Return to Question 4 where you were told that the average distance all students live from Murphy Elementary School is 6.4 miles. Suppose you learn that Jason, Sam, Clint, and Cori all travel to school each day on a different school bus. The students they included in their samples were students they talked to on their respective school buses. The fact that there is a systematic tendency for the sample averages they obtained (i.e., 9.7 miles, 12.2 miles, 10.4 miles, and 7.8 miles) to overestimate the true population average of 6.4 miles is an illustration of the concept of ___________.

A. categorization B. nonresponse C. stratification D. bias E. variability