= Solution
First inspect overlap and post-match balance using <standardized mean differences>, distributions, and interactions for every pre-treatment covariate. Material imbalance or UTCs outside the control support shows that the design is extrapolating and cannot justify conditional exchangeability on those variables.
Second perform a <negative control outcome> analysis using apprenticeship outcomes from before conversion, or another outcome that UTC status could not yet affect. A nonzero estimated effect indicates remaining selection or differential trends. Neither diagnostic proves absence of hidden confounding, so a quantitative <sensitivity analysis for unmeasured confounding> is also useful.
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