Research on nonsampling errors that may systematically bias survey estimates upwards or downwards is difficult to conduct, and the available information is rarely complete or definitive about the magnitude of the biases. Hence, users are rarely in a position to adjust estimates of interest to correct for nonsampling errors. Nonetheless, users should acknowledge known nonsampling errors in their uses of the ACS data.
As examples of possible biases in the ACS, a comparison of the C2SS with the 2000 long-form sample found significantly lower estimates of median income in the C2SS than in the long-form sample, while comparisons of the C2SS and the 2001–2003 ACS test surveys with the CPS consistently found significantly lower estimates of unemployment in the ACS surveys than in the CPS (see Section 2-B.2.e). Further research is needed to determine which survey is more accurate.
c. Carefully consider the pros and cons of alternative strategies for extracting value from ACS 5-year period estimates for very small areas, such as aggregating small-area estimates into estimates for larger, user-defined areas.
Large cities and counties should use ACS 5-year period estimates for census tracts and block groups as building blocks to define larger areas that are meaningful for analysis and for which 5-year period estimates are sufficiently precise. For example, a city might aggregate census tracts into several planning areas, or it might use combinations of block groups that do not necessarily respect census tract boundaries. Statistical mapping techniques may help identify which tracts and block groups would be most useful to combine into subareas for analyzing such phenomena as commuting patterns. For user-defined subareas, a city might ask the Census Bureau to develop 3-year period estimates for large population groups to obtain more information on trends.
Small governmental units may not be able to satisfy their data needs by aggregating 5-year period estimates into larger areas. However, with due care they may be able to work with 5-year period estimates for large population groups in their jurisdiction and 5-year period estimates for smaller groups for a larger area, such as their county, to assess changes in the composition of their own area. Small governmental units might also ask the Census Bureau to develop ACS estimates for their area for periods longer than 5 years.