“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.
Materials and Methods
We designed a self-administered questionnaire to compare women's understanding of risk expressed in two different ways: rates per 1000 women or proportions with a numerator of one. To avoid the potential effect of anxiety related to serious risks2,3 such as Down syndrome, we chose bladder infection as our clinical event. Without individual counseling, many respondents would not have understood the term Down syndrome. The anonymous questionnaire began with a simple example, showing respondents how to identify and circle the larger of two choices (the height of two buildings). We then used rates of trisomy 21 at birth at maternal ages of 35 years (one in 384) and 40 years (one in 112) cited by the California Department of Health Services, Genetic Diseases Branch,13 for the cystitis questions. The first question asked respondents to identify which rate of bladder infection was higher: 1 in 384 or 1 in 112 persons. In the next question, we posed the same comparison, this time using rates: 2.6 or 8.9 per 1000 women (the same proportions converted into rates). We also asked women to give their ages and educational attainments. To avoid bias in responses to the second question from experience gained by answering the first question, we randomly varied the sequence of the two questions. We used a table of random numbers to generate the sequence of questions.
After approval of the project by the Committee on Human Research of the University of California, San Francisco and San Francisco General Hospital, we had clinic staff translate the questionnaire into Spanish and Chinese. We field-tested the translations to ensure accuracy. We then distributed questionnaires in all three languages to the waiting rooms of three obstetric and gynecologic clinics affiliated with University of California, San Francisco. Code numbers on the questionnaires identified clinical sites. The first site, San Francisco General Hospital, serves a large immigrant population; many of the women in this population speak Spanish or the Chinese dialects of Cantonese or Mandarin. We also distributed questionnaires in the obstetrics and gynecology resident continuity clinic at Moffitt Hospital of University of California, San Francisco and in the faculty private practice at that hospital. Women in the continuity clinic population tend to be English-speaking but less educated, whereas the private practice patients are largely English-speaking and highly educated. At each site, we asked the clinic staff to encourage women to complete the four-question form while waiting for their appointments. For the two main questions, respondents could choose either of the risk expressions or “don't know.” Our analysis focused on correct responses, and we grouped nonresponses with “don't know” and incorrect answers. We excluded from the survey women whose primary language was other than English, Spanish, or Chinese and those who were illiterate or previously had completed a questionnaire for the study.
We analyzed the results using public-domain software (Epi Info 6, USD, Inc., Stone Mountain, GA). The sample size was one of convenience; the total number of questionnaires completed from June through August of 1997 was 633. The primary outcome measure was the ability of respondents to identify the higher of two risks (probability of Down syndrome at maternal age 35 years versus 40 years) portrayed as a proportion or rate. We calculated 95% confidence intervals (CIs) for correct response rates and compared proportions using χ2 tests. We also examined the potential effect of age (in 10-year intervals), primary language, and educational attainment on the outcome measure. We calculated crude and adjusted relative risks of correct answers using the Mantel-Haenszel procedure. In addition, we did a paired analysis with the McNemar test.
The clinical sites provided a heterogeneous group of women. The largest number of respondents came from San Francisco General Hospital (442), followed by the faculty practice at Moffitt Hospital (137) and the resident continuity clinic at Moffitt Hospital (54). Most women (367 of 633, 58%) used English as their primary language; 211 (33%) returned Spanish questionnaires, and 55 (9%) chose Chinese (Table 1). Most respondents were of reproductive age, although the Chinese-speaking women were older on average. In general, English-speaking women were more educated than the other women. Nearly two-thirds of English-speaking respondents had completed at least some college. In contrast, only 18% of Spanish-speaking women and 27% of Chinese-speaking women had education beyond high school. Three women did not supply their age, and ten did not disclose their educational attainment.
Women understood rates better than proportions (Table 2). Overall, 73% (95% CI 70, 77) correctly identified the higher risk in rate format, in contrast with 56% (95% CI 52, 60) who correctly answered the same question framed as proportions. The crude relative risk (RR) of correct assessment with the rate format was 1.3 times (95% CI 1.2, 1.4; P < .001) that of proportion format.
Superiority of the rate format was consistent across all age groups, primary languages, and educational levels (data not shown). Controlling singly for the effect of age, language, and education did not alter the RR estimate for the rate format, which remained 1.3. As shown in Figure 1, the likelihood of correctly judging the larger of two risks was related directly to educational attainment (P < .001). However, at each educational level the 95% CI around the percent correct responses for rates and proportions did not overlap. Among women with 9 or fewer years of school, only half could evaluate the rates correctly, and less than a third could assess the proportions.
