Small differences in course sequencing may have broad effects on undergraduate science learning. In the current research, we developed an analytical approach for assessing questions about course sequencing using educational data sets, and we applied it to questions about the Psychology major. This study examined the relationships between student achievement (grades) in psychology courses taken before and after methodological courses. We used a longitudinal institutional dataset involving thousands of students across seven cohorts, and control for demographics, SAT achievement, and prior psychology GPA. We found that two courses were especially important: Achievement in statistics and research methods courses related to grades in subsequent advanced seminars, lab courses, and overall psychology GPA. Additionally, relations between research methods achievement and topical course grades were stronger when those courses were taken after versus before research methods, further reducing the likelihood of hidden third variable explanations. The same was not true for most other introductory courses, although it was found for biopsychology, which may be because biopsychology (which also includes neuroscience) is relevant across many areas of psychology, similar to research methods. These correlational findings suggest that requiring students to take research methods and biopsychology early on in the major, and ensuring success in these courses, may enhance subsequent learning. More broadly, this research provides a template for data-based approaches to course sequencing questions within any undergraduate major.