Strengthening Argument on Tutoring Impact

A recent study found that students who attended after-school tutoring sessions scored an average of 15 points higher on the math section of an exam compared to those who did not attend any sessions. The study concluded that attending after-school tutoring is beneficial for improving math scores.

However, one of the researchers pointed out that the study did not account for whether students who attended tutoring were already performing better in math before attending the sessions or if they had other advantages, such as previous tutoring experiences or extra resources available at home.

To strengthen the argument that after-school tutoring improves math scores, which of the following pieces of information would be most helpful?