Current techniques for census adjustment involve the "synthetic assumption" that
undercount rates are constant within "post-strata" across geographical areas. A poststratum
is a subgroup of people with given demographic characteristics; poststrata are chosen
to minimize heterogeneity in undercount rates. This paper will use 1990 census data
to assess the synthetic assumption. We find that heterogeneity within poststrata
is quite large, with a corresponding impact on local undercount rates estimated by
the synthetic method. Thus, any comparison of error rates between the census and
adjusted counts should take heterogeneity into account.