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Collider-Stratification bias at the start of a study (‘M-bias’)
| Selection Restriction Before Start of Study Arising from Collider Stratification Bias | ||
|---|---|---|
| Bias | Causal Graph | |
| 1 | Problem: Sample selection may induce M-bias (M-bias): threat to external validity |  |
| 2 | Response: (a) Do not restrict sample; (b) condition on descendent of U1 or U2 |  |
