A double robust approach to causal effects in case-control studies

In a recent issue of the Journal, VanderWeele and Vansteelandt (Am J Epidemiol. 2011;174(10):1197-1203) discussed an inverse probability weighting method for case-control studies that could be used to estimate an additive interaction effect, referred to as the "relative excess risk due to interaction." In this article, we reinforce the well-known disadvantages of inverse probability weighting and comment on the desirability of the described parameter. Further, we review an existing double robust estimator not considered by VanderWeele and Vansteelandt, the case-control-weighted targeted maximum likelihood estimator, which has improved properties in comparison with a previously described inverse-probability-weighted estimator. This targeted maximum likelihood estimator can be used to target various parameters of interest, and its implementation has been described previously for the risk difference, relative risk, and odds ratio.