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We explored the statistical properties of short-term X-ray variability usinglong-exposure {\it XMM-Newton} data during high X-ray variability phases ofblazars S5 0716+714 and PKS 2155-304. In general, hardness ratio showscorrelated variations with the source flux state (count rate), but in a fewcases, mainly the bright phases, the trend is complex with correlation andanti-correlation both, indicating spectral evolution. Stationarity testssuggest the time series as non-stationarity or have trend stationarity. Exceptfor one, none of the histograms fit resulted in a reduced-\(\chi^2 \sim 1\) fora normal and log-normal profile but a normal profile is favored in general. Onthe contrary, the Anderson-Darling test favors lognormal with a test-statisticvalue lower for log-normal over normal for all the observations, even if out ofsignificance limits. None of the IDs show linear RMS-flux relation. Thecontrary inferences from widely used different statistical methods indicatethat a careful analysis is needed while the complex behavior of count rate withhardness ratio suggests spectral evolution over a few 10s of kilo-secondsduring bright phases of the sources. In these cases, the spectrum extractedfrom whole observation may not be meaningful for spectral studies and certainlynot a true representation of the spectral state of the source.

Short-term X-ray Variability during Different Activity Phases of Blazars S5 0716+714 and PKS 2155-304