Exchangeability
In this section, we’ll introduce the exchangeability assumption (also known as the ignorability assumption) and discuss its relation to confounding.
Exchangeable subjects
The main idea behind exchangeability is the following: the treated subjects, had they been untreated, would have experienced the same average outcome as the untreated did (being actually untreated) and vice versa (Hernán & Robins, 2020).
Formally speaking, exchangeability is usually defined as:
In the preceding formula, and are counterfactual outcomes under and respectively, and is a vector of control variables. If you’re getting a feeling of confusion or even circularity when thinking about this definition, you’re most likely not alone. According to Pearl (2009), many people see this definition as difficult to understand.
At the same time, the core idea behind it is simple: the treated and the untreated need to share all the relevant characteristics...