Table 2
Definitions of different types of treatment effect
| Effect | Potential outcome notation | Description |
| Average treatment effect (ATE) |
| The difference between the average outcome when everyone is exposed, and the average outcome when nobody is. |
| Average treatment effect in the treated (ATT) |
| The ATE in the subpopulation of individuals who were actually exposed. |
| Average treatment effect in the untreated (ATU/ATUT) |
| The ATE in the subpopulation of individuals who were actually unexposed. |
| Intention-to-treat effect (ITT) |
| Average effect of being assigned to (but not necessarily receiving) the exposure. |
| Complier average causal effect (CACE) or local average treatment effect (LATE) |
| The ATE among the ‘compliers’, that is, the subpopulation whose exposure status was affected by the assignment mechanism. |
A denotes actual exposure status (a=1 for exposed, a=0 for unexposed). Z denotes assignment to the exposure, which may or may not have been adhered to (z=1 for assignment to the exposure, z=0 for assignment away from the exposure).