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  "path": "/t/collider-in-rct-subgroup-analysis/28689#post_15",
  "publishedAt": "2026-05-17T20:02:23.000Z",
  "site": "https://discourse.datamethods.org",
  "textContent": "If the variable is pre-randomization and solely prognostic for the outcome, then adjust in the analysis. All that stratification aka subgrouping (or adding a product term in regression which is the equivalent of stratification) achieves is overfitting and nothing else useful can be gained from this procedure. This is a form of effect heterogeneity called risk magnification.\n\nTo be an effect modifier, the variable must be associated with the intervention in a post-randomization sense as well as prognostic for the outcome, otherwise how would it modify the effect of the intervention on the outcome?",
  "title": "Collider in RCT Subgroup Analysis"
}