Published online by Cambridge University Press: 12 April 2016
In this paper results from a monitoring programme of a large sample of quasars comprising regular yearly observations over a period of 23 years are presented. Structure functions of the light curves are calculated and compared with predictions for models of quasar variability of current interest. These include recently published models of varibility from accretion disk instability, variability from starbursts or supernovae, and variations caused by the microlensing effect of compact bodies along the line of sight. The analysis favours the accretion disk model for low luminosity AGN, but suggests that the variations of more luminous quasars are dominated by microlensing.