Table 4 Trial conduct solutions to treatment contamination

Clinicians deliver both active and control treatments

Recruiting groups of clinicians, each one of which is responsible for a single treatment

Monitoring contamination using supervision/therapy session recordings

Formalising differences between interventions, e.g. using structured manual during therapist training

Asking clinicians not to use intervention content when treating those in control arm

Providing active intervention within the research project rather than health service

Using a script for contact with control participants during treatment

Clinicians not involved in active intervention treating participants in both trial arms

Blinding usual care clinicians

Confining intervention to provision by specialist clinicians

Communication between clinicians in different trial arms

Asking clinicians not to share details of the intervention with each other

Communication between participants in different trial arms

Holding treatment sessions at different times / in different locations

Staggering the scheduling of data collection appointments / reducing waiting time so that participants do not meet in waiting room

Allocating separate therapists / modes of delivery for individual and group therapies when usual group therapy was shared by participants in both arms

Asking participants not to share contents of intervention with others

Excluding potential participants who know someone else attending screening

Holding separate sessions of existing group treatments for participants in separate trial arms in order to prevent contact

Restricting the release of intervention materials in order to reduce the chance of their being shared with control participants

Recruiting participants in blocks and providing one treatment at a time, with no new participants recruited during the final week of each period in order to maintain separation between trial arms

Participants switching clinicians and therefore trial arms

Preventing referrals for add-on care by clinicians who are members of study team

Avoiding transfer of participants between clinicians

Participants seeking treatment outside the trial

Informing participants only about the treatment they were allocated to receive (Zelen’s design)

Promising the intervention to control participants at the end of follow-up

Active treatment is available to some extent within the healthcare system

Making intervention distinct from usual care by adapting one or other

Establishing common treatment for all participants

Excluding institutions that already offer some aspect of the intervention