It has become common to find a convenience store that also sells gas. These data describe the sales over time at a franchise outlet of a major U.S. oil company. Each row summarizes sales for one day. This particular station sells gas, and it also has a convenience store and a car wash. The column labeled Sales gives the dollar sales of the convenience store, and the column Volume gives the number of gallons of gas sold. Formulate the regression model with dollar sales as the response and number of gallons sold as the predictor. When checking the conditions for using the SRM, remember that these data are sequential.
(a) Give a confidence interval for the difference in average sales in the convenience store between days on which the station sells 2,000 gallons of gas and those on which the station sells 3,000 gallons.
(b) Is $1,800 an unusually low level of sales in the convenience store on a day that the pumps record having sold 3,000 gallons?