From the article: > Of the 3,800 students in our classes (from 2000 to 2006), this study included only the 494 who used the C programming language and who also

From the article: > Of the 3,800 students in our classes (from 2000 to 2006), this study included only the 494 who used the C programming language and who also completed all 10 programming exercises. That should answer your first question. As for the defects: I’m not sure (but perhaps defect-free = passing all tests?).