Dr. Chen is studying why students perfоrm pооrly on mаth exаms in her district. She notices thаt students who eat breakfast before school score an average of 15 points higher on math tests than students who skip breakfast. The correlation is strong and consistent across multiple schools (r = 0.78, p < 0.001). Dr. Chen concludes: "Skipping breakfast causes poor math performance. We should mandate that all students eat breakfast before taking math exams." What is the BEST criticism of Dr. Chen's conclusion?