11.55 Table 11.6 gives the national air-quality data for the years 1980–2000 (National Air Quality and Emissions Report, 2003). All values are measured in ppm.

a Plot the air quality data versus year. Describe any trends observed in the air quality measurements over the years data were collected.

b Make a scatterplot matrix of all the air quality variables. Comment on the nature of relations among these variables.

c Estimate the strongest relation. Assess the fit of the model using .