Eighteen studies (number of supervisees ranged from one to 274) were included in the review. Eight studies were quantitative. Three studies were qualitative. Seven studies used mixed methods. Quality scores ranged from 2 (poor) to 5 (very good), with most studies rated as average and only two studies rated as "very good".
Two qualitative studies of average quality reported that self-awareness was enhanced by supervision.
Four average quality studies and one very good quality study suggested that both individual and group supervision was associated with a range of improved counselling skills.
One poor quality study, three average quality studies and one very good quality study reported positive effects in counsellors' self-efficacy. One average quality study also assessed whether self-efficacy increased as working alliances became stronger, but found no evidence to support this.
One average quality and one poor quality study suggested that the timing and amount of supervision affected the process and outcome of supervision.
Two average quality studies reported that awareness, development and changes in theoretical orientation reflected the supervisees' ongoing development.
One poor quality study found support to be beneficial. Two average quality studies and one poor quality study found mixed effects for support on client outcomes.