An education researcher wonders if young students better learn arithmetic facts using physical flashcards or a new online game. The researcher recruits 505050 students for a study. The study allows parents to choose which learning tool they'd like their child to use: flashcards or the online game. Each student takes a pre-test and a post-test, and at the end of the study, students who used flashcards show significantly better outcomes than students who used the online game.
The researcher concludes that flashcards were more effective than the online game for this group of students.
What is a confounding variable that makes their conclusion inappropriate?