A counseling psychologist developing a technique to reduce procrastination has students time their procrastination for a week and uses this as a pretest measure of procrastination. Students then attend a workshop in which they are instructed to do a specific warm-up exercise for studying by focusing on a pleasant activity. Students again time their procrastination for a week, and the time from this second week is the posttest measure.
-If the psychologist finds that the sum of squared deviations from the mean of the difference scores of the sample is 135,the estimated population variance would be
A)135 / 10 = 13.5
B)135 / 9 = 15.0
C)10 / 135 = .074
D)9 / 135 = .067

Question

Quizplus · Accepted Answer

The formula to estimate population variance from sum of squared deviations is:
Estimated population variance = sum of squared deviations / degrees of freedom
Here, the sample has 10 students (pretest and posttest) and we are finding the variance of difference scores between pretest and posttest, so the degrees of freedom is 9 (10-1). 
Therefore, estimated population variance = 135 / 9 = 15.0. 
Hence, the best choice is B.

A Counseling Psychologist Developing a Technique to Reduce Procrastination Has