Developments in assessment occur for four reasons. Firstly the content of the subject areas themselves changes with the changing needs of society and the developing perceptions of teachers and others working in education. Secondly our technical knowledge of the development of children's cognitive abilities enables us to direct our assessment more exactly. Both these aspects of assessment are more appropriately dealt with in specialist works which can cover the content of each subject area separately. Thirdly our understanding of the processes of assessment enables our techniques of assessment to become more effective, though validity and reliability have to a certain extent to be traded against each other. Lastly, the nature of society itself is constantly changing and forcing on us a reconsideration of whether our methods of assessment fulfil its needs. Assessment at 16 can provide one of the most effective means of causing teachers to implement change in the classroom.