  I show that Tremaine-Weinberg (TW) measurements of bar pattern speeds are
sensitive to errors in the position angle of the disc, PA_disc. I use an
$N$-body experiment to measure these errors; for typical random PA_disc errors,
the resulting scatter in the measured values of the dimensionless bar speed
parameter $R$ (defined as the ratio of the corotation radius to the bar
semi-major axis) is of order the observational. I also consider how the
systematic PA_disc errors produced by disc ellipticities affect TW
measurements. The scatter produced by these errors may be significant,
depending on the ellipticity distribution. Conversely, by using the sample of
TW observations, I find that an upper limit of the typical disc (density)
ellipticity is 0.07 at the 90 per cent confidence level, which is in good
agreement with previous measurements. Taken together, the random and systematic
scatter suggest that the intrinsic distribution of $R$ of gas-poor early-type
barred galaxies may be as narrow as that of the gas-rich later-types.
