A researcher is studying the effects of a diet low in artificial flavoring on the hyperactivity of school-age children. A group of children identified as having ADHD are all placed on a diet low in artificial flavoring and accessed regularly. At the end of the program she evaluates the change in the children's level of hyperactivity. She finds that the average hyperactivity score had decreased significantly from baseline over the year's time. Which threat to internal validity should she be most concerned about