We can actually get a much better line on Castagna's clanger rate than would normally be the case. This is because he has two contemporaries in Butler and Rioli who have mainly played a similar role in a similar period across a roughly similar amount of matches. Over their careers, all average between 11.2 and 11.5 disposals per match. Rioli goes at 1.9 clangers per match on average, Butler 2.0 and Castagna 2.1.
An even better comparison may be the 2017 season when all were fit and firing off good preparations and beside Butler missing two matches they all played every match. I think they all played almost all of their ground time that season as forwards. This comparison allows us to exclude Rioli's 2018 for example where he missed half the season and took time to find his feet once he returned to AFL football. In 2017 Rioli averaged 10.8 disposals and 1.8 clangers. Castagna averaged 10.8 disposals and 2.2 clangers. Butler averaged 11.3 disposals and 2.0 clangers. So from similar amounts of disposals Butler commits about 10% more clangers than Rioli, and Castagna commits about a further 10% more clangers than Butler, in 2017 at least.
I think these figures are about what we might expect from our impressions of watching the three players play. Clangers of course are only one measure of player error. And then there are clangers and clangers. A turnover to an opponent running unchecked towards his teams' goal is worse than missing a target and say putting it out of bounds, in terms of scoreboard impact at least.
The last observation I have on this is Castagna's clanger curve. He appears to be steadily improving, again what you might expect as a young player gains experience and skills. From 2017 to 2018 Castagna's disposal average dropped about 5% from 10.8 per match to 10.3 per match. His clanger average however dropped about 14% from 2.2 per match to 1.9 per match. Comparing 2018 to 2019 so far, his disposal rate this year is up on average by about 36% from 10.3 per match to 14 per match. His clangers have risen by about 31% from 1.9 per match to 2.5 per match. So each year he seems to be improving a little on a clangers per disposals basis.
