F IL E A sports enthusiast wants to examine the factors that influence a quarterback's salary (Salary). In particular, he wants to assess the influence o f the pass com pletion rate (PC), the total touchdow ns scored (TD), and a quarterback's age (Age) on Salary. He uses 2009 data, a portion o f which is shown in the accompanying table; the com plete data set can be found on the text website, labeled Quarterback Salaries.
Name Salary (in $ millions) PC TD Age Philip Rivers 25.5566 65.2 28 27 Jay Cutler 22.0441 60.5 27 26 Tony Romo 0.6260 63.1 26 29 SOURCE: USA T o d a y d a ta b a s e fo r s a la rie s ; N F L .c o m fo r o th e r d a ta .

a. Estimate and interpret the model: Salary = β0 + β1PC +
β 2TD + β3Age +

e. Show th a t this m odel is preferable to a m odel that uses log o f salary as the response variable.

b. Consider the quadratic effect o f Age by adding Age2 in the regression. Use a partial F te s t to determ ine the jo in t statistical significance o f Age and Age2.