  We present an analysis of X-ray variability in a sample of 156 radio quiet
quasars taken from the ROSAT archive, covering a redshift range 0.1 < z < 4.1.
A maximum likelihood method is used to constrain the amplitude of variability
in these low signal to noise light curves. Through combining these data in
ensembles we are able to identify trends in variability amplitude with
luminosity and with redshift. The decline in variability amplitude with
luminosity identified in local AGN (z < 0.1) is confirmed out to z=2. There is
tentative evidence for an increase in QSO X-ray variability amplitude towards
high redshifts (z > 2) in the sense that QSOs of the same X-ray luminosity are
more variable at z > 2. We discuss possible explanations for this effect. The
simplest explanation may be that high redshift QSOs are accreting at a larger
fraction of the Eddington limit than local AGN.
