In an experiment studying the effects of exercise on mood, participants exercised for various durations over ten days. The raw scores for the hours of exercise were 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, and the PHQ-9 scores were 14, 12, 10, 8, 7, 5, 4, 3, 2, 1 respectively. What is the standard deviation of the PHQ-9 scores?
1) 3.22
2) 4.15
3) 5.44
4) 7.33