Does LTE network performance vary between providers? The file LTE contains the download speed (in Mbps) on LTE connections of AT&T and Verizon providers in 31 U.S. large metro markets.

Source: Data extracted from State of Mobile Networks: USA, August 2016, available at bit.ly/2aYXxox.

a. At the 0.05 level of significance, is there evidence of a difference in the mean download speed between AT&T and Verizon?

b. What assumption is necessary to perform this test?

c. Use a graphical method to evaluate the assumption made in (a).

d. Construct and interpret a 95% confidence interval estimate of the difference in the mean download speed between AT&T and Verizon.