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The Role of Numeracy in Understanding the Benefit of Screening Mammography

Lisa M. Schwartz, MD, MS; Steven Woloshin, MD, MS; William C. Black, MD; and H. Gilbert Welch, MD, MPH
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From Department of Veterans Affairs Medical Center, White River Junction, Vermont; and Dartmouth Medical School, Hanover, New Hampshire. Disclaimer: The views expressed herein do not necessarily represent those of the Department of Veterans Affairs or the U.S. government. Acknowledgments: The authors thank the study participants. They also thank Kathy Herbst for organizational help, Betty Acheson for assistance with the Women Veterans Registry, and Harold C. Sox, MD, for reviewing the manuscript. Grant Support: Drs. Schwartz and Woioshin were supported by the Department of Veterans Affairs Fellowship Program in Ambulatory Care and by a New Investigator Award from the Department of Defense Breast Cancer Research Program (RP#950610). Dr. Welch was supported by a Department of Veterans Affairs Career Development Award in Health Services Research and Development. Requests for Reprints: Lisa M. Schwartz, MD, MS, VA Outcomes Group (111B), Department of Veterans Affairs Medical Center, White River Junction, VT 05009. Current Author Addresses: Drs. Schwartz, Woloshin, and Welch: VA Outcomes Group (111B), Department of Veterans Affairs Medical Center, White River Junction, VT 05009.

Copyright ©2004 by the American College of Physicians

Ann Intern Med. 1997;127(11):966-972. doi:10.7326/0003-4819-127-11-199712010-00003
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Background: Quantitative information about risks and benefits may be meaningful only to patients who have some facility with basic probability and numerical concepts, a construct called numeracy.

Objective: To assess the relation between numeracy and the ability to make use of typical risk reduction expressions about the benefit of screening mammography.

Design: Randomized, cross-sectional survey

Setting: A simple random sample of 500 female veterans drawn from a New England registry.

Intervention: One of four questionnaires, which differed only in how the same information on average risk reduction with mammography was presented.

Measurements: Numeracy was scored as the total number of correct responses to three simple tasks. Participants estimated their risk for death from breast cancer with and without mammography. Accuracy was judged as each woman's ability to adjust her perceived risk in accordance with the risk reduction data presented.

Results: 61% of eligible women completed the questionnaire. The median age of these women was 68 years (range, 27 to 88 years), and 96% were high school graduates. Both accuracy in applying risk reduction information and numeracy were poor (one third of respondents thought that 1000 flips of a fair coin would result in <300 heads). Accuracy was strongly related to numeracy: The accuracy rate was 5.8% (95% CI, 0.8% to 10.7%) for a numeracy score of 0, 8.9% (CI, 2.5% to 15.3%) for a score of 1, 23.7% (CI, 13.9% to 33.5%) for a score of 2, and 40% (CI, 25.1% to 54.9%) for a score of 3.

Conclusions: Regardless of how information was presented, numeracy was strongly related to accurately gauging the benefit of mammography. More effective formats are needed to communicate quantitative information about risks and benefits.


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Figure 1.
Overview of the task presented to a woman completing the survey and our measures of her ability to accurately apply risk reduction information.

The relation between this measure of accuracy and numeracy was subsequently assessed.

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Figure 2.
Distributions of women's estimates of the benefit of mammography according to how information on risk reduction was framed.

Respondents provided numerical answers (___ out of 1000); these were summarized into ranges for presentation. The black bars represent the actual risk reduction given in the information presented.

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Figure 3.
Accuracy rate for each numeracy score.

Bars represent 95% Cls; numbers in parentheses are the number of study participants with the given score.

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