A nurse is interested in whether there is a relationship between daily levels of stress and likelihood of a heart attack. The stress scores (predictor variable) and risk of heart attack values (criterion variable) from a small sample of participants are depicted below (Values below are valid for this question only!)

Stress Score Risk of Heart Attack

(Higher values = more stress) (Higher values = greater risk)

4 7

5 9

9 4

0 2

What is the standard deviation of the predictor variable?