This study sought to create a dispositional attribution model to describe differences in the development of statistical proficiency. To what extent can differences in psychological dispositions explain differences in the development of statistical proficiency and, in particular, students' understanding of hypothesis testing? A framework to describe statistical proficiency was created. Study subjects were undergraduates who have taken an algebra-based statistics course. The three emergent themes found in student discussions of hypothesis testing were, how students consider the experimental design factors of a hypothesis test situation, what types of evidence students find convincing, and what students understand about p-values.