Toomey E, et al. – This trial entailed the establishment of the fidelity of delivery, of a complex self–management intervention. Furthermore, it investigated the reasons for these findings using a convergent/triangulation design. The data revealed that the intervention was delivered with high fidelity. Evidence was yielded with regard to the fidelity assessment methods within complex behaviour change interventions. A combination of quantitative methods was found to be suitable for the analysis of fidelity of delivery. A mixed methods approach presented a more insightful comprehension of fidelity and its influencing factors.