Amanda is conducting an observational study regarding the correlation between high school start time and the average first period year-end grade for all students at the school. She decides to observe two high schools in North Carolina, Williams High and Valley High, for one school year. During the school year in which Amanda is observing, Williams High School begins first period at 7:35 am, while Valley High School begins first period at 9:00 am. At the end of the school year, Amanda finds that the average first period year-end grade for all students at Williams High School is ten points lower than the average first period year-end grade for all students at Valley High School. Amanda then repeats the observational study eight times with other high schools in the state of North Carolina. All studies yield the same results. What generalization can be made from the results of the observational study?