Solution
= Solution
Let $Y_s(1)$ and $Y_s(0)$ be the <potential outcomes> for the apprenticeship proportion at school $s$ if it is or is not converted to a UTC. The policy-relevant estimand is the average causal effect
$$
\mathbb E\{Y_s(1)-Y_s(0)\}
$$
over the population of schools eligible for conversion. Ideally this would be identified by a <Randomized controlled trial> of conversion, or by an observational design that reproduces its exchangeability.