A researcher is interested in how school exercise programs can lead to better weight management in high school students. All the high school seniors are forced to run one mile a day before lunch. At the end of the semester, the researcher looks at weight management (gain/loss) and how fast each student ran his or her miles. He finds that kids who ran faster lost the most weight and assumes he should make students run faster next year. Unfortunately (for the researcher), most of the kids who ran fast were also in cross country. What type of problem does this present?