  We present an analysis of X-ray variability in a flux limited sample of QSOs.
Selected from our deep ROSAT survey, these QSOs span a wide range in redshift
($0.1<z<3.2$) and are typically very faint, so we have developed a method to
constrain the amplitude of variability in ensembles of low signal-to-noise
light curves. We find evidence for trends in this variability amplitude with
both redshift and luminosity. The mean variability amplitude declines sharply
with luminosity, as seen in local AGN, but with some suggestion of an upturn
for the most powerful sources. We find tentative evidence that this is caused
by redshift evolution, since the high redshift QSOs ($z>0.5$) do not show the
anti-correlation with luminosity seen in local AGN. We speculate on the
implications of these results for physical models of AGN and their evolution.
Finally, we find evidence for X-ray variability in an object classified as a
narrow emission-line galaxy, suggesting the presence of an AGN.
