In earlier chapters we have criticized the selection system on several grounds. Here we have to assume that the present system will operate in most areas for some time to come, and to ask the fundamental question: How far does it select and reject the right pupils? To what extent are pupils wrongly placed in the grammar and secondary modern school? No easy straightforward answer to these questions is possible since they imply involved technical considerations. The basic method of assessing the validity of selection is as follows. Pupils selected by entry tests or examinations are followed up in their secondary-school years. Then their success in the secondary school is assessed by internal or external examinations, such as the G.C.E., and these follow-up measures of success are correlated with the entry data.