Ignorability, also called exchangeability, allows adjustment for covariates when potential outcomes are independent of treatment assignment conditional on those covariates. Under this assumption, the adjustment formula can estimate the average treatment effect without bias. In an RCT, random assignment makes treatment groups comparable in characteristics such as age, socioeconomic class, and sex. section 10.3
Keep these distinctions in view:
- The assumption appears under several names, including strong ignorability and exchangeability; some treatments also include positivity under this label. section 10.3
- In the potential-outcomes framework, ignorability provides the condition for applying the adjustment formula; DAGs instead use the back-door criterion to identify suitable adjustment sets. section 10.3 section 10.6
Comparing treatment groups
Suppose an RCT assigns patients to treatment or control. For any age a, randomization implies approximately equal probabilities of observing that age in both groups: P(Age = a | T = 1) = P(Age = a | T = 0). This balance illustrates why treatment assignment can be treated as independent of measured patient characteristics. section 10.3

