Dr Drib. one of the things that makes it difficult to measure an author's improvement over time is that the public often doesn't get to see where they started. For example, by the time most trad' pub authors are actually published they have already written a lot. (Some Indie published authors might be publishing the first thing that appears on the screen, so over time they might offer better source for scientific study of this phenomena.
)
I was interested to read a couple of the collections put out by Terry Pratchett (
A Blink of the Screen, and
Dragons at Crumbling Castle: And Other Stories) that gave glimpses of some of his earlier writing. In amongst these are ideas that obviously became more fully fledged works later (eg:
Rincemangle, the Gnome of Even Moor becomes
Truckers of the Bromeliad trilogy). If ever I needed convincing that even the best authors get better with work, this was it.