Measuring school value added with administrative data: the problem of missing variables

The UK Department for Education (DfE) calculates contextualised value added (CVA) measures of school performance using administrative data that contain only a limited set of explanatory variables. Differences on schools’ intake regarding characteristics such as mother’s education are not accounted for due to the lack of background information in the data. In this paper we use linked survey and administrative data to assess the potential biases that missing control variables cause in the calculation of CVA measures of school performance. We find that ignoring the effect of mother’s education leads DfE to erroneously over-penalise low achieving schools that have a greater proportion of mothers with low qualifications and to over-reward high achieving schools that have a greater proportion of mothers with higher qualifications. This suggests that collecting a rich set of controls in administrative records is necessary for producing reliable CVA measures of school performance.