I have downloaded data on family income, educational attainment, and nativity by county from the 1970 census (broken down by race/ethnicity). I’ve noticed that for some variables a value of “-1” is listed. For example, the variable for the number of males who completed 1-3 years of high school gives a value of “-1” for some counties. It is not clear to me what this value means as it is not listed in the codebook that came with the data. The data came specifically from the “1970_Cnt4Pb”.
The -1 value in the census tract Sex by Race table is an indicator for data suppression. The Bureau used suppression in 1970-1980 to maintain respondent confidentiality. The Bureau put a -1 (or -2 value) in the first cell of a tabulation to indicate suppression, and then they “blanked” out the remaining cells in the tabulation. For more information about 1970 suppression, see pages 189-190 of 568 in this PDF.