index among the students. What is the numerical value of the correlation between percent of classes attended and grade index? (Use 2 decimal places.)=

A study of class attendance and grades among first-year students at a state university showed that in general students who attended a higher percent of their classes earned higher grades. Class attendance explained 81% of the variation in grade index among the students. What is the numerical value of the correlation between percent of classes attended and grade index? (Use 2 decimal places.)=