  We describe a method for forecasting errors in interferometric measurements
of polarization of the cosmic microwave background (CMB) radiation, based on
the use of the Fisher matrix calculated from the visibility covariance and
relation matrices. In addition to noise and sample variance, the method can
account for many kinds of systematic error by calculating an augmented Fisher
matrix, including parameters that characterize the instrument along with the
cosmological parameters to be estimated. The method is illustrated with
examples of gain errors and errors in polarizer orientation. The augmented
Fisher matrix approach is applicable to a much wider range of problems beyond
CMB interferometry.
