You are seeing a 12-year-old boy who has attention-deficit/hyperactivity disorder (ADHD). Like so many of his peers, he loves video games and spends many hours a day playing. You know that gamers spend an average of 8 hours each week playing. Besides the lost opportunity to exercise, you begin to wonder about the effect of video games on behavior. Are children who play video games more likely to have ADHD? Embedded in your question are two possible lines of inquiry: 1) Does video game playing cause ADHD? and 2) Are children who have ADHD more likely to play video games? Each question suggests a slightly different study design (Table).

In cross-sectional study designs, data collection occurs at one point in time. In longitudinal study designs, subjects are followed over time, with data collection occurring at predetermined intervals or at set events (eg, graduation from high school). The most...

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