Descriptions of four studies are given. In each of the studies, the two populations of interest are female students at a particular university and male students at the university. Which of these studies have samples that are independently selected?
Study 1: To determine if there is evidence that the mean amount of time spent playing online video games differs for the two populations, a random sample of 20 female students and a random sample of 50 male students are selected.
Study 2: To determine if the mean number of hours worked per week differs for the two populations, a random sample of female students who have a brother who also attends the university is selected. The female student is included in the sample of females, and her brother is included in the sample of males.
Study 3: To determine if the mean amount of time spent using campus recreational facilities differs for the two populations; a random sample of female students is selected. A separate random sample of the same size is selected from the population of male students.
Study 4: To determine if the mean amount of money spent on housing differs for the two populations, a random sample of female students is selected. Each student in this sample is asked how far away from campus she lives. For each of these female students, a male student who lives about the same distance from campus is identified and included in the sample of male students.