  In the last decade, a number of experiments dealt with the problem of
measuring the arrival direction distribution of cosmic rays, looking for
information on the propagation mechanisms and the identification of their
sources. Any deviation from the isotropy may be regarded to as a signature of
unforeseen or unknown phenomena, mostly if well localised in the sky and
occurring at low rigidity. It induced experimenters to search for excesses down
to angular scale as narrow as 10 degrees, disclosing the issue of properly
filtering contributions from wider structures. A solution commonly envisaged in
these years based on time-average methods to determine the reference value of
cosmic ray flux. Such techniques are nearly insensitive to signals wider than
the time-window in use, thus allowing to focus the analysis on medium- and
small-scale signals. Nonetheless, often the signal cannot be excluded in the
calculation of the reference value, what induce systematic errors. The use of
time-average methods recently brought to important discoveries about the
medium-scale cosmic ray anisotropy, present both in the northern and southern
hemisphere. It is known that the excess (or the deficit) is observed as less
intense than in reality and that fake deficit zones are rendered around true
excesses, because of the absolute lack of knowledge a-priori of which signal is
true and which is not. This work is an attempt to critically review the use of
time average-based methods for observing extended features in the cosmic-ray
arrival distribution pattern.
