A control group must include only cases that could have shown the effect

Evidence

The strongest statement along these lines is Lemke’s (1995, 29) claim that relevant dyads matter because they comprise the correct referent group, and thus function as a true control group, against which war dyads are compared. The reason the set of relevant dyads is the correct referent group for war dyads is that it is only the relevant dyads that might have had a war. (Lemke and Reed 2001, 128) It is worth noting that the concept of “political relevance” has the typical three-level concept structure. — Gary Goertz, Social Science Concepts, 112

Categorization

• XREF: Generalizes the counterfactual/at-risk population problem in causal inference — connects to sampling and selection-on-the-outcome issues in any observational design.
• SEED: Article angle: ‘The control group that couldn’t have happened’ — how studying only real cases inflates apparent causes across epidemiology, criminology, and war studies.