In a multi-study naturalistic quasi-experiment involving 269 students in a semester-long introductory philosophy course, we investigated the effect of teaching argument diagramming (AD) on students' scores on argument analysis tasks. An argument diagram is a visual representation of the content and structure of an argument. In each study, all of the students completed pre- and post-tests containing argument analysis tasks. During the semester, the treatment group was taught AD, while the control group was not. Methodological problems with the first study were addressed in the second. The results were that among the different pre-test achievement levels, the scores of low-achieving students who were taught AD increased significantly more than the scores of low-achieving students who were not taught AD, while the scores of the high-achieving students did not differ significantly between the treatment and control groups. The results for intermediate-achieving students were mixed. The implication of these studies is that learning AD significantly improves low- and intermediate-achieving and students' ability to analyze arguments. (Contains 1 note, 6 tables, and 1 figure.)