Mixed methods in intervention research is an increasingly important methodological approach that combines quantitative and qualitative methods to better understand intervention effects, implementation processes, and contextual influences. Although its use has expanded considerably in recent years, the methodological quality of mixed methods integration in intervention research remains insufficiently examined. The aim of this doctoral dissertation is to develop a conceptual and theoretical model for assessing the quality of mixed methods integration in intervention research, examine its analytical applicability, and apply it to a sample of empirical studies. The study draws on methodological literature in mixed methods research, intervention research, and mixed methods intervention studies, and combines model development, preliminary expert consultation, and a systematic methodological review of empirical intervention studies. The proposed model includes dimensions, components, criteria, and indicators that make it possible to assess the quality of mixed methods integration across the research process. The literature analysis shows that methodological sources emphasize quality criteria unevenly. Applying the model to empirical intervention studies indicates that the quality of mixed methods integration is generally modest, although gradual improvement can be observed over time. The analysis also shows a predominance of quantitative approaches and uneven integration across different phases of the research process. The dissertation contributes to methodological discussions on quality in mixed methods intervention research by offering an analytical framework and practical guidance for improving the design, implementation, and evaluation of mixed methods studies. Particular attention is given to methodological transparency in reporting integration procedures, as many empirical studies do not explicitly explain the logic of linking data sources. The proposed model distinguishes more precisely between the formal presence of mixed methods and the methodological quality of integration in empirical research practice. It also emphasizes the need for clearer methodological guidelines to support more consistent application across intervention research.
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