It's omitting a lot of details of the methodology.
Is this only including students who did all the problems, ran the test cases, and submitted all the statistics they asked for? That would tend to exclude all the low performers.
And what happened with the defects? Did they just go unfixed, and data for broken programs is included, or did they have to rework them until all tests pass?
> 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?).
Is this only including students who did all the problems, ran the test cases, and submitted all the statistics they asked for? That would tend to exclude all the low performers.
And what happened with the defects? Did they just go unfixed, and data for broken programs is included, or did they have to rework them until all tests pass?