Data Dictionary:  Census 1980 
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Survey: Census 1980
Data Source:  U.S. Census Bureau 
Data set: Summary Tape File 3 (STF3)
Table:  T91. Aggregate Income (In 1979 Dollars) for Unrelated Individuals [1] 
Universe: Unrelated Individuals 15 Years And Over
Table Details
T91.  Aggregate Income (In 1979 Dollars) for Unrelated Individuals  
Universe: Unrelated Individuals 15 Years And Over  

Relevant Documentation:
Excerpt from:  Social Explorer; U.S. Census Bureau; Census of Population and Housing, 1980: Summary Tape File 3 [machinereadable data file] / conducted By the U.S. Bureau of the Census. Washington: Bureau of the Census [producer and distributor], 1982. 
Summary Tape File 3 > Summary Tape File 3  Part II > Glossary > Income in 1979 > Mean income 
The mean income is the amount obtained by dividing the total income of a particular statistical universe (termed "aggregate income" in STF documentation) by the number of units in that universe. Thus, mean family income is obtained by dividing total family income by the total number of families. Mean income for persons is obtained by dividing the total income of persons (including patients or inmates in institutional quarters) by the number of persons with income. When the mean income for an area or population subgroup is a net loss, the dollar amount is shown preceded by a sinus sign (e.g., $123).
Care should be exercised in using and interpreting mean income values in the statistics for small subgroups of the population. Since the mean is strongly influenced by extreme values in the distribution, it is especially susceptible to the effects of sampling variability, misreporting, and processing errors. The median is not affected by extreme values and is, therefore, a better measure than the mean when the population base is small. The mean, nevertheless, is shown for most smallarea tabulations because, when weighted according to the number of cases, the means can be added to obtain summary measures for areas and groups other than those shown.
Care should be exercised in using and interpreting mean income values in the statistics for small subgroups of the population. Since the mean is strongly influenced by extreme values in the distribution, it is especially susceptible to the effects of sampling variability, misreporting, and processing errors. The median is not affected by extreme values and is, therefore, a better measure than the mean when the population base is small. The mean, nevertheless, is shown for most smallarea tabulations because, when weighted according to the number of cases, the means can be added to obtain summary measures for areas and groups other than those shown.