Absolute risk reduction is correctly given by

Absolute risk reduction is correctly given by

• A. ARR = [a/(a+b)] − [c/(c+d)]
• B. ARR = [c/(c+d)] − [a/(a+b)]
• C. ARR = [a/(a+d)] − [c/(c+d)]
• D. ARR = [c/(c+d)] − [a/(a+d)]

Answer: (B)

Absolute risk reduction measures how much the treatment lowers the risk of the outcome compared with no treatment. It is the difference between the event rate in control and the event rate in treatment. In a 2x2 table, the control group event rate is c/(c+d) and the treatment group event rate is a/(a+b). So ARR = c/(c+d) − a/(a+b). This positive value means the treatment reduces risk; a negative ARR would imply increased risk with treatment. The other formulas either reverse the subtraction or mix proportions from different groups, which would not represent the actual difference in risk between control and treatment within each group.

Absolute risk reduction measures how much the treatment lowers the risk of the outcome compared with no treatment. It is the difference between the event rate in control and the event rate in treatment.

In a 2x2 table, the control group event rate is c/(c+d) and the treatment group event rate is a/(a+b). So ARR = c/(c+d) − a/(a+b). This positive value means the treatment reduces risk; a negative ARR would imply increased risk with treatment.

The other formulas either reverse the subtraction or mix proportions from different groups, which would not represent the actual difference in risk between control and treatment within each group.