lation sizes, subsampling rates for addresses that must be followed up in person, and estimated 5-year rates of sample cases after subsampling; Table 2-4 provides counts of governmental units by type and population size.

Data Collection

Each month the residential housing unit addresses in the ACS sample with mailable addresses—about 95 percent of each month’s sample of 250,000 addresses—are sent a notification letter followed 4 days later by a questionnaire booklet. A reminder postcard is sent out 3 days after the questionnaire mailing. Whenever a questionnaire is not returned by mail within the following 3 weeks, a second questionnaire is mailed to the address. If there is still no response and if the Census Bureau is able to obtain a telephone number for the address, then trained interviewers conduct telephone follow-up using computer-assisted telephone interviewing (CATI) equipment. Telephone follow-up is also used to obtain missing information from households that mailed back incomplete questionnaires. About 33 percent of mail questionnaires in 2005 required telephone follow-up because key items were missing or because the household reported more members than there was room to provide information (U.S. Census Bureau, 2006:7-9).

For samples of addresses for which no mail or CATI responses are received after 2 months, or the postal service returned the questionnaire because it could not be delivered as addressed, or the address is not in street name and number format and so was not mailed out in the first place (for example, post office box or rural route addresses), interviewers are sent into the field with laptop computers. They visit housing units in person (or, in about 20 percent of cases, make contact by telephone) and collect the ACS data through computer-assisted personal interviewing (CAPI). The personal interview follow-up is conducted on a sample basis in order to save costs: about two-thirds of nonmailable addresses and between one-third and one-half of mailable addresses in each census tract—depending on the expected mailback and CATI response rate for the census tract—are followed up in person. Interviewers also visit group quarters in person to collect data for residents, using paper-and-pencil questionnaires.

An important difference between the ACS and 2000 census long-form-sample data collection procedures is that all nonresponding housing units were included in the long-form follow-up operations. Long-form-sample questionnaires were sent out in mid-March 2000, preceded by a notification letter and followed by a reminder postcard. For every address for which a questionnaire was not returned by mail, temporary interviewers (enumerators) went into the field to try to obtain responses in the period of late April-June. The enumerators were often not successful in obtaining

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