A group of researchers is measuring foot traffic in a museum. Recently one half of the museum was redone in order to encourage more foot traffic. Researchers are counting the number of attendees in this new zone and comparing it to other zones that have not been redone. On the first day that researchers are recording foot traffic, several buses of schoolchildren arrive at the museum unexpectedly for a field trip. After a day of testing, the new zone appears to be getting just as much foot traffic as the other zones. Which problem is most likely affecting this quasi-experiment