Do upload and download speeds vary between cities and Internet service providers? The file Internet Provider Speeds contains the upload and download speeds in Mbps in 40 different cities for four Internet service providers, AT&T, T-Mobile, Sprint, and Verizon, as measured in November 2017. In this problem, you have decided to focus on the difference in the download times between AT&T and Verizon.
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.