When information is missing or inconsistent, the Census Bureau uses a method called imputation to assign values. Imputation relies on the statistical principle of "homogeneity," or the tendency of households within a small geographic area to be similar in most characteristics. For example, the value of "rented" is likely to be imputed for a housing unit not reporting on owner/renter status in a neighborhood with multiunits or apartments where other respondents reported "rented" on the census questionnaire. In past censuses, when the occupancy status or the number of residents was not known for a housing unit, this information was imputed.

The data on residence 5 years earlier were derived from answers to long-form questionnaire Item 15, which was asked of a sample of the population 5 years old and over. This question asked for the state (or foreign country), U.S. county, city or town, and ZIP Code of residence on April 1, 1995, for those people who reported that on that date they lived in a different house than their current residence. Residence 5 years earlier is used in conjunction with location of current residence to determine the extent of residential mobility of the population and the resulting redistribution of the population across the various states, metropolitan areas, and regions of the country.

On the Puerto Rico questionnaire, people living in Puerto Rico in 1995 were asked to report the name of the municipio (county equivalent); the city, town or village; and the ZIP Code where they lived. People living in the United States in 1995 were asked to report the name of the city, county, state, and ZIP Code where they lived. People living outside Puerto Rico or the United States were asked to report the name of the foreign country or U.S. Island Area where they were living in 1995.

When no information on previous residence was reported for a person, information for other family members, if available, was used to assign a location of residence in 1995. All cases of nonresponse or incomplete response that were not assigned a previous residence based on information from other family members were imputed the previous residence of another person with similar characteristics who provided complete information on residence 5 years earlier.

The tabulation category, "Same house," includes all people 5 years old and over who did not move during the 5 years as well as those who had moved but by Census Day had returned to their 1995 residence. The category, "Different house in the United States," includes people who lived in the United States 5 years earlier but lived in a different house or apartment from the one they occupied on Census Day. These movers are then further subdivided according to the type of move.

In most tabulations, movers within the U.S. are divided into three groups according to their previous residence: "Different house, same county," "Different county, same state," and "Different state." The last group may be further subdivided into region of residence in 1995. An additional category, "Abroad," includes those whose previous residence was in a foreign country, Puerto Rico, American Samoa, Guam, the Commonwealth of the Northern Mariana Islands, or the U.S. Virgin Islands, including members of the armed forces and their dependents. Some tabulations show movers who were residing in Puerto Rico or one of the U.S. Island Areas in 1995 separately from those residing in foreign countries.

In most tabulations, movers within Puerto Rico are divided into two groups according to their 1995 residence: "Same municipio," and "Different municipio." Municipio of previous residence in Puerto Rico is not available for people living in the United States in 2000. Other tabulations show movers within or between metropolitan areas similar to the stateside tabulations.

Some special tabulations present data on inmigrants, outmigrants, and net migration. "Inmigrants" are generally defined as those people who moved into an area. In some tabulations, movers from abroad may be included in the number of inmigrants; in others, only movers within the United States are included. "Outmigrants" are people who moved out of a specific area to some other place in the United States. Movers who left the United States are not available to be included in any tabulations. "Net migration" is calculated by subtracting the number of outmigrants from the number of inmigrants. The net migration for the area is net inmigration if the result is positive and net outmigration if the result is negative. In the tabulations, net outmigration is indicated by a minus sign (-).

Inmigrants and outmigrants for states include only those people who did not live in the same state at both dates; that is, they exclude people who moved between counties within the same state. Thus, the sum of the inmigrants to (or outmigrants from) all counties in any state is greater than the number of inmigrants to (or outmigrants from) that state. However, in the case of net migration, the sum of the nets for all the counties within a state equals the net for the state. In the same fashion, the net migration for a division or region equals the sum of the nets for the states comprising that division or region, while the number of inmigrants and outmigrants for that division or region is less than the sum of the inmigrants or outmigrants for the individual states.

The number of people who were living in a different house 5 years earlier is somewhat less than the total number of moves during the 5-year period. Some people in the same house at the two dates had moved during the 5-year period but by the time of the census had returned to their 1995 residence. Other people who were living in a different house had made one or more intermediate moves. For similar reasons, the number of people living in a different county, metropolitan area, or state, or the number moving between nonmetropolitan areas, may be understated.