changed to a current residence concept, as is more common in polls and other person-based surveys. Specifically, the ACS residence concept is based on a “2-month rule:” people who live for more than 2 months at a sample address are assumed to be residents of that unit. The rule is intended to be prospective as well as retrospective—that is, people who have lived in a unit for more than 2 months at the time of the ACS interview and people who have just moved into the unit and expect to stay there for more than 2 months are considered residents of the unit.

The Census Bureau has identified three exceptions to this general concept (U.S. Census Bureau, 2006:6-2, 6-3): (1) children younger than college age who are away at boarding schools or summer camps are to be considered residents of their parents’ or caregivers’ homes; (2) children who live under joint custody agreements and move often between the residences of their parents are to be considered current residents of the sample unit at which they are staying when contact is made; and (3) commuter workers who stay in a residence close to their work and return regularly to a family residence are to be considered residents of the family residence and not the work residence. In addition, people staying at a unit at the time of the interview who have no other place to stay are to be considered residents of the unit.

While the 2-month rule generally seems reasonable, it is not clear why 2 months was chosen and not another value (for example, 1 month or 3 months). Some of the exceptions to the 2-month rule, particularly for commuter workers, also do not have a clear conceptual basis. In addition, while the 2-month rule acknowledges that not everyone stays in the same “usual residence” all the time (for example, people with summer and winter homes, commuter workers), it does not address other kinds of situations in which people have multiple residences. Examples include people with weekday and weekend residences, people who live and travel throughout the year in recreational vehicles, and people who move among the residences of several relatives or friends.

The 2-month residence rule is applied at the time the data collection takes place. For example, if no mail return comes back from a sampled address and there is no success with CATI, but the address is included in CAPI in the third month of data collection, respondents are asked about residence under the 2-month rule at the time of the interview. Thus, the reference period is a function of the time of interview rather than a fixed time interval related to the month of mailout.

The CATI and CAPI computerized instruments may include questions to enable the ACS residence rules to be applied as intended. However, the mail questionnaire does not clearly or fully explain these rules, as shown in Box 4-3. The accompanying guide for respondents does not provide further instruction (see U.S. Census Bureau, 2006:App. B, which reprints the mail



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