A complier is a unit with potential treatment receipts and : binary encouragement changes its receipt from no treatment to treatment. This pair defines a principal stratum, rather than a behavior inferred from one observed assignment.
For example, a unit that participates exactly when invited is a complier. Under instrumental-variable monotonicity, which excludes defiers,Thus, with randomized assignment, the difference in treatment-receipt rates estimates the complier fraction. To interpret an outcome-rate ratio as the local average treatment effect, one additionally needs the relevant independence and exclusion assumptions and a nonzero complier fraction. An observed encouraged participant may instead be an always-taker.[ref]
Principal Stratification in Causal Inference by Constantine E. Frangakis and Donald B. Rubin. Section 2, cross-classification by intermediate potential values
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