The coefficient β1 measures the effect of TSP exposure on
mortality, after controlling for the available covariates. Consistent
estimation of β1 requires that unobserved determinants of mortality
do not covary with TSPj after adjustment for Xj. Thus, the
conventional approach rests on the assumption that linear adjustment
for the limited set of variables available in the census
removes all sources of confounding. With data from the United
States, Chay et al. (9)