a crossover trial is planned to evaluate the impact of an educational intervention program to reduce alcohol consumption in patients determined to be risk for alcohol problems. The plan is to measure alcohol consumption ( the number of drinks on a typical drinking day) before the intervention and then again after participants complete the educational intervention program. How many participants would be required to ensure that a 95% confidence interval for the mean difference in the number of drinks is within two drinks of the true mean? Assumed that the standard deviation of the difference in the mean number of drinks is 6.7 drinks.