Objectives: We sought to address denominator neglect (i.e. the focus on the number of treated and nontreated patients who died, without sufficiently considering the overall numbers of patients) in estimates of treatment risk reduction, and analyzed whether icon arrays aid comprehension.
Methods: We performed a survey of probabilistic, national samples in the United States and Germany in July and August of 2008. Participants received scenarios involving equally effective treatments but differing in the overall number of treated and nontreated patients. In some conditions, the number who received a treatment equaled the number who did not; in others the number was smaller or larger. Some participants received icon arrays.
Results: Participants-particularly those with low numeracy skills-showed denominator neglect in treatment risk reduction perceptions. Icon arrays were an effective method for eliminating denominator neglect. We found cross-cultural differences that are important in light of the countries' different medical systems.
Conclusions: Problems understanding numerical information often reside not in the mind but in the problem's representation. These findings suggest suitable ways to communicate quantitative medical data.