American Community Survey (ACS)

Data users often want to compare the most current ACS data with data from Census 2000 and from previous ACS releases. Some comparisons might be inappropriate due to differences in questions or methods. This page provides guidance on these issues.

Users should take a few things into consideration.

1. Geographic boundaries for the area of interest may have changed. For more information, visit How Multiyear Estimates are Produced.
2. Although the ACS produces estimates of population and housing, it is the Census Bureau's Population Estimates Program that produces and disseminates the official estimates of the population for the nation, states, counties, cities and towns and estimates of housing units for states and counties. The ACS should be used to examine housing characteristics and demographic, social, and economic characteristics of the population.
3. The weighting for the group quarters (GQ) population is controlled at the state level, but not at the sub-state level. Users may observe greater fluctuations in year-to-year ACS estimates of the GQ population at sub-state levels than at state levels. Substantial changes in the substate ACS GQ estimates can impact substate ACS estimates of total population characteristics.
4. There are global differences that exist between the ACS and Census 2000. These include differences in residence rules, universes, and reference periods.

General Guidance

Comparing 2006-2008 ACS 3-Year Data with Census 2000 Data - The Census Bureau supports comparisons made between the 2006-2008 ACS 3-year data and Census 2000 data but cautions users about important differences between the two data sets. Item by item guidance is provided below.

Comparing 2006-2008 ACS 3-Year Data with 2008 ACS 1-Year Data - When comparing estimates across geographies or small population groups, users should compare the same period length for each estimate. Whenever you are comparing state-level data with sub-state data for an area with a population below 65,000 you should use the 3-year estimates for the state to compare with the 3-year estimates for the sub-state area.

Comparing 2006-2008 ACS 3-Year Data with 2005-2007 ACS 3-Year Data - Because of the complexities of interpretation, the Census Bureau discourages direct comparisons between estimates for overlapping periods. Ideally comparisons should be made based on non-overlapping periods (e.g. comparing a 2005-2007 ACS 3-year estimate with a 2008-2010 ACS 3-year estimate).

Users who are interested in comparing overlapping multiyear estimates should refer to The ACS Compass Products handbooks, which include appendixes on using multiyear estimates and making comparisons (Appendix 1 and Appendix 4). The State and Local Governments handbook [PDF 1.2MB] provides the most detailed guidance and a case study on this issue. For additional information about the challenges of comparing overlapping multiyear estimates see: Statistical Issues of Interpretation of ACS 1-, 3-, and 5-year Period Estimate. [PDF 679KB]

Item-Specific Guidance

Census Bureau subject matter specialists have determined that for most population and housing subjects available in both ACS 3-year data products and Census 2000 Summary File 3, comparisons can be made. Specific item-level guidance is provided in the table below. Guidance is given for each item in the 2006-2008 ACS compared with Census 2000 data.

Some topics, which were new or substantially modified on the 2008 ACS, will not have estimates in the 2006-2008 ACS 3-year data products. These include: health insurance, marital history, veteran's service connected disability rating, and disability. The first 3-year estimates for these topics will be available in 2011 when the 2008-2010 ACS 3-year estimates are released. An overview of the changes that were made to the questionnaire and the impact on the estimates that were observed in the 2006 ACS Content Test is available in the New and Modified Content on the 2008 ACS Questionnaire: Results of Testing Prior to Implementation.

* Each subject area is listed with its 2-digit code. This code corresponds to the second and third characters of the ACS table number. For example, Table B08303 - Travel Time to Work - has the digits of "08" in the second and third position. This corresponds to the subject Journey to Work; Workers; and Commuting.