A telemarketing firm has recorded the households that have purchased one or more of the company's products. These number in the millions. The firm would like to conduct a survey of purchasers to acquire information about their attitude concerning the timing of the telephone calls. The president of the company would like to know the views of all pur- chasers but would also like to compare the attitudes of people in the West, South, North, and East. Describe a suitable sampling plan.