The manager of a supermarket would like to evaluate the effectiveness of a large freezer unit currently used to store excess supplies of various frozen food items in the supermarket's inventory. Specifically, the manager wants to determine whether the current freezer is maintaining the valuable store inventory at a roughly constant temperature of 7 degrees. To make this evaluation, he asks his assistant to take 6 temperature readings within the freezer at various points in the day for a total of 30 days. These measurements are given in file P20_13.xlsx.

a. Generate and interpret X and R charts for the given data. Do these data indicate the presence of one or more problems with the operation of this freezer? Explain.

b. How might you explain the trend in the R chart for subsamples 21 through 30?

c. What advice would you give to the supermarket manager regarding this freezer?