I was wondering how the distribution of Lyman alpha absorbers (as seen in the "forest" of absorption lines of a quasar spectrum) varies with red-shift (corresponding to evolution with the age of the universe). I've found various sources which mention in passing that the frequency of absorbers in a given spectrum vary with redshift according to a power law, which sounds interesting. But it seems that the constants involved and hence the redshift distribution of absorption systems varies for different objects. Is that really right, and if so how can that be? I had expected that the average number of clouds between two given redshift values would be about the same in all directions, so all objects should show similar distributions. Can anyone point me to a recent reference on this? Most of the papers I can find are around 1997-2001.