To better understand the learning and assessment of introductory computer science, we performed five complementary analyses on over 28 thousand student responses from the 2004 and 2009 Advanced Placement Computer Science (AP CS) A exams. Analyzing the questions from the 1988 AP CS exam, Reges hypothesized that a small set of questions, which were highly correlated with success on the rest of the exam, might be used to assess students' CS ability before taking a CS class. The pattern Reges observed was not replicated on the 2004 and 2009 AP CS A exams. Our hypotheses for why the pattern was not replicated advances Reges' alternative hypothesis regarding the importance of students' mental model of program execution.