Question: A major company would like to assess the impact of using a professional trainer to conduct a confidence-building workshop with their salespeople. A sample of 18 workers is obtained. Half (n = 9) attend the workshop and the other half (n = 9) serves as a control group. Two weeks later, each of the participants is given a questionnaire measuring their level of self-confidence. The data are as follows:

Controls Workshop

M = 17.5 M = 22.7

SS = 290 SS = 358

Make an interval estimate of the mean difference so that you are 80% confident that the population mean difference is in your interval.