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) 6 8 7 6 4 4 1 2 What is the standard deviation of the predictor variable?