Problem: Two doctoral students from the University of Sepulveda wanted to see if an intervention to improve social behavior was effective with students with ADHD. They conducted the intervention with three students with ADHD that were selected from the gifted and talented children program. The intervention was highly effective with these three students, and therefore the doctoral students concluded that the social behavior intervention was effective with children with ADHD. There is a problem in the study due to external validity. This problem was?