A "frequent buyer" program is designed to reward customers who regularly purchase a company's products or services. The main factor for segmentation in such a program would be the volume of business that the customers generate. This is because the purpose of these programs is to encourage repeat business and increase customer loyalty by offering rewards or benefits to those who make purchases more frequently or in larger amounts.
References: While I cannot directly verify the answer from the ASQ documentation, the concept of customer segmentation based on the volume of business is a common practice in developing loyalty or frequent buyer programs. This approach is supported by various marketing and quality management resources that discuss customer relationship strategies123.