We conducted a study to demonstrate that formal training in debugging helps students develop skills in diagnosing and removing defects from computer programs. To accomplish this goal in an assembly language course, we designed multiple activities to enhance students' debugging skills. These activities included debugging exercises, debugging logs, development logs and reflective memos, and collaborative assignments. In a previous paper, we reported positive qualitative results. Students agreed that formal debugging training enhanced their debugging skills. In this paper, we present positive quantitative results that support our previous qualitative results. Students who completed the optional debugging exercises spent significantly less time on debugging their programs than those who did not. Furthermore, we develop a model of debugging abilities and habits based on students' comments in their debugging logs, development logs, reflective memos, and evaluation surveys. Students and educators could use the model to diagnose students' current debugging skills and take actions to enhance their skills.