Each week coaches in the National Football League (NFL) face a decision during the game. On fourth-down, should the team punt the ball or go for a first-down? To aid in the decision-making process, statisticians at California State University, Northridge, developed a regression model for predicting the number of points scored (y) by a team that has a first-down with a given number of yards (x) from the opposing goal line (Chance, Winter 2009). One of the models fit to data collected on five NFL teams from a recent season was the simple linear regression model, E(y) = β0 + β1x The regression yielded the following results:  = 4.42 - 048x, r2 = .18.
a. Give a practical interpretation of the coefficient of determination, r2.
b. Compute the value of the coefficient of correlation, r, from the value of r2. Is the value of r positive or negative? Why?