Cell Phones. The number of cell phone users has increased dramatically since 1987. According to the Semi-annual Wireless Survey, published by the Cellular Telecommunications & Internet Association, the mean local monthly bill for cell phone users in the United States was $48.16 in 2009. Last year’s local monthly bills, in dollars, for a random sample of 75 cell phone users are given on the WeissStats site. Use the technology of your choice to do the following.

a. Obtain a normal probability plot, boxplot, histogram, and stemand-leaf diagram of the data.

b. At the 5% significance level, do the data provide sufficient evidence to conclude that last year’s mean local monthly bill for cell phone users decreased from the 2009 mean of $48.16? Assume that the population standard deviation of last year’s local monthly bills for cell phone users is $25.

c. Remove the two outliers from the data and repeat parts

(a) and (b).

d. State your conclusions regarding the hypothesis test.

Extending the Concepts and Skills