  This paper presents the results of a study of X-ray spectral and flux
variability on time scales from months to years, of the 123 brightest objects
(including 46 type-1 AGN and 28 type-2 AGN) detected with XMM-Newton in the
Lockman Hole field. We detected flux variability with a significance >3sigma in
~50% of the objects, including 68+-11% and 48+-15% among our samples of type-1
and type-2 AGN. However we found that the fraction of sources with best quality
light curves that exhibit flux variability on the time scales sampled by our
data is >80%, i.e the great majority of the AGN population may actually vary in
flux on long time scales. The mean relative intrinsic amplitude of flux
variability was found to be ~0.15 although with a large dispersion in measured
values, from ~0.1 to ~0.65. The flux variability properties of our samples of
AGN do not significantly depend on the redshift or X-ray luminosity of the
objects and seem to be similar for the two AGN types. Using a broad band X-ray
colour we found that the fraction of sources showing spectral variability with
a significance >3sigma is ~40% i.e. less common than flux variability. Spectral
variability was found to be more common in type-2 AGN than in type-1 AGN with a
significance >99%. This result is consistent with the fact that part of the
soft emission in type-2 AGN comes from scattered radiation, and this component
is expected to be much less variable than the hard component. The observed flux
and spectral variability properties of our objects cannot be explained as being
produced by variability of one spectral component alone, for example changes in
the continuum shape associated with changes in the mass accretion rate, or
variability in the amount of X-ray absorption. At least two spectral components
must vary in order to explain the X-ray variability of our objects.
