= Solution
The raw difference is generally not a <causal effect>. Existing UTCs may differ systematically from non-UTCs in location, admissions, pupil composition, prior attainment, resources, and the local apprenticeship market. These are possible <confounders> of school type and outcome. The comparison also weights the 43 UTCs and roughly 3500 controls very differently from a clearly defined target population. Without <conditional exchangeability>, <positivity in causal inference>[positivity], and <consistency in causal inference>[consistency], the difference mixes the effect of UTC status with selection bias.
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