Different ray paths through a turbulent plasma can produce stochastic Faraday rotation leading to depolarization of any linearly polarized component. Simple theory predicts that the average values of the Stokes parameters decay according to and ∝exp(-δ_ι), with δ_ι∝λ~4. It is pointed out that a Definitive test for such depolarization is provided by the fact that remains constant while ~2 + ~2 decreases ∝exp(-2δ_ι). The averages to which this effect, called polarization covariance, should apply are discussed; it should apply to spatial averages over a polarization map or temporal averages over a data set, but not to beamwidth and bandwidth averages that are intrinsic to the obser- vation process.
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