Using plausible group sizes to communicate information about medical risks.

OBJECTIVE: To make informed health decisions, patients must understand and recall risks, which often involve ratios with large denominators. Grasping the meaning of such numbers may be difficult, because of limited exposure to large groups of people in either our evolutionary history or daily life.
METHODS: In an experiment (n=98), we investigated whether medical risks are easier to understand and recall if their representation is based on small, evolutionarily plausible groups of people, and whether this representation especially helps patients with low numeracy.
RESULTS: Participants-especially those with low numeracy-often disregarded and incorrectly recalled denominators of ratios representing medical risks when the denominators involved were large. Risks were easier to understand and recall if their representation was based on smaller, evolutionarily plausible groups of people.
CONCLUSIONS: Our results extend previous literature on the role of numeracy in understanding health-relevant risk communications by showing the importance of using plausible group sizes to communicate these risks to people with low numeracy. Our results also support the notion that problems in risk perception occur because of inappropriate presentation formats rather than cognitive biases. PRACTICE IMPLICATIONS: Our findings suggest suitable ways to communicate quantitative medical data-especially to people with low numeracy.