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) 7 3 0 3 9 3 0 9 What is the standard deviation of the criterion variable?