  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 $<Q>$, $<U>\propto\exp(-\delta_l)$, with
$\delta_l\propto\lambda^4$. It is pointed out that a definitive test for such
depolarization is provided by the fact that $<Q^2+U^2>$ remains constant while
$<Q>^2+< U>^2$ decreases $\propto\exp(-2\delta_l)$. 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
observation process. Observations of depolarization would provide statistical
information on fluctuations in the turbulent plasma along the line of sight,
specifically, the variance of the rotation measure. Other effects that can also
cause depolarization are discussed.
