Question: A school counselor tests the level of depression in fourth graders in a particular class of 20 students. The counselor wants to know whether the kind if students in this class differs from that of fourth graders in general at her school. On a test, a score of 10 indicates severe depression, while a score of 0 indicates no depression. From reports, she is able to find out about past testing. Fourth graders at her school usually score 5 on the scale, but the variation is not known. Her sample of 20 fourth graders has a mean depression score of 4.4.

The counselor calculates the unbiased estimate of the population’s variance to be 15. What is the variance of distribution of means?

a. 15/20 = 0.75

b. 15/19 = 0.79

c. 15²/20 = 11.25

d. 15²/19 = 11.84