According to this review (22 page PDF) of selected, filtered and codified research papers, "the highest areas of focus within the research questions related to the PD program itself, implementation, instructors, and helping the instructor design a quality course." Meanwhile another major thread "examined instructor perceptions, outcomes, and characteristics." Interestingly, "the PD program itself should be the largest focus of universities seeking to improve professional development at their institutions." This actually makes sense; if you can't get professional development right, you probably shouldn't be advising people on how to do professional development. Additionally, "the type of professional development and the content within it should rely heavily on the needs of the participants at a given institution." I say this about learning generally, but somehow get a lot of pushback.