On the basis of their employment status or activity during the week of March 24 to 30, 1940, all persons 14 years old and over were classified into two large groups: (a) Persons in the labor force, including those employed (at work or with a job, except on public emergency work), those on public emergency work, and those seeking work; and (b) persons not in the labor force, including those engaged in own home housework, those in school, those unable to work, inmates of institutions, other persons not in the labor force, and those whose employment status was not reported.
In the interpretation of the data for persons on public emergency work, allowance must be made for the misclassification in the census returns of some public emergency workers, as a result of which the number of such workers is considerably understated.

Three major race classifications are distinguished in this bulletin, namely, white, Negro, and other races. Persons of Mexican birth or ancestry who were not definitely Indian or of other nonwhite race were returned as white in 1940. In tables 3a and 6a statistics for the nonwhite (Negroes and other nonwhite races combined) are shown separately for tracts with 250 or more nonwhite persons. Occupation statistics for nonwhites are presented for Southern cities only. The white population is divided into two nativity groups, native and foreign born. A person born in the United States or in any of its territories or possessions is counted as native. The foreign-born white are presented by country of birth. The country of birth classification is based on political boundaries as of January 1, 1937, and therefore includes such countries as Austria; Czechoslovakia, and Poland.
The age classification is based on the age of the person at his last birthday before the date of the census, that is, the age of the person in completed years.