The paired analysis confirmed the superiority of the rate format. In Figure 2, cell a includes those women who correctly identified the larger of two risks with both formats. Cell d contains those who failed with both formats. If the rate and proportion formats were equally easy to understand, then the number of women who answered correctly in one format but not in the other should have been equal in cells b and c. The χ2 value for these discordant cells was 64.17, indicating P < .001.
Our evidence contradicts conventional wisdom of genetic counselors5,13–15 and physicians16 that women understand proportions better than rates. Given the national scope of genetic counseling and antenatal screening and the critical decisions based on them, that discrepancy is troubling. By using a suboptimal format for expressing risk, counselors and clinicians might be causing unnecessary confusion.
Our survey had several strengths. We attempted to improve the generalizability of the study by using the three most common languages spoken by our patients. Respondents ranged from recent immigrants with little education to highly educated professionals in a university community. We tried to avoid ascertainment bias by randomly varying the sequence of the two principal questions.
How counselors frame the expression of risk and design individual counseling15 might influence women's comprehension. According to one report,15 “The mode in which this type of information is explained to counsellees is also of the utmost importance. Many individuals seen for counselling have little or no concept of probability theory.” Hence, we framed the comparison in an objective, written questionnaire to avoid that potential influence.
The paired analysis provided the most robust comparison of the formats. Many women (129) did not understand either format. However, if the two formats were equally understandable, we expected equal numbers of women in cells b and c of Figure 2 (those who were correct with only one of the two formats). However, more than three times as many women (151) judged risks correctly with rates alone, compared with only proportions (41). Each woman served as her own control in this analysis, so bias could not account for that difference. The likelihood that those results were due to chance is less than one in a quadrillion. Neither bias nor chance can account for our results, so we conclude that the observed differences are real.
Our survey had several weaknesses as well. The voluntary, unsupervised nature of the survey probably led to selection bias in responses. Those who thought they were unable to answer the questions were probably less likely to turn in their forms than were other women. Thus, the proportion of correct responses here is probably spuriously high. All respondents came from a single city, so replication of our research in other locales should be a priority. The questionnaire was written, so the results cannot be extrapolated to women who cannot read. The number of Chinese respondents was small, limiting the precision of estimates for that population. We gathered little demographic information on respondents. Exposure to courses in mathematics or probability, experience with mathematics in employment, duration of residence in the United States, and other information might have refined our evaluation. We intentionally limited the survey instrument to four questions, hoping its brevity would encourage voluntary participation.
We intentionally chose a nonthreatening hypothetical example for the study instead of Down syndrome. Emotions can impair understanding2,3; therefore we believe comprehension might be poorer in actual genetic counseling practice, so our results might be optimistic. Women at risk also might pay more attention to counseling, resulting in better understanding.
Illiteracy poses a major problem in health care.17–19 For example, a quarter of the adult population cannot read simple written materials, and the average reading ability of the nation is at the eighth-grade level.18 Innumeracy, the mathematical equivalent of illiteracy, is also endemic—and problematic in health care.20 A large portion of the public lacks functional knowledge of fractions, large numbers, or percentages.21 Facility with percentages is limited, despite universal exposure to these terms during school. Students have difficulty learning percentages, and their comprehension does not grow as they progress in school.22 Both school children and adults have difficulty transforming data into percentages. Indeed, in one study22 a third of adult women with less than a college education did not recognize that 1/1000 is less than 1%. Most respondents in our study were younger than 40 years. Mathematical ability is better among younger women,23 so women's understanding in this study might be higher than that of the general population of women.
Against that background, our finding that women struggle even more with proportions is not surprising.6 Confusion between proportions and odds complicates the issue further. For example, “one in two” is not equivalent to the odds of “one to two,” which is one in three (33%). Fortunately, the error from confusing odds and proportions becomes large only when risk is very high.14
Although using proportions for genetic counseling5,13–16 has been standard for decades, we can find no scientific evidence to support that convention. Recipients of genetic counseling prefer numeric to nonnumeric information.24 Our responsibility is to provide this numeric information in an understandable manner. Alternative means of conveying risk, such as use of pictograms, risk ladders, or pie charts, might be more useful for women with limited facility with numbers.23 However, given the choice of conveying risk as rates or as proportions, physicians and counselors should choose rates because they are much better understood.
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