A compact disc (CD) manufacturer wanted to determine which of two different cover designs for a newly released CD will generate more sales. The manufacturer chose 70 stores to sell the CD. Thirty-five of these stores were randomly assigned to sell CDs with one of the cover designs and the other 35 were assigned to sell the CDs with the other cover design. The manufacturer recorded the number of CDs sold at each of the stores and found a significant difference between the mean number of CDs sold for the two cover designs. Which of the following gives the conclusion that should be made based on the results and provides the best explanation for the conclusion?

Since the sample size for each group was large enough, it is reasonable to conclude that cover designs was the main influence. There could be other factors, but since 35 stores were randomly chosen in each group, it is likely that the other factors were present in equal amounts in both groups. The only variable distinct in the groups was the presence of one of two cover designs.