Potential sources of bias
As with all psychological intervention research, interventions can only be single-blinded and therefore completely unbiased effects are not possible. As both interventions were delivered by one person, this minimised the issue of whether “therapy differences” are attributable to “therapist differences” (e.g., confidence, experience). However, this raises the question of whether facilitator beliefs about likely efficacy of each group played a role. Both groups were designed to maximise effectiveness, discussed with a clinical supervisor, and delivered in the knowledge that fidelity would be assessed—which was, as discussed, rated as very high. An important issue for subsequent trials would be maximising facilitator and participant “buy-in” to both approaches to not skew favour toward either approach. There is always the risk that participants who consent to take part in research are not fully representative of the wider population who may be referred to groups in clinical services. In addition, those who self-referred by responding to online invitations may differ in a number of ways from others living with ABI.