An institute studies traffic delays. Data the institute published for the year 2001 include information on the Total Delay per Person (hours per year spent delayed by traffic), the Average Arterial Road Speed (mph), the Average Highway Road Speed (mph), and the Size of the city (small, medium, large, very large). The regression model based on these variables is shown in the accompanying table. The variables Small, Large, and Very Large are indicators constructed to be 1 for cities of the named size and 0 otherwise. Complete parts a and b.
a) Why is there no coefficient for Medium?
b) Explain how the coefficients of? Small, Large, and Very Large account for the size of the city in the model.