We present a Monte Carlo analysis of the recently introduced variability in
dices tau (Tompkins 1999) and I (Zhang et al. 2000 & Torres et al. 2001) fo
r gamma -ray sources. We explore different variability criteria and prove t
hat these two indices, despite the very different approaches used to comput
e them, are statistically correlated (5 to 7 sigma). This conclusion is mai
ntained also for the subset of AGNs and high latitude (\b \ > 10 deg) sourc
es, whereas the correlation is lowered for the low latitude ones, where the
influence of the diffuse galactic emission background is strong.