Objective: To assess patients' ability to compare magnitudes of Down syndrome risk at maternal ages of 35 and 40 years, expressed as rates or as proportions.
Methods: We used a self-administered, anonymous questionnaire that posed the same comparison in two different formats: 2.6 versus 8.9 per 1000 women (rates) and one in 384 versus one in 112 women (proportions). The study setting included several university-affiliated obstetrics and gynecology outpatient clinics in San Francisco, California. A total of 633 women, whose primary languages were English, Spanish, or Chinese, participated. The main outcome measure was correct identification of the larger of two risks.
Results: Women were more successful with rates (463 of 633 respondents, 73%) than with proportions (353 of 633 respondents, 56%). A paired analysis, in which each woman served as her own control, found risk assessment to be significantly better with rates than with proportions (P < .001). Women with little formal education had difficulty understanding risks framed either way.
Conclusion: The traditional use of proportions to express risk in genetic counseling lacks scientific basis. Rates were easier to understand than proportions, regardless of respondents' age, language, and education.
“Every definition of genetic counseling includes mention of the need to give parents an accurate recurrence rate for the condition of concern: imparting such information is a sine qua non of counseling.”1 Conveying probabilities of genetic abnormalities and other birth defects, however, is often difficult. Many patients have a binary view of risk: a bad outcome either will occur, or it will not. Additional hurdles to understanding probability include emotional shock,2,3 low social class,4 low educational attainment, and limited knowledge of biology.5 Differences in language and culture pose other barriers.6 With a few exceptions,7 genetic counseling literature shows that patients' understanding of risk is limited.2,4,6,8–11 We need to improve how we convey the concept of risk.
Landmark epidemiologic studies12 described the risk of genetic abnormalities in scientific format: rates of disease per unit of population exposed to the risk (commonly per 1000 persons). Genetic counselors attempting to make those risks more understandable (Hook EB, written communication, July 8, 1997) transformed those rates into proportions with a numerator of one and shifting denominators (eg, three per 1000 becomes one in 333). However, proportions with large denominators are confusing. As noted by Walker,6 “To many, 1/400 sounds higher than 1/200 because the denominator is bigger. To any of us, 1 in 20 may sound higher than 5%, or vice versa.” Because effective communication of risk is important and little is known about the effectiveness of different strategies, we conducted this study. Our a priori hypothesis was that women would understand rates better than proportions.