Our findings have several limitations. First is the question of generalizability. Our study samples were chosen deliberately to ensure the participation of people across a broad spectrum of age, income, and formal education. Because almost all participants were white and English speakers, we cannot be certain how the primer would perform in a more ethnically diverse setting. Also, because all participants were paid—those recruited from an actual health care setting and those recruited from the community—we cannot be certain how unpaid persons would respond to the primer. Second, there may be concern regarding the appropriateness of the control booklet. We chose an educational booklet (7) (published by the U.S. Department of Health and Human Services Agency for Healthcare Research and Quality) that was similar to the primer in length and tone. Because the control booklet did not teach readers about interpreting risk, it worked as a “placebo” with respect to the main outcome measure (that is, the tests results on medical data interpretation in the control group should reflect existing abilities). Third, in both trials, completion rates were lower in the primer group than in the control group, particularly in the low SES trial. This observation is not hard to explain: The primer covered more challenging material (it required readers to do math) than did the control book. However, completion rates were high overall (95% vs. 98% in the high SES trial and 85% vs. 97% in the low SES trial).