Providing post-16 education cost-effectively becomes more challenging for smaller comprehensive schools, because of the number of courses needed to cover a broader curriculum with comparatively fewer students. This is why schools have tended to get larger and also why many local authorities have organised secondary education into 11–16 schools, with the post-16 provision provided by sixth form colleges and further education colleges. Comprehensive schools do not select their intake on the basis of academic achievement or aptitude, but there are demographic reasons why the attainment profiles of different schools vary considerably.