= Never-taker
{title2=$(D(0),D(1))=(0,0)$}
A never-taker is a unit whose <potential treatment receipts> are $D(0)=D(1)=0$: it receives no <treatment> under either value of a binary assignment or encouragement $Z$. This is the $(0,0)$ <principal stratum>, defined before observing which assignment actually occurs.
For example, a unit may decline participation whether or not it receives an invitation. Among units with observed $Z=0,D=0$, <never-takers> are mixed with <compliers>, so a single observation does not reveal the <principal stratum>. Under an exclusion restriction through treatment receipt, assignment has no outcome effect on a <never-taker> because receipt stays fixed; that is an additional causal assumption, not part of the definition.<Principal Stratification in Causal Inference by Constantine E. Frangakis and Donald B. Rubin>[Section 2, principal strata defined by joint intermediate potential values]{ref}
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