To evaluate the quality of care, particularly in the case of new forms of healthcare interventions, the healthcare services to be provided are defined and documented in advance. The presented explanatory sequential mixed methods design combines quantitative and qualitative data collection and the analysis enables a deeper understanding of a new healthcare intervention. Using the example of the POET-Pain project, which investigates the effect of a perioperative transitional pain service (TPS), the methodological application of the explanatory sequential mixed methods design is demonstrated in order to present the structural and process evaluation of the new healthcare intervention (in this case TPS) and to understand its influence on the quality of care. The mixed methods design presented enables the results of the quantitative phase to be interpreted and expanded in depth using qualitative data, which leads to a comprehensive understanding of the subject matter (second pillar of health services research).