But there is another hypothesis that explains the data equally well: students who register
themselves in a course in "Techniques of Problem Solving" and are then selected for a
study about problem solving, will do better than students who register themselves in a
course in "structured programming" and are then selected for a study in problem solving. (!)
Surely this is the most elementary type of error that one could make, loading the dice by not
matching the test and control groups. Not only not matching them, but actually choosing for
the test group students who have self-selected for their interest in the very subject matter
that is allegedly being tested by the study.