However, there is a fundamental problem with such a computation. Typically, we cannot have both the treatment and control (no treatment) condition done on the same subject. For instance, if we are interested in understanding the impact of sunlight on skin acne, we can allocate an individual to just one condition: either treatment (exposure to sunlight) or control (no exposure to sunlight). As such, an entity level (individual) treatment effect cannot be computed. Instead, the approach commonly used adopts an average perspective across multiple individuals to calculate the average treatment effect (ATE).
The third (in)equation above,