  A recent paper (Nodland and Ralston, PRL 78, 3043, astro-ph/9704196) claims
to have detected evidence for birefringence in the propagation of radio waves
across cosmological distances. In order to assess the statistical significance
of their results, the authors analyze simulated data sets, finding a stronger
correlation in the real data than in the simulations. Unfortunately, the
procedure used for generating the simulated data sets is based on an incorrect
null hypothesis. Furthermore, the correct null hypothesis would lead to a
stronger correlation in the simulated data sets, weakening the case for
birefringence. We conclude that the paper's analysis does not provide
statistically significant evidence of birefringence.
