Use the data file Combaro.xls to complete the following exercises. Determine whether the relationship between productivity (Prdtvty) and days absent (DysAbsnt) is linear. Develop a regression model for productivity using days absent, job satisfaction (JobSat) and unpaid overtime (UOv- Time). Is this a valid model? If not, why? Now include a new variable in the dataset. Head this variable ‘Days absent squared’. Obtain the data for this column by squaring the corresponding entry for ‘Days absent’. (Do this for the first entry and simply copy down.) Rerun the regression with the previous three independent variables plus the new variable ‘Days absent squared’. Check the t statistics and the p-values for all four independent variables. Based on the p-values, comment on the validity of the model.