An article from the American Journal of Public Health 2 reports the results from a randomized study designed to evaluate the efficacy of an intervention targeted to young African American men who had tested positive for a sexual transmitted infection (STI) at a large urban health clinic. A representative sample of 266 men was randomized to be in either the intervention group (n=141) or the control group (n=125). The primary outcome under study was reinfection, defined as testing positive for another STI during the 3-month follow-up period. At the end of the study, 45 of the 141 subjects in the intervention group were reinfected with a STI, as compared to 63 of the 125 subjects in the control group.

The estimated risk difference for reinfection within 3 months is -0.185 (-18.5%) for the intervention group compared to the control group. How could you explain to the health commissioner of city with an estimated 1,000 STI infected African American young men? a. If the intervention was given there would be an estimated 185 fewer STI reinfections (over 3 months) than if the intervention was not given. b. If the intervention was given there would be an estimated 185 more STI reinfections (over 3 months) than if the intervention was not given. c. Each participant in the intervention program would reduce his personal risk of reinfection (over 3 months) by a factor of 0.185 relative to his personal risk of reinfection if he did not participate in the intervention program. d. Each participant in the intervention program would increase his personal risk of reinfection (over 3 months) a factor of 1.185 relative to his personal risk of reinfection if he did not participate in the intervention program. e. There is no way to explain this because risk differences cannot be negative.