Assessing student knowledge based on their writing using traditional qualitative methods is time-consuming. To improve speed and consistency of text analysis, we present our mixed methods development of a machine learning predictive model to analyze student writing. Our approach involves two stages: first an exploratory sequential design, and second an iterative complex design. We first trained our predictive model using qualitative coding of categories (ideas) in student writing. We next revised our model based on feedback from instructor-users. The model itself highlighted categories in need of revision. The contribution to mixed methods research lies in our innovative use of the machine learning tool as a rapid, consistent additional coder, and a resource that can predict codes for new student writing.