As you know, we have been doing some work for Ms. Mini Driver, the Director of Operations at MetroArea Trucking, on how location affects the maintenance expenses for the trucks in the fleet. We have received an additional contract to further analyze the fleet’s maintenance expenses. Ms. Mini Driver would like us to analyze the entire truck data set (see attachment), which includes last year’s maintenance expense, the mileage, age, and type of truck, as well as the location (based either in city or out of city) of where the truck is based. Ms. Mini Driver wants us to provide her with an analysis of what factors affect maintenance expenses and how much each affects the expenses.

I’d like you to develop your own optimal regression model by choosing your own variables and going through your own model-refining process before seeing what a stepwise regression routine produces for an optimal model. This process should give you a better feel for how the variables contribute to the maintenance expense, which should be helpful when you interpret your models.

1. Start with a full model without any interaction terms and record your findings in the chart below. I would like you to begin this way because there are situations when interaction terms aren’t really worth their trouble, whereas in others they are.
2. Run the reduced model with the significant variables that you get from the full model, again without any interaction terms. Record your findings in the chart.
3. Start over with a full model with all interaction terms. Record your findings.
4. Run a reduced model with the significant variables only. Record your findings.
5. Now run a full model with all interaction terms using a stepwise regression routine. Record your findings.
6. Write a memo to me stating what you think the model should be and why, including a description of how you went about finding your model. Be sure to include your supporting evidence (you will find the chart helpful here). Comment on the quality of your model and then interpret your model, explaining which variables significantly affect maintenance expenses and how much each affects the expenses.

Attachment: Data file C10 Truck.xls [.rda]