The Environmental Protection Agency (EPA) believes that the air quality in a city is directly related to the number of serious respiratory diseases. The agency regresses the number of reported cases of respiratory diseases per 1,000 population on the city’s air quality index (ranges from 0 to 100; high scores indicate pollution) for 150 cities. It finds
 = 15.7 + .7X sb = .04 Sy|x = 5.1 r2 = .71
Write a memo interpreting these results. What other factors affect this relationship?