From April 24 through September 11, 2000, the U.S. Census Bureau surveyed a sample of about 314,000 housing units (about 1.4 million census and A.C.E. records in various areas of the country, including Puerto Rico) to estimate the number of people and housing units missed or counted more than once in the census and to evaluate the final census counts. Temporary bureau staff conducted the surveys by telephone and in-person visits. The A.C.E. sample consisted of about 12,000 “clusters” or geographic areas that each contained about 20 to 30 housing units. The bureau selected sample clusters to be representative of the nation as a whole, relying on variables such as state, race and ethnicity, owner or renter, as well as the size of each cluster and whether the cluster was on an American Indian reservation. The bureau canvassed the A.C.E. sample area, developed an address list, and collected response data for persons living in the sample area on Census Day (April 1, 2000). Although the bureau’s A.C.E. data and address list were collected and maintained separately from the bureau’s census work, A.C.E. processes were similar to those of the census. After the census and A.C.E. data collection operations were completed, the bureau attempted to match each person counted by A.C.E. to the list of persons counted by the census in the sample areas to determine the number of persons who lived in the sample area on Census Day. The results of the matching process, together with the characteristics of each person compared, provided the basis for statistical estimates of the number and characteristics of the population missed or improperly counted by the census. Correctly matching A.C.E. persons with census persons is important because errors in even a small percentage of records can significantly affect the undercount or overcount estimate. Matching over 1.4 million census and A.C.E. records was a complex and often labor-intensive process. Although several key matching tasks were automated and used prespecified decision rules, other tasks were carried out by trained bureau staff who used their judgment to match and code records. The four phases of the person matching process were (1) computer matching, (2) clerical matching, (3) nationwide field follow- up on records requiring more information, and (4) a second phase of clerical matching after field follow-up. Each subsequent phase used additional information and matching rules in an attempt to match records that the previous phase could not link. (first phase) (second phase) Computer matching took pairs of census and A.C.E. records and compared various personal characteristics such as name, age, and gender. The computer then calculated a match score for the paired records based on the extent to which the personal characteristics were aligned. Experienced bureau staff reviewed the lists of paired records, sorted by their match scores, and judgmentally assigned cutoff scores. The cutoff scores were break points used to categorize the paired records into one of three groups so that the records could be coded as a “match,” “possible match,” or one of a number of codes that defines them as not matched. Computer matching successfully assigned a match score to nearly 1 million of the more than 1.4 million records reviewed (about 66 percent). Bureau staff documented the cutoff scores for each of the match groups. However, they did not document the criteria or rules used to determine cutoff scores, the logic of how they applied them, and examples of their application . As a result, the bureau may not benefit from the possible lessons learned on how to apply cutoff scores. When the computer links few records as possible matches, clerks will spend more time searching records and linking them. In contrast, when the computer links many records as possible matches, clerks will spend less time searching for records to link and more time unlinking them. Without documentation and knowledge of the effect of cutoff scores on clerical matching productivity, future bureau staff will be less able to determine whether to set cutoff scores to link few or many records together as possible matches. (first phase) (second phase) During clerical matching, three levels of matchers—including over 200 clerks, about 40 technicians, and 10 experienced analysts or “expert matchers”—applied their expertise and judgment to manually match and code records. A computer software system managed the workflow of the clerical matching stages. The system also provided access to additional information, such as electronic images of census questionnaires that could assist matchers in applying criteria to match records. According to a bureau official, a benefit of clerical matching was that records of entire households could be reviewed together, rather than just individually as in computer matching. During this phase over a quarter million records (or about 19 percent) were assigned a final match code. The bureau taught clerks how to code records in situations in which the A.C.E. and census records differed because one record contained a nickname and the other contained the birth name. The bureau also taught clerks how to code records with abbreviations, spelling differences, middle names used as first names, and first and last names reversed. These criteria were well documented in both the bureau’s procedures and operations memorandums and clerical matchers’ training materials, but how the criteria were applied depended on the judgment of the matchers. The bureau trained clerks and technicians for this complex work using as examples some of the most challenging records from the 1998 Dress Rehearsal person matching operation. In addition, the analysts had extensive matching experience. For example, the 4 analysts that we interviewed had an average of 10 years of matching experience on other decennial census surveys and were directly involved in developing the training materials for the technicians and clerks. (first phase) (second phase) The bureau conducted a nationwide field follow-up on over 213,000 records (or about 15 percent) for which the bureau needed additional information before it could accurately assign a match code. For example, sometimes matchers needed additional information to verify that possibly matched records were actually records of the same person, that a housing unit was located in the sample area on Census Day, or that a person lived in the sample area on Census Day. Field follow-up questionnaires were printed at the National Processing Center and sent to the appropriate A.C.E. regional office. Field follow-up interviewers from the bureau’s regional offices were required to visit specified housing units and obtain information from a knowledgeable respondent. If the household member for the record in question still lived at the A.C.E. address at the time of the interview and was not available to be interviewed after six attempts, field follow-up interviewers were allowed to obtain information from one or more knowledgeable proxy respondents, such as a landlord or neighbor. (first phase) (second phase) The second phase of clerical matching used the information obtained during field follow-up in an attempt to assign a final match code to records. As in the first phase of clerical matching, the criteria used to match and code records were well documented in both the bureau’s procedures and operations memorandums and clerical matchers’ training materials. Nevertheless, in applying those criteria, clerical matchers had to use their own judgment and expertise. This was particularly true when matching records that contained incomplete and inconsistent information, as noted in the following examples. Different household members provided conflicting information. The census counted one person—the field follow-up respondent. A.C.E. recorded four persons—including the respondent and her daughter. The respondent, during field follow-up, reported that all four persons recorded by A.C.E. lived at the housing unit on Census Day. During the field follow-up interview, the respondent’s daughter came to the house and disagreed with the respondent. The interviewer changed the answers on the field follow-up questionnaire to reflect what the daughter said— the respondent was the only person living at the household address on Census Day. The other three people were coded as not living at the household address on Census Day. According to bureau staff, the daughter’s response seemed more reliable. An interviewer’s notes on the field follow-up questionnaire conflicted with recorded information. The census counted 13 people—including the respondent and 2 people not matched to A.C.E. records. A.C.E. recorded 12 people—including the respondent, 10 other matched people, and the respondent’s daughter who was not matched to census records. The field follow-up interview attempted to resolve the unmatched census and A.C.E. people. Answers to questions on the field follow-up questionnaire verified that the daughter lived at the housing address on Census Day. However, the interviewer’s notes indicated that the daughter and the respondent were living in a shelter on Census Day. The daughter was coded as not living at the household address on Census Day, while the respondent remained coded as matched and living at the household address on Census Day. According to bureau staff, the respondent should also have been coded as a person that did not live at the household address on Census Day, based on the notes on the field follow-up questionnaire. A.C.E., census, or both counted people at the wrong address. The census counted two people—the respondent and her husband—twice; once in an apartment and once in a business office that the husband worked in, both in the same apartment building. The A.C.E. did not record anyone at either location, as the residential apartment was not in the A.C.E. interview sample. The respondent, during field follow-up, reported that they lived at their apartment on Census Day and not at the business office. The couple had responded to the census on a questionnaire delivered to the business office.