The BBC reports on a study by two psychologists, purporting that staying hydrated can improve grades:
Students who bring water into the examination hall may improve their grades, a study of 447 people found.
Controlling for ability from previous coursework results, researchers found those with water scored an average of 5% higher than those without.
The study, from the universities of East London and Westminster, also noted that older students were more likely to bring in water to exam halls
I don’t believe an RCT is needed to answer every question out there, but it is a little silly in instances like this where a simple intervention could test the same hypothesis: just hand out water bottles to a random group of students before an exam, and see who performs better.
Surely, even controlling for ability (lagged dependent variable, anyone?) students who choose to bring water into exams might be different in some unobservable way. Of course, this doesn’t stop the researchers from making policy recommendations.