For a sample of 100 recent credit-card customers of a company's on-line store, the following data are available: the dollar amount of merchandise ordered during the visit, how much time (minutes) the customer spent at the site during the purchase visit, and whether the customer used a Visa credit card.
Describe the format of a linear regression equation for estimating the dollar amount of the order on the basis of the other variables described above. If any qualitative variables are used, indicate the strategy for coding them.