Table 2.
Principal Stratification on the Basis of Potential Mediator Status
| Principal stratum membership (Ci) | Potential mediator |
Potential outcome |
Individual causal effect given Ci | ||
|---|---|---|---|---|---|
| Mi(0) | Mi(1) | Yi(Ci, 0) | Yi(Ci, 1) | ||
| n (never-improver) | 0 | 0 | Yi(n, 0) | Yi(n, 1) | Yi(n, 1) − Yi(n, 0) |
| f (forward-improver) | 0 | 1 | Yi(f, 0) | Yi(f, 1) | Yi(f, 1) − Yi(f, 0) |
| b (backward-improver) | 1 | 0 | Yi(b, 0) | Yi(b, 1) | Yi(b, 1) − Yi(b, 0) |
| a (always-improver) | 1 | 1 | Yi(a, 0) | Yi(a, 1) | Yi(a, 1) − Yi(a, 0) |
Note. C = principal stratum membership; M = mediator; Y = outcome.