A study found that students who applied for and used vouchers to attend private schools in Milwaukee graduate at a higher rate than those students enrolled in Milwaukee public schools. The study found that those who used vouchers when they entered the ninth grade had a graduation rate of 64 percent compared to 36 percent of students in public schools. The authors do note that they could not locate a significant number of students from both groups who moved out of the area between ninth-grade and graduation. Critique the external validity of this study. Describe external validity as part of your answer.