The paper "Are Tenure Track Professors Better Teachers?" by David N. Figlio, Morton O. Schapiro and Kevin B. Soter (NBER, September 2013) concludes that part time adjunct staff make better university teachers than full time professors. This is a welcome finding for those of us who are adjunct lecturers and hardly surprising. However, I suspect it has more to do with adjuncts having more, and more up-to-date, training in teaching, than anything to do with full/part-time status. Adjuncts are likely to be newer to the institution, so more likely to have been through the teacher-training program which institutions run and also less able to avoid doing it (as Professors can). The paper does not appear to address the teaching qualifications of the staff in the research.
There may also be an element of priorities in the difference. When I suggested to one professor that improved teacher training for staff would improve student outcomes, they explained to me that staff were selected and promoted based on their research publication record: quality of teaching was not important. I was a little shocked by this, but if that is what the system is telling staff is important, they can't be blamed for acting accordingly. This may change as student feedback on individual courses and for institutions becomes more common.