The use of questionnaires to evaluate educational initiatives is widespread, but often problematic. This paper examines four aspects of an evaluation survey carried out with very able pupils attending out-of-school classes: ethics, design, bias and interpretation. There is a particular focus on the interpretation and analysis of pupils' answers to open questions. Conclusions are drawn from this analysis which will help teachers and others to take a careful and critical approach to their use of questionnaires in educational evaluation.