These data are estimates of the actual figures that would have been obtained from a complete count. Estimates derived from a sample are expected to be different from the 100-percent figures because they are subject to sampling errors. A detailed discussion of the sample design and calculations of errors can be found in Appendix C of the 1990 Census Summary Tape File Technical Documentation. The data for 1960 are for census tracts and their processing is incomplete. See 1960info.txt .
Counties are the primary political divisions of most states.
In some states these divisions are not called counties, so county equivalents are used, such as parishes in Louisiana, boroughs and census areas in Alaska, independent cities in Maryland, Missouri, Nevada, and Virginia. Other areas can be treated as county equivalents in census tabulations. The counties found in these data are as presented as they were for each census year. The only exception for this is that the Name of the County as given in 1970 comes from the 1980 data. Although the terminology has changed, the concept of metropolitan statistical areas has remained the same since the 1950's when it was first established.