Source: U.S. Census Bureau  

Release: Racial Dissimilarity Index

Units:  Percent, Not Seasonally Adjusted

Frequency:  Annual

The Racial Dissimilarity Index measures the percentage of the non-hispanic white population in a county which would have to change Census tracts to equalize the racial distribution between white and non-white population groups across all tracts in the county.

Starting with the 2016 observations, the calculation has been changed so that counties with only one census tract have missing data. Zero values represent counties where the proportions of non-white population and non-hispanic white population are the same. Multiyear estimates from the American Community Survey (ACS) are "period" estimates derived from a data sample collected over a period of time, as opposed to "point-in-time" estimates such as those from past decennial censuses. ACS 5-year estimate includes data collected over a 60-month period. The date of the data is the end of the 5-year period. For example, a value dated 2014 represents data from 2010 to 2014. However, they do not describe any specific day, month, or year within that time period.

Multiyear estimates require some considerations that single-year estimates do not. For example, multiyear estimates released in consecutive years consist mostly of overlapping years and shared data. The 2010–2014 ACS 5-year estimates share sample data from 2011 through 2014 with the 2011–2015 ACS 5-year estimates. Because of this overlap, users should use extreme caution in making comparisons with consecutive years of multiyear estimates.

Please see "Section 3: Understanding and Using ACS Single-Year and Multiyear Estimates" on publication page 13 (file page 19) of the 2018 ACS General Handbook for a more thorough clarification.