Figure 1.

The causal models of truly randomized clinical trials and biased randomized clinical trials. X represents the subject covariates; Y is the sampling of a subject to a trial; T indicates the treatment; and Z is the outcome. The black arrows represent causal dependencies between variables. A. In the causal model for truly randomized clinical trials, no dependency should exist between X and Y. Thus, the observed probability of outcome Z given the treatment is a good estimate of whether the treatment causes the outcome. B. In the causal model for biased randomized clinical trials, an arrow exists between X and Y′, which indicates the dependence. Thus, invalid causal inferences may be estimated for treatment efficacy among some subpopulations and result in unfair and avoidable population health disparities.