If you're interested in other things IPRA 'investigates', some people have FOIA'ed Chicago police misconduct complaints and created visualizations based on the data[1].
Devils advocate but wouldn't they investigate every shooting incident, so you'd expect the vast majority to be clean.
I know every discharged weapon in the UK is followed by immediate officer suspension and investigation, and I suspect very few are actually ruled as bad and rightly so.
[1] http://invisible.institute/police-data/