These data describe sales over time at a franchise outlet of a major US oil company. (The data fle has values for two stations. For this exercise, use only the 283 cases for site 1.) Each row summarizes sales for one day. This particular station sells gas, and it also has a convenience store and a car wash. The response Sales gives the dollar sales of the convenience store. The explanatory variable Volume gives the number of gallons of gas sold, and Washes gives the number of car washes sold at the station.
(a) Fit the multiple regression of Sales on Volume and Washes. Do both explanatory variables improve the ft of the model?
(b) Which explanatory variable is more important to the success of sales at the convenience store: gas sales or car washes? Do the slopes of these variables in the multiple regression provide the full answer?
(c) Find the variance inflation factor and interpret the value that you obtain.
(d) One of the explanatory variables is just barely statistically significant. Assuming the same estimated value, would a complete lack of collinearity have made this explanatory variable noticeably more statistically significant?