Choice of study design and method, as in all research fields, is dependent on the research/evaluation question and what is appropriate to the evaluation context and population. As a result, there are many different designs and methods which can be used in process evaluation. 

Both quantitative and qualitative methods have an important place in process evaluation. Implementation data on fidelity, dose and reach are often collected quantitatively through structured observations of intervention activities, routine monitoring data or via self-report data (e.g. implementer checklists and beneficiaries questionnaires). These data might also be corroborated and adaptation of interventions explored via qualitative interviews with implementers.  

Mechanisms of impact can be explored via qualitative data collection, such as interviews or focus groups with implementers and beneficiaries of interventions. These might also be explored quantitatively via mediator analysis. Contextual factors impacting on implementation can be explored via qualitative data collection, for example interviews with national and local level implementers. Acceptability might be explored quantitatively via self-report surveys or qualitatively via interviews, focus groups or observations with implementers and beneficiaries. Qualitative interviews with beneficiaries could also be useful for exploring contextual outcomes.

There are strengths and weaknesses to all designs and methods, and in designing your process evaluation it is important to consider which methods will best address your question and be most appropriate to your context and population group.