A supermarket chain traditionally advertises by mailing circulars to 
some of the households in their area and by having the circulars 
inserted in the local paper.  The file smsales.sas7bdat gives the 
amount of money spent mailing circulars (MAIL) and having them 
included in the local paper (NEWS), along sales (SALES) for the week 
covered by the circulars.  All amounts are in thousands of dollars. 

    What is the expected change in sales for each additional $1,000 
spent on mailing circulars? 
    
    What is the expected change in sales for each additional $1,000 
spent on placing circulars in newspapers? 
    
    Research questions:  
    
    Is the money on mailings well spent?

    Is the money on newspapers well spent?
    
    Does the effect of spending additional money on mailings change 
according to how much is being spent on newspapers or is it the same 
regardless of what is spent on newspapers?  Does the effect of 
spending additional money on newspapers change according to how much 
is being spent on mailings or is it the same regardless of what is 
spent on mailings?  

    How should the company spend its advertising budget?