An industrial engineer conjectures that a major difference between successful and unsuccessful companies is the percentage of their manufactured products returned because of defectives. In a study to evaluate this conjecture, the engineer surveyed the quality control departments of 35 successful companies (identified by the annual profit statement) and 35 unsuccessful companies. The companies in the study all produced products of a similar nature and cost. The successful companies selected had a mean percentage of total output returned by customers in the previous year of 5.51 with a standard deviation of 2.74, and the unsuccessful companies had a mean of 7.13 with standard deviation of 2.32. Is there sufficient evidence that successful co