Differing levels of productivity of labour and barriers to mobility, insofar as they affect the demand and the supply of labour, go a long way to explaining wage differentials between occupations. There are also wage differentials within occupations which could be due to the incremental scales which are common within organisations, the influence of performance-related pay or regional differences in the availability of labour or in the cost of living. The existence of incremental scales is largely based on an acceptance of the idea that with time and experience workers become more productive.