I put some effort into researching this on a statistical basis. To cut it short: I can show you nothing. For whatever I looked, the picture is always the same: No development, no trend, no correlation. It is presented below, since I had done it. But the worth is dubious, you may as well skip this section. Quite depressing for one who set out to discover hidden wonders in the world of numbers.
-- "Robert", analyzing the footnotes in the work of Terry Pratchett