The 2011 ACS detailed tables for race were modified to include additional categories for detailed American Indian and Alaska Native groups, Asian groups, and Native Hawaiian and Other Pacific Islander groups. As a result, not all tabulated race estimates are directly comparable from 2008-2010 to 2011-2013.

Note that in 2011 the ancestry edit was changed to allow for more general responses, such as American, Arab, or European, and more race and Hispanic responses.

Modest changes were made in 2009 to the region and country groupings. Specifically, Macau (code 225) is now included under the definition of China, and Jan Mayen (code 121) and Svalbard (code 135) are included under the definition of Norway. Learn more in the 2009 ACS Code List.

Field of degree was added to the ACS questionnaire in 2009, so this subject is not available in the 2008-2010 ACS 3-year estimates.

Methodological changes to data collection in 2013 may have affected language data for 2010-2013. Users should use caution when comparing 2011-2013 with 2008-2010. For more information on the effects of these methodological changes on the language data, visit the 2013 language user note.

Period of Service:
In 2013, the period of military service categories were revised. Four peacetime categories were collapsed into two categories. Data from this question can still be compared across years.

Veterans Status:
The Census Bureau introduced an improved veteran status question in the 2013 ACS questionnaire. Accordingly, we recommend using caution when making comparisons of the estimate of the veteran population from 2013 or later with data from prior years. For more information on the revised question and its evaluation in the 2010 ACS Content Test, see the 2010 ACS Content Test Evaluation Report Covering Veteran Status. Learn more information about the evaluation of the new veterans question on the 2013 ACS. Additional information can also be found on the Veteran Statistics website.

In 2010, we added an instruction to the food stamp question incorporating the new program name - Supplemental Nutrition Assistance Program (SNAP). Estimates on this topic can still be compared across years.

From 2008 to 2012 ACS industry data were coded based on the 2007 North American Industry Classification System (NAICS), while the 2013 ACS industry data use the 2012 revision of the NAICS codes. To allow for the creation of 2011-2013 tables, industry data in the 2011-2013 multiyear file was recoded to 2013 census industry codes. Because of the census industry code changes, we recommend using caution when comparing 2011-2013 industry data with 2008-2010 industry data. For more information on census industry code changes, please visit the Industry and Occupation website.

The census occupation codes for 2010 and later years are based on the 2010 revision of the Standard Occupational Classification (SOC). To allow for the creation of 2008-2010 tables, occupation data in the 2008-2010 multiyear file was recoded to 2010 census occupation codes. For more information on the Census occupation code changes, please visit Industry and Occupation website.

Changes made to the 2008 ACS question wording and translation to the Spanish questionnaire resulted in an inconsistency in the Plumbing Facilities data. Therefore, the complete plumbing for Puerto Rico was suppressed since some of the underlying data were determined to be unreliable. In the 2013 Puerto Rico Community Survey, a new set of questions to assess whether a home had complete plumbing was introduced. Until a determination of how well the new questions performed, the plumbing data for Puerto Rico will be suppressed. For more detailed information about Plumbing Facilities, see the Plumbing Facilities section of the "Subject Definitions."

There was a change to the ACS group quarters (GQ) estimation methodology beginning in 2011 to improve sub-state estimates for the total GQ population. This improvement is evidenced in the 2011-2013 ACS data. Due to the change to the ACS group quarters estimation methodology, the 2008-2010 ACS group quarters data should be compared with caution to the 2011-2013 ACS group quarters data. Large fluctuations may be observed at the sub-state level when comparing 2008-2010 to 2011-2013 ACS estimates for the total GQ population.

Computer and Internet use was added to the ACS in 2013, so these subjects are not available in the 2008-2010 ACS 3-year estimates or the 2011-2013 ACS 3-year estimates.