Dr. vandepolder is conducting a study on mid-west american college-students' radio- listening habits. he doesn't have the resources to survey all college students in the mid-west, so he uses census data to randomly select a group of several thousand students from michigan, wisconsin, illinois and minnesota to participate in the study. in this study, the group of all mid-west american college students is called the while the group randomly selected from michigan, wisconsin, illinois and minnesota is called the