you probably noticed then, that the yearbooks before 1969 (iirc) do not contain hour information. before then they used to record things like total placements and RVs (back-calls) tho i would imagine it would be a little trickier to incorporate that into your research. i dont know if detailed hour information exists at all before then.
when i compiled these stats for myself a while ago, i WAS amazed by how regular the avg hours stats is, compared to other stats that peak and trough wildly. avg hours flows up and down in steady curves and seemed like an excellent general indicator. avg studies is another smooth curve tho id imagine there are additional variables affecting this one.
mox