Golf handicaps are used to allow players of differing abilities to play against one another in a fair match. Recently a sample of golfers was selected in an effort to develop a model for explaining the difference in handicaps. One independent variable of interest is the number of rounds played per year. Another is whether or not the player is using an “original” name brand club or a copy. In recent years, a number of smaller golf club manufacturers have attempted to copy major golf club designs and sell “copies” of original clubs such as the Big Bertha by Calloway. To incorporate the type of club used, which of the following methods could be used?