  In the presence of mixing between massive neutrino states, the distortion of
the electron spectrum in beta decay is, in general, a function of several
masses and mixing angles. For $3\nu$-schemes which describe the solar and
atmospheric neutrino data, this distortion can be described by a single
effective mass, under certain conditions. In the literature, two different
definitions for the effective mass have been suggested. We show that for
quasi-degenerate mass schemes (with an overall mass scale $m$ and splitting
$\Delta m^2$) the two definitions coincide up to $(\Delta m^2)^2/m^4$
corrections. We consider the impact of different effective masses on the
integral energy spectrum. We show that the spectrum with a single mass can be
used also to fit the data in the case of $4\nu$-schemes motivated, in
particular, by the LSND results. In this case the accuracy of the mass
determination turns out to be better than $(10 - 15)%$.
