NUMERACY AND THE USE AND MISUSE OF DENOMINATORS IN RISK PERCEPTIONS

NUMERACY AND THE USE AND MISUSE OF DENOMINATORS IN RISK PERCEPTIONS

   Purpose: Greater numeracy generally leads to better medical judgments and decisions, but it can lead to number overuse and worse performance. The objective of this study was to examine whether risk perceptions of the flu would differ based on numeracy and the presence/absence of a denominator.

   Methods: Respondents (N=148) were recruited from a large Internet sample diverse in age and education and randomized into one of three experimental conditions. All respondents were told that 9 people exposed to the flu will catch it. Control respondents received no additional information. In two experimental conditions, respondents also received denominator information (9 people out of 100 [or 1,000] people exposed to the flu will catch it).  They were then asked “How great do you think the risk of catching “Helsinki flu” would be? 0=no risk to 7=very high risk” and “How do you feel about the 9 people who would catch Helsinki flu next winter? -3 = Extremely Bad, +3 = Extremely good.”

   Results: Numeracy, experimental condition, and their interaction significantly influenced risk perceptions (p's<.05). Among the less numerate, more risk was perceived with larger comparison groups (see table for risk-perception means). As hypothesized, they were sensitive to how many people were exposed. The highly numerate instead appeared to compare provided numbers to derive their risk perceptions. In particular, they perceived more risk when a denominator was present and with larger proportions (see table). Affect to the 9 people partially mirrored the risk-perception results.

	Risk-perception means		Affect means
	Less numerate	More numerate	Less numerate	More numerate
Control (9 people)	2.6	1.5	-0.9	-0.6
Small denominator (9 out of 100 exposed)	2.8	3.1	-0.9	-0.9
Large denominator (9 out of 1000 exposed)	3.4	2.4	-1.6	-0.6

   Conclusions: Holding constant the expected effects of flu exposure (9 people will get it), respondents were nonetheless sensitive to a provided denominator. Less numerate individuals appeared to derive affect and risk perceptions from the number of exposed individuals, whereas those with higher numeracy derived responses from a comparison of numerator to denominator. Implications for shared decision making and risk communication to people with different numeracy levels, as well as potential strategies to eliminate differences will be discussed.