  In optical interferometry, the visibility squared modulus are generally
assumed to follow a Gaussian distribution and to be independent of each other.
A quantitative analysis of the relevance of such assumptions is important to
help improving the exploitation of existing and upcoming multi-wavelength
interferometric instruments. Analyze the statistical behaviour of both the
absolute and the colour-differential squared visibilities: distribution laws,
correlations and cross-correlations between different baselines. We use
observations of stellar calibrators obtained with AMBER instrument on VLTI in
different instrumental and observing configurations, from which we extract the
frame-by-frame transfer function. Statistical hypotheses tests and diagnostics
are then systematically applied. For both absolute and differential squared
visibilities and under all instrumental and observing conditions, we find a
better fit for the Student distribution than for the Gaussian, log-normal and
Cauchy distributions. We find and analyze clear correlation effects caused by
atmospheric perturbations. The differential squared visibilities allow to keep
a larger fraction of data with respect to selected absolute squared
visibilities and thus benefit from reduced temporal dispersion, while their
distribution is more clearly characterized. The frame selection based on the
criterion of a fixed SNR value might result in either a biased sample of frames
or in a too severe selection.
