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 \leq \alpha \leq 0.03 for 0.01 \leq L / L _ { Edd } \leq 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 simulations in which MHD turbulence is the primary viscosity mechanism .