Qualitative and quantitative independent researches on the combined or single use of psychotherapy and pharmacotherapy in treatment of major depression
Creswell and Clark (2007) discussed when researchers decide the approach for mixing the two approaches within their mixed methods designs. I suggest you should read the following book because it may help you.
Creswell, J. W., & Clark, V. L. P. (2007). Designing and conducting mixed methods research. Thousand Oaks, CA: Sage.
No doubt, Creswell, J. W., & Clark, V. L. P. (2007). Designing and conducting mixed methods research. Thousand Oaks, CA: Sage., is awesome book in this regard.