The accompanying summary quantities for x 5 particulate pollution (mg/m3

) and y 5 luminance (.01 cd/

m2

) were calculated from a representative sample of data that appeared in the article “Luminance and Polarization of the Sky Light at Seville (Spain) Measured in White Light” (Atmospheric Environment [1988]: 595–599).

n 5 15 a x 5 860 a y 5 348 a x2 5 56,700 a y2 5 8954 axy 5 22,265

a. Test to see whether there is a positive correlation between particulate pollution and luminance in the population from which the data were selected.

b. What proportion of observed variation in luminance can be attributed to the approximate linear relationship between luminance and particulate pollution?