  We estimate a value of the viscosity parameter in AGN accretion discs for the
PG quasar sample. We assume that optical variability on time-scales of months
to years is caused by local instabilities in the inner accretion disc.
Comparing the observed variability time-scales to the thermal time-scales of
alpha-disc models we obtain constraints on the viscosity parameter for the
sample. We find that, at a given L/L_Edd, the entire sample is consistent with
a single value of the viscosity parameter, alpha. We obtain constraints of 0.01
< alpha < 0.03 for 0.01 < L/L_Edd < 1.0. This narrow range suggests that these
AGN are all seen in a single state, with a correspondingly narrow spread of
black hole masses or accretion rates. The value of alpha we derive is
consistent with predictions by current simulation s in which MHD turbulence is
the primary viscosity mechanism.
