A tutoring company has offices where students physically attend a test prep course. The company recently created an online test prep course, and they wonder if it produces similar or different outcomes than their in-person course. They recruit 50 students who are willing to try the online course. On average, those 50 students produced significantly better outcomes than students historically do in the in-person course. The company concludes that the online course produced better outcomes than the in-person course for these students. Which of these concerns makes their conclusion inappropriate?