This research was conducted via literature review. Most of the literature reviewed were either observational studies or survey data, with some quasiexperimental research sprinkled in.
Unfortunately, police training is not currently standardized across states or even across state departments. This means that the data collected may not be very generalizable. That said, the fact that some departments report inadequate training is a good enough reason to require reforms across all departments, since it will not meaningfully inconvenience departments who are already implementing good training techniques.
It would not be ethical to manipulate violent encounters with police for the purpose of experiments, which means that it is difficult to prove direct causation between any of these factors and police violence or brutality.