APPENDIX C.
                             Accuracy of the Data 

CONTENTS 

Confidentiality of the Data....................................... C-1 

Editing of Unacceptable Data...................................... C-1 

Sources of Error.................................................. C-1 

CONFIDENTIALITY OF THE DATA 

To maintain confidentiality required by law (Title 13, United States
Code), the Bureau of the Census applies a confidentiality edit to
assure published data do not disclose information about specific
individuals, households, and housing units. The result is that a small
amount of uncertainty is introduced into some of the census
characteristics to prevent identification of specific individuals,
households, or housing units. The edit is controlled so that the counts
of total persons, totals by race and American Indian tribe, Hispanic
origin, and age 18 years and over are not affected by the
confidentiality edit and are published as collected. In addition, total
counts for housing units by tenure are not affected by this edit. 

  The confidentiality edit is conducted by selecting a sample of census
households from the 100-percent data internal census files and
interchanging its data with other households that have identical
characteristics on a set of selected key variables but are in different
geographic locations within the same State. To provide more protection
for "small areas," a higher sampling rate was used for these
areas. The net result of this procedure is that the data user's ability
to obtain census data, particularly for small areas and subpopulation
groups, has been significantly enhanced. 

EDITING OF UNACCEPTABLE DATA 

The objective of the processing operation is to produce a set of
data that describes the population as accurately and clearly as
possible. To meet this objective, questionnaires were edited during
field data collection operations for consistency, completeness, and
acceptability. Questionnaires were also reviewed by census clerks for
omissions, certain inconsistencies, and population coverage. For
example, write-in entries such as "Don't know" or "NA" were
considered unacceptable. For some district offices, the initial edit
was automated; however, for the majority of the district offices, it
was performed by clerks. As a result of this operation, a telephone or
personal visit follow-up was made to obtain missing information.
Potential coverage errors were included in the follow-up, as well as a
sample of questionnaires with omissions or inconsistencies. 

  Subsequent to field operations, remaining incomplete or inconsistent
information on the questionnaires was assigned using imputation
procedures during the final automated edit of the collected data.
Allocations, or computer assignments of acceptable codes in place of
unacceptable entries or blanks, are needed most often when an entry for
a given item is lacking or when the information reported for a person
or housing unit on that item is inconsistent with other information for
that same person or housing unit. As in previous censuses, the general
procedure for changing unacceptable entries was to assign an entry for
a person or housing unit that was consistent with entries for persons
or housing units with similar characteristics. The assignment of
acceptable codes in place of blanks or unacceptable entries enhances
the usefulness of the data. 

  Another way in which corrections were made during the computer editing
process was through substitution; that is, the assignment of a full set
of characteristics for a person or housing unit. When there was an
indication that a housing unit was occupied, but the questionnaire
contained no information for the people within the household, or the
occupants were not listed on the questionnaire, a previously accepted
household was selected as a substitute, and the full set of
characteristics for the substitute was duplicated. The assignment of
the full set of housing characteristics occurred when there was no
housing information available. If the housing unit was determined to be
occupied, the housing characteristics were assigned from a previously
processed occupied unit. If the housing unit was vacant, the housing
characteristics were assigned from a previously processed vacant unit. 

SOURCES OF ERROR 

In any large-scale statistical operation, such as the 1990 decennial
census, human- and machine-related errors occur. These errors are
commonly referred to as nonsampling errors. Such errors include not
enumerating every household or every person in the population, not
obtaining all required information from the respondents, obtaining
incorrect or inconsistent information, and recording information
incorrectly. In addition, errors can occur during the field review of
the enumerators' work, during clerical handling of the census
questionnaires, or during the electronic processing of the
questionnaires. 

  To reduce various types of nonsampling errors, a number of techniques
were implemented during the planning, development of the mailing
address list, data collection, and data processing activities. Quality
assurance methods were used throughout the data collection and
processing phases of the census to improve the quality of the data. A
reinterview program was designed to minimize the errors in the data
collection phase for enumerator-filled questionnaires. 

  Several coverage improvement programs were implemented during the
development of the census address list and census enumeration and
processing to minimize undercoverage of the population and housing
units. These programs were developed based on experience from the 1980
decennial census and results from the 1990 decennial census testing
cycle. In developing and updating the census address list, the Census
Bureau used a variety of specialized procedures in different parts of
the country. 

  - For larger urban areas, the Census Bureau purchased and coded
    address lists, had the United States Postal Service (USPS) review and
    update this list, and conducted a dependent canvass and update
    operation. Prior to mailout, local officials were given the opportunity
    to examine block counts of address listings (local review) and identify
    possible errors, and the USPS conducted a final review. 

  - For small cities and suburban and selected rural parts of the
    country, the Census Bureau created the address list through a listing
    operation that occurred in 1988 and 1989. For the addresses listed in
    1988, the USPS reviewed and updated this list, and the Census Bureau
    reconciled USPS corrections through a field check; prior to mailout,
    the USPS conducted a final review of these addresses, and local
    officials participated in reviewing block counts of address listings. 

Coverage improvement programs continued during and after mailout.
The Census Bureau (rather than the USPS) delivered census
questionnaires in the rural and seasonal housing areas listed in 1989
and in inner-city public housing developments. Computer and clerical
edits and telephone and personal visit followups contributed to
improved coverage. 

  If the 1990 census is not subject to count adjustment, the population
counts shown in 100-percent data products will be tabulated from the
entries for persons on all questionnaires. These counts will not be
subject to sampling error. If count adjustment is done, a discussion of
the count adjustment methodology and the appropriate methods for
calculating sampling errors of adjusted counts can be found in appendix
H. (Housing unit counts will not be subject to count adjustment.)