In order to study precession and interstellar magnetic field variations, we measured the polarized position angle of 81 pulsars at several-month intervals for four years. We show that the uncertainties in a single-epoch measurement of position angle is usually dominated by random pulse-to-pulse jitter of the polarized subpulses. Even with these uncertainties, we find that the position angle variations in 19 pulsars are significantly better fitted (at the 3 {sigma} level) by a sinusoid than by a constant. Such variations could be caused by precession, which would then indicate periods of ~ (200 - 1300) d and amplitudes of ~(1 - 12) degrees. We narrow this collection to four pulsars that show the most convincing evidence of sinusoidal variation in position angle. Also, in a handful of pulsars, single discrepant position angle measurements are observed which may result from the line of sight passing across a discrete ionized, magnetized structure. We calculate the standard deviation of position angle measurements from the mean for each pulsar, and relate these to limits on precession and interstellar magnetic field variations.
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