MCAT Question: A study examined the relationship between smartphone use and mental…

A study examined the relationship between smartphone use and mental health in adolescents. Researchers recruited 500 teenagers from a single high school and tracked their smartphone usage time and self-reported anxiety and depression symptoms over a period of three months. They found a significant positive correlation between higher smartphone use and increased anxiety and depression scores. Based on these findings, the researchers concluded that excessive smartphone use causes anxiety and depression in adolescents. Which of the following factors MOST LIKELY limits the generalizability of the study's conclusions to the broader adolescent population?