Evaluating public health interventions: 7. Let the subject matter choose the effect measure: Ratio, difference, or something else entirely

We define measures of effect used in public health evaluations, which include the risk difference and the risk ratio, the population-attributable risk, years of life lost or gained, disability-adjusted life years, quality-adjusted life years, and the incremental costeffectiveness ratio. Except for the risk ratio, all of these are absolute effect measures. For constructing externally generalizable absolute measures of effect when there is superior fit of the multiplicative model, we suggest using the multiplicative model to estimate relative risks, which will often be obtained in simple linear form with no interactions, and then converting these to the desired absolute measure. The externally generalizable absolute measure of effect can be obtained by suitably standardizing to the risk factor distribution of the population to which the results are to be generalized. External generalizability will often be compromised when absolute measures are computed from study populations with risk factor distributions different from those of the population to whom the results are to be generalized, even when these risk factors are not confounders of the intervention effect.