Listwise deletion (LWD) appears more common than imputation: our review of leading journals finds that about 50% of articles state or imply LWD, versus 7% describing an imputation model. Assuming oracle regression with all required values observed consistently estimates the population average treatment effect, we ask when complete-case regression remains consistent. With constant effects, we offer two checks: whether covariates and the missing indicator satisfy the adjustment criterion in an augmented causal diagram (DAG), and whether the estimator pools valid within-covariate comparisons appropriately, potentially using inverse treatment-propensity weighting (IPW). For heterogeneous effects, assuming conditional effects carry over to the population, we reweight comparisons to the population covariate distribution. Simulations and empirical illustrations from American politics, comparative politics, and international relations demonstrate the framework's use and show why a missing-at-random (MAR) classification, although useful for assessing imputation, does not by itself establish whether complete-case regression consistently estimates the target population ATE. Thus, we recommend adding the missing indicator and its causes to causal models, then checking adjustment and weighting for LWD.

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