  The physical mechanisms driving angular momentum transport in accretion discs
are still unknown. Although it is generally accepted that, in hot discs, the
turbulence triggered by the magneto-rotational instability is at the origin of
the accretion process in Keplerian discs, it has been found that the values of
the stress-to-pressure ratio (the alpha "viscosity" parameter) deduced from
observations of outbursting discs are an order of magnitude higher than those
obtained in numerical simulations. We test the conclusion about the
observation-deduced value of alpha using a new set of data and comparing the
results with model outbursts. We analyse a set of observations of dwarf-nova
and AM CVn star outbursts and from the measured decay times determine the
hot-disc viscosity parameter alpha_h. We determine if and how this method is
model dependent. From the dwarf-nova disc instability model we determine an
amplitude vs recurrence-time relation and compare it to the empirical
Kukarkin-Parenago relation between the same, but observed, quantities. We found
that all methods we tried, including the one based on the amplitude vs
recurrence-time relation, imply alpha_h ~ 0.1 - 0.2 and exclude values an order
of magnitude lower. The serious discrepancy between the observed and the
MRI-calculated values of the accretion disc viscosity parameter alpha is
therefore real since there can be no doubt about the validity of the values
deduced from observations of disc outbursts.
