Second, delinquency rates for each relative income group (lower, middle, and higher) were calculated as the weighted sum of delinquencies divided by the weighted sum of mortgages, where the weights equal each Zip code's share of the population in census tracts of the particular relative income group.
Relative income is based on the 2000 decennial census and is calculated as the median family income of the census tract divided by the median family income of its metropolitan statistical area or nonmetropolitan portion of the state. Both approaches yield virtually identical results.