4.6. The myth of quasar variability
One last difficulty associated with estimating H0 is implicit in the fact that only 10% of the hundred or so known lensed quasars appear in figure 1. When we think of quasar variability we think of 3C273 or 3C279 and variations of a magnitude or more. But those quasars are well known precisely because of their variability. Most quasars are considerably more boring. Systematic studies of quasar variability at optical wavelengths consistently give rms fluctuations of order 10 - 15% on proper timescales of one year ([Cristiani et al. (1996)]; [Vanden Berk et al. (2004)]).
The good news is that a few quasars vary more than this. The bad news is that most vary even less than this. The statistics may be somewhat better for radio quasars, but then radio loud systems constitute only 10% of all quasars. While the small amplitude of quasar variations may give graduate students (mostly those working at radio wavelengths) an opportunity to show their skill in beating down their observational errors, flat lightcurves don't lead to offers of prestigious postdocs. It should be noted that there is considerable room for improvement in the optical lightcurves, for which the observational errors are much larger than photon statistics would imply. Our list of major difficulties in measuring H0 has grown to include the following: