(b) Explain in language that management can understand why they cannot be confident that sales rose by 7.9% and, in fact, may have dropped. Identify the best explanation. The marketing manager is correct. Sales are definitely up this year. The 95% confidence interval contains zero. In fact, because the confidence interval contains negative numbers, it is possible that the mean number of units sold last year was greater than the mean number of units sold this year. Since the 95% confidence interval contains negative numbers, the study is invalid and no real conclusion can be drawn. Because the upper bound of the 95% confidence interval is further away zero than the lower bound of the 95% confidence interval, the sales rose by more than 7.9%.