Apparently forgetting that correlation is not causation, Associated Press promotes a study that says more sleep leads to better performance in schools compared to all-night cram sessions the night before an exam.

It reaches this conclusion based on the fact that people who stay up all night have statistically better grades.

This is an uncontrolled study. Rather than take two randomly-selected students and have one stay up and the other go to bed, it asks people after the fact about their habits. While it shows a link between sleep and grades, it does not show that the lack of sleep while cramming causes a decrease in grades.

The study could be simply explained away by the fact that students who do poorly tend to procrastinate to the last minute and do all-night cramming. There’s no evidence that getting them to bed earlier would improve their grades, because nobody has actually tested for that.

AP (and the Globe) should know better than this. Comments attached to the Globe story pounced on it immediately. Why didn’t a journalist?