Does group size matter? An evaluation of the impact of group size on DBT skills training outcomes

Abstract

NHS mental health services continue to experience high demand and limited capacity, contributing to long waiting times and poorer outcomes for service users. Dialectical behaviour therapy (DBT) skills groups are an established intervention for emotion dysregulation, yet little is known about whether group size influences treatment outcomes. This comparative service evaluation examined whether larger DBT skills training groups (20 participants), delivered as a standalone intervention, produced outcomes and retention comparable to standard–sized groups (12 participants), to provide preliminary evidence regarding the feasibility of increasing service capacity. Routinely collected data were analysed for 206 adults who attended standalone DBT skills training groups, delivered in cohorts of either 12 participants ( n = 103) or 20 participants ( n = 103). Emotion regulation difficulties were assessed pre– and post–intervention using the Difficulties in Emotion Regulation Scale (DERS–18). Drop-out rates were used as an indicator of retention. Independent samples t -tests indicated no significant differences in treatment outcomes between smaller and larger groups ( p =.300, d = 0.15). Drop-out rates were comparable across formats (36% vs 43%). Larger DBT skills–training groups produced comparable outcomes and retention to the service’s standard–sized groups. These findings offer preliminary feasibility signals that increasing group size may help expand access, although controlled research is required before wider implementation.

Key learning aims