Marginal versus conditional causal effects

Conditional methods of adjustment are often used to quantify the effect of the exposure on the outcome. As a result, the stratums-specific risk ratio estimates are reported in the presence of interaction between exposure and confounder(s) in the literature, even if the target of the intervention on the exposure is the total population and the interaction itself is not of interest. The reason is that researchers and practitioners are less familiar with marginal methods of adjustment such as inverse-probability-weighting (IPW) and standardization and marginal causal effects which have causal interpretations for the total population even in the presence of interaction. We illustrate the relation between marginal causal effects estimated by IPW and standardization methods and conditional causal effects estimated by traditional methods in four simple scenarios based on the presence of confounding and/or effect modification. The data analysts should consider the intervention level of the exposure for causal effect estimation, especially in the presence of variables which are both confounders and effect modifiers.