Interpretation of the Individual Effect Under Treatment Spillover

Some interventions are intended to benefit both individuals and the groups to which they belong. When a treatment given to one person exerts a causal effect on others, the treatment is said to exhibit spillover, dissemination, or interference. However, defining meaningful causal effects under spillover can be challenging. In this commentary, we discuss the meaning of the "individual effect," a quantity proposed to summarize the effect of treatment on the person who receives it, when spillover may be present.