Before revision:  Under the assumption of selection on observables, we consider treatment effects of the population, of sub-populations , and of alternative populations that may have alternative covariate distributions. We further consider the decomposition of a total effect into a direct effect and an indirect effect ( as mediated by a particular mechanism) .
After revision:  Under the assumption of selection on observables, we consider treatment effects of the population, of sub-populations , and of alternative populations ; (ii) the decomposition of a total effect into a direct effect and an indirect effect ( as mediated by a particular mechanism) .