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We present the results of a study of different statistical methods currentlyused in the literature to analyse the (micro)variability of active galacticnuclei (AGNs) from ground-based optical observations. In particular, we focuson the comparison between the results obtained by applying the so-called C andF statistics, which are based on the ratio of standard deviations andvariances, respectively. The motivation for this is that the implementation ofthese methods leads to different and contradictory results, making thevariability classification of the light curves of a certain source dependent onthe statistics implemented.  For this purpose, we re-analyse the results on an AGN sample observed alongseveral sessions with the 2.15m "Jorge Sahade" telescope (CASLEO), San Juan,Argentina. For each AGN we constructed the nightly differential light curves.We thus obtained a total of 78 light curves for 39 AGNs, and we then appliedthe statistical tests mentioned above, in order to re-classify the variabilitystate of these light curves and in an attempt to find the suitable statisticalmethodology to study photometric (micro)variations. We conclude that, althoughthe $C$ criterion is not proper a statistical test, it could still be asuitable parameter to detect variability and that its application allows us toget more reliable variability results, in contrast with the F test.

Microvariability in AGNs: study of different statistical methods - I. Observational analysis