GMAT Critical Reasoning (CR) | Re: QOTD #18 The Georgetown Public School recommends that all its students

The Georgetown Public School recommends that all its students take an active interest in playing chess. This is because, according to a recent medical study, those students who played chess on an average performed better in tests of general intelligence than those who did not play chess. Thus the school contends that playing chess will boost up the intelligence of its students.

Which of the following raises the most serious doubt about the