1. An evaluation of workshop training in motivational interviewing for addiction and mental health clinicians.
Baer J.S., Rosengren D.B., Dunn C.W. et al. Drug and Alcohol Dependence: 2004, 73(1), p. 99–106.
US study suggests that when it comes to choosing therapists, choosing the 'right' people who have not been trained in motivational interviewing would be better than choosing the 'wrong' people who have been trained; the former not only start at a higher level, but are more able to benefit from and retain training.

2. How does motivational interviewing work? Therapist skill predicts client involvement within motivational interviewing sessions.
Moyers T.B., Miller W.R., Hendrickson S.M.L. Journal of Consulting and Clinical Psychology: 2005, 73(4), p. 590–598.
Analysis of counselling session recordings from therapists trained in motivational interviewing suggests that the important quality of seeming 'genuine' can suffer if training mandates unnaturally withholding normal responses; however, departing from these tenets is risky unless done by a socially skilled therapist.