A researcher is interested in the mean mileage for all school buses in a school district. Forty buses are randomly selected from a computer printout that contains every bus in the district. For each bus they record the name of the school it is used for, mileage, and age. It is calculated that 32.5% of the 40 buses are over 15 years old. With 99% confidence they find that the mean mileage for all school buses in the school district is between 85,261 and 134,890 miles. What is the qualitative variable in this study