  The evaluation of the nuclear matrix elements (NME) of the two-neutrino
double beta ($2\nu\beta\beta$) decay and neutrinoless double beta
($0\nu\beta\beta$) decay using the proton-neutron quasiparticle random-phase
approximation (pnQRPA) is addressed. In particular, the extraction of a proper
value of the proton-neutron particle-particle interaction parameter, $g_{\rm
pp}$, of this theory is analyzed in detail. Evidence is shown, that it can be
misleading to use the experimental half-life of the $2\nu\beta\beta$ decay to
extract a value for $g_{\rm pp}$. Rather, arguments are given in favour of
using the available data on single beta decay for this purpose.
