Microvariability in AGNs: study of different statistical methods - I.
  Observational analysis
  We present the results of a study of different statistical methods currently
used in the literature to analyse the (micro)variability of active galactic
nuclei (AGNs) from ground-based optical observations. In particular, we focus
on the comparison between the results obtained by applying the so-called C and
F statistics, which are based on the ratio of standard deviations and
variances, respectively. The motivation for this is that the implementation of
these methods leads to different and contradictory results, making the
variability classification of the light curves of a certain source dependent on
the statistics implemented.
  For this purpose, we re-analyse the results on an AGN sample observed along
several 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 applied
the statistical tests mentioned above, in order to re-classify the variability
state of these light curves and in an attempt to find the suitable statistical
methodology to study photometric (micro)variations. We conclude that, although
the $C$ criterion is not proper a statistical test, it could still be a
suitable parameter to detect variability and that its application allows us to
get more reliable variability results, in contrast with the F test.
