To ensure high quality data, we continuously review the estimates produced by the American Community Survey. Whenever anomalies are discovered, we investigate to learn and find out its causes and make methodological changes, as appropriate, to improve the ACS estimates. These changes in methodology may result in large differences in ACS estimates for a small number of geographic areas. Since ACS weighting is controlled at the county level, small changes in methodology are more likely to affect sub-county estimates. For example, a modification in the weighting methodology to improve sub-county estimates resulted in an unexpected large drop in the estimates of housing units for Anaheim, California. This modification was implemented in 2003. The large drop in the housing unit estimate occurred in 2003 from levels estimated in 2001 and 2002.