities, only 15% of the younger adults and 18% of the older adults provided accurate estimates in at least 1 of the tasks. When information was presented in terms of natural frequencies, 55% of the younger adults and 58% of the elderly participants gave correct estimates. This effect occurred without explicit training. Furthermore, participants with higher numeracy scores performed better in the estimation tasks than those with lower numeracy scores.
Conclusions: Natural frequencies help elderly and young patients-including those with lower numeracy skills-to understand positive predictive values of medical screening tests.