A manager wants to determine the effectiveness of a new training program designed to increase employee productivity (measured by the number of units produced during a one week period). Data is collected from two independent random samples of employees. One group went through the existing training program and the other group went through the new training program. Since the company is relatively small, the sample sizes for both groups were 20 each. The manager has productivity measures for each person in the two groups. The most appropriate test to determine whether or not the new training method increased productivity would be