= Constantine E. Frangakis
{c}
<Constantine E. Frangakis> and <Donald B. Rubin> introduced <principal stratification> as a framework for defining <causal effects> in problems involving a post-treatment variable.<Principal Stratification in Causal Inference by Constantine E. Frangakis and Donald B. Rubin>[Section3, principal strata and principal causal effects]{ref}
For example, let $S(0)$ and $S(1)$ describe whether the same person would comply under two treatment assignments. A principal stratum fixes the pair $(S(0),S(1))$, and a treatment comparison within it refers to the same underlying people under both assignments. Only one component is observed for each person, so defining this <estimand> leaves an identification problem requiring further assumptions or information. Conditioning only on observed compliance need not compare the same kinds of people.<Principal Stratification in Causal Inference by Constantine E. Frangakis and Donald B. Rubin>[Definition and latent principal-stratum membership]{ref}
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