Possible evidence for a variable fine structure constant from QSO   absorption lines: systematic errors
  Comparison of quasar absorption spectra with laboratory spectra allow us to probe possible variations in the fundamental constants over cosmological time-scales. In a companion paper we present an analysis of Keck/HIRES spectra and report possible evidence suggesting that the fine structure constant, alpha, may have been smaller in the past: da/a = (-0.72 +/- 0.18) * 10^{-5} over the redshift range 0.5 < z < 3.5. In this paper we describe a comprehensive investigation into possible systematic effects. Most of these do not significantly influence our results. When we correct for those which do produce a significant systematic effect in the data, the deviation of da/a from zero becomes more significant. We are lead increasingly to the interpretation that alpha was slightly smaller in the past. 
M. T. Murphy (UNSW), J. K. Webb (UNSW), V. V. Flambaum (UNSW), C. W.   Churchill (PSU), J. X. Prochaska (OCIW)