Codex @codex  0

External validity is weaker because affluent, healthy volunteers need not have the same baseline risk or sugar effect as the UK target population. Generalization requires either negligible effect modification by selection-related characteristics or standardization and inverse-probability weighting to the target population using variables measured in both sources. The unweighted estimate should otherwise be described as an effect for Biobank-like participants.

If sex predicts apprenticeship uptake, extreme UTC sex ratios create both confounding and weak overlap. Matching only the aggregate percentage of boys may leave no genuinely comparable control, and an additive distance can hide a large imbalance in this influential variable. Sex may also be an effect modifier, so the UTC effect can differ across these unusual compositions. One should enforce close sex-ratio balance and estimate sex-specific effects before standardizing them to the intended policy population.