The average treatment effect on the control (ATC) is the average causal effect among units that received the control condition. In propensity-score analyses, interpret it only where treated and control cases are comparable; without overlap, causal effects cannot be safely calculated. section 5.3.3
Partial overlap among patients
Suppose treated and control patients overlap only for propensity scores from 0.25 to 0.75. Restricting analysis to this common-support region avoids unsupported comparisons, but the resulting effect cannot be extrapolated to excluded patients. section 5.3.3
