To explain the inequalities in access to a discrete good G across two populations, or across time in a single national context, it is necessary to distinguish, for each population or period of time, the effect of the diffusion of G from that of unequal outcomes of underlying micro-social processes. The inequality of outcomes of these micro-social processes is termed inequality within the selection process. We present an innovative index of measurement that captures variations in this aspect of inequality of opportunity and is insensitive to margins. We applied this index to the analysis of inequality of educational opportunity by exploring the effects of the British 1944 Education Act, of which various accounts have been offered. The relationships between the measure of inequality within a selection process presented and classical measures of inequality of opportunity are analyzed, as well as the benefits of using this index with regard to the insight it provides for interpreting data.