  We explored the statistical properties of short-term X-ray variability using
long-exposure {\it XMM-Newton} data during high X-ray variability phases of
blazars S5 0716+714 and PKS 2155-304. In general, hardness ratio shows
correlated variations with the source flux state (count rate), but in a few
cases, mainly the bright phases, the trend is complex with correlation and
anti-correlation both, indicating spectral evolution. Stationarity tests
suggest the time series as non-stationarity or have trend stationarity. Except
for one, none of the histograms fit resulted in a reduced-\(\chi^2 \sim 1\) for
a normal and log-normal profile but a normal profile is favored in general. On
the contrary, the Anderson-Darling test favors lognormal with a test-statistic
value lower for log-normal over normal for all the observations, even if out of
significance limits. None of the IDs show linear RMS-flux relation. The
contrary inferences from widely used different statistical methods indicate
that a careful analysis is needed while the complex behavior of count rate with
hardness ratio suggests spectral evolution over a few 10s of kilo-seconds
during bright phases of the sources. In these cases, the spectrum extracted
from whole observation may not be meaningful for spectral studies and certainly
not a true representation of the spectral state of the source.
