Over the past 17 years a large scale quasar monitoring programme has been underway at the Royal Observatory, Edinburgh, using COSMOS measures of a long sequence of plates from the UK 1.2 m Schmidt telescope in Australia. 3 or 4 plates were taken each year, and quasar candidates were selected solely on the basis of variability, with a minimum amplitude of 0.35 mag. over the 17 years of the survey. Of the 1000 or so quasars detected in this way, redshifts were available for about 300, which formed the sample to be investigated. Inspection of the light curves suggested a typical timescale of variation of around 5 to 10 years. To investigate the cause of variability, the timescales of subsamples were measured using the timevarying autocorrelation function (ACF).