  The standard field-theoretical approach to the slow-roll inflation is
introduced. We then show as, in order to calculate the mean square of the
canonical gauge invariant quantum fluctuations associated to a generic field,
the logarithm of the scale factor has to be used as the time variable in the
Fokker-Planck equation in the stochastic approach. Then we compute the growth
of different test fields with a small effective mass during slow-roll
inflationary models, comparing the results with the one for the gauge invariant
canonical fluctuation associated to the inflaton, the Mukhanov variable. We
find that in most of the single fields inflationary models such fluctuation
grows faster than any test field with a non-negative effective mass, with the
exception of hybrid models.
