The McArthur Co. produces special vacuum cleaners for conveniently cleaning the inside of cars. About a thousand of these are produced every month with stamped serial numbers and stored serially in a stock room. Once a month, an inspector does a quality control check on fifty of these. When he certifies them as to quality, the units are released from the stock room for sale. The production and sales managers, however, are not satisfied with the quality control check, since quite often, many of the units sold are returned by customers because of various types of defects, What would be the most useful sampling plan to test the fifty units?