A transportation strategist wanted to compare the traffic congestion levels across three continents: Asia, Europe, and North America. The file CongestionLevel contains congestion level, defined as the increase (%) in overall travel time when compared to a free flow situation (an uncongested situation) for 30 cities in each continent. (Data extracted from "TomTom Traffic Index," bit.ly/1GNRRk1.)

a. At the 0.05 level of significance, is there evidence of a difference in the mean congestion level across continents?

b. If the results in (a) indicate that it is appropriate to do so, use the Tukey-Kramer procedure to determine which continents differ in congestion level

c. What assumptions are necessary in (a)?

d. At the 0.05 level of significance, is there evidence of a difference in the variation of the congestion level across continents?