INTRODUCTION: There is limited research on applying theory to retention in complex intervention trials. To address this gap, this study aimed to qualitatively examine retention in the METHODS: Nine focus group discussions were used to generate data from RESULTS: Our findings highlighted the contextual issues influencing the trial, including participants' socioeconomic circumstances, and the presence or absence of social support, the trial complexity and participant burden. Issues related to controlled motivation comprised challenges of staying in contact, financial incentives and food, health services provided and other incentives. We also identified aspects of the trial supporting participants' psychological needs of autonomy, competence and relatedness, which in turn contributed to their intrinsic autonomous motivation. These included participants' interest in the trial and its relevance to them
participants' sense of agency, meaning and purpose through their involvement
the building of their knowledge and awareness about their health
relating to other participants and the relationships built with staff and being treated well. CONCLUSION: SDT provides a helpful frame for a contextualised understanding of the complexity of retention of