One of the reasons the article adds to our knowledge of the relationship between participation in sport and the academic performance of student athletes is that the author, Katie Schultz, has data on individual athletes. This allows her to employ a basic but powerful test of the impact of sporting participation. There are 5,580 athletes in her data set. However, her attention is primarily on about 50% of the full sample. She examines those athletes who play one sport in one season. (Her statistical tests reveal that there is no sample selection issue in this approach.) Her data means she can look at an individual athlete and compare their performance when they were involved in sport with their academic performance during their off-season. And, she can do this for over 2,000 athletes.
Schultz also conducts her analysis for various decompositions of the GPA. For example, she examines the relationship between in-season and off-season for the core GPA (Maths, Science, English and History). She also examines the relationship using the maths/science part of the core and the English/history part of the core separately. It is in this latter decomposition that she finds that it is the English/history GPA is the primary channel for single-sport athletes performing below their best. Schultz argues that this might be the result of English/history requiring greater time (due to the way they are delivered and assessed). The maths/science GPA core improves for single-sport athletes during the in-season.
Those who take the time to read the Schultz paper will find that it also explores the relationship between sporting participation and the choice of non-core subjects. They might also enjoy the way she establishes the softer/easier subjects.
Finally, it is worth noting that Schultz finds that "being an athlete corresponds to obtaining a 0.49 point higher GPA compared to a nonathlete". It is precisely because of findings like this that it is important to focus on the change in the academic performance of individual athletes where they are engaged in sport. It is a paper well worth reading.
[I have previously posted on some aspects of the relationship between sport and academic performance in an Irish context here and here.]