A number of challenges and issues have accompanied the Bureau's expansion of both the sample survey measurement program and the intercensal population estimates program. Specifically, integration of the ACS results and independent population estimates reveal methodological differences pertaining to: (1) residence concepts; (2) temporal concepts; and, (3) race and ethnic definitions. Since the primary purpose of this paper is to assess the comparability of the ACS data and intercensal population estimates it is important to highlight these differences now.
First consider residence concepts as they are defined in each data source. The ACS survey as it was implemented in the four 1996 test sites relies on a "current residence" concept. Although very close to a pure de facto, "who slept here last night" concept, there is one main difference; the ACS aims to include everyone who is currently living or staying at a unit, with the exception of people who "usually" or "really" live somewhere else (e.g. their de jure residence) and are gone from that usual residence for two months or less. To introduce consistency in the assignment of persons to a residence the ACS established the "two-month" rule.
The ACS two-month rule defines the current residence of all persons, with only three apparent exceptions. First, children below the college level away at school are considered residents of their parental home. College students' current residence is established by the two-month rule. Second, children who live under joint custody agreements and move often between the separate residences of their parents are considered to be current residents of the sample unit if they are staying there when the contact with the unit is made. Finally, in the instance of commuter workers, persons who stay in a residence close to their work and return regularly to another residence, usually with family, are considered to be current residents of the family residence, not the work-related one. 9
On the other hand, intercensal population estimates rely on the residence concept of the decennial census, where persons are required to have a de jure "usual residence." This is defined as the place they live and sleep most of the time or the place they consider to be their usual home. Although implied, but never explicitly stated on the census form, "usual residence" is assumed to be the place a person spends six months or more of the year. 10 Since the 1950 census, college students have been enumerated at the place of their college rather than where their parents live and where they may return to during holidays and summer. 11
For the majority of the population, their de facto and de jure residence will be the same. There are, however, certain segments of the population where the de facto and the de jure residence will be different. Most notable are the "snowbirds." A snowbird is a generic label for a person who lives in one place for an extended portion of the year and another for the remainder. For all intents and purposes a snowbird is a person who resides at a seasonal residence for at least some portion of the year. The decennial census has typically handled the "snowbird situation" by determining which residence is the "usual" address as of the census date and assigning the household to that location, even if they are staying at the "nonusual" location at the time of contact. The ACS, on the other hand, will count the snowbirds where they are found.
Although the number of "snowbirds" is not great, the phenomenon is geographically specific and presents a challenge in integrating ACS and population estimates at the county level. The ACS does recognize that appreciable differences may exist for areas where large numbers of people spend several months of the year in units that are not their primary residence, for example Florida, Arizona, and in beach or mountain vacation areas. A working group within the Population Division at the Census Bureau is currently investigating this possibility.
Another methodological variation between the ACS and intercensal population estimates that warrants discussion is the reference period applied to characteristics. To illustrate, the ACS annual survey results constitute "average" annual estimates. On the other hand, intercensal population estimates are based on the characteristics at one particular point in time.
The annual ACS results represent the cumulative results of the twelve month interview cycle. For areas with a seasonal component, variations are included in the annual ACS picture. The intercensal population estimates represent a picture at one point in time. Although the intercensal estimates uses July 1st as its reference point the snapshot date is not exact.
Going back to the estimates methodology, the intercensal estimates begin with the decennial census population. For the county totals and state estimates the annual components of population change are added and subtracted. Because the population estimates are heavily grounded to the decennial census the estimate begins with the snapshot of the usual residence on census day, April 1st. For the 1990 year, the population is moved forward to July 1st using estimates of the components for the April-July period. Once the population is moved forward to July 1st, the remaining subnational estimates are estimated for each July 1st date using annual estimates of the components of population change.
Although the components of birth, death and international migration represent the actual July to July period, the estimates of internal migration are not as neatly tied to the actual July 1st to July 1st period. Because the internal migration component is an estimate heavily based on tax return information received from January through August the reference period is not as straight forward. However, in the intercensal estimates environment we assume these annual estimates of internal migration apply to the July 1st to July 1st period.
In sum, likening the ACS to a video and intercensal estimates to a still photograph simplifies the explanation of reference period differences. The ACS results can be thought of as a video that runs over the course of a twelve month period which displays the demographic, social, and household characteristics of the population. As a sample survey running in different phases, the ACS results are an "average" of the population's characteristics over a twelve month period. Intercensal estimates, on the other hand, are analogues to a still picture taken at one particular point in time, July 1st. In theory the estimates derived by both sources should be similar, both display characteristics of the population over approximately the same period in the past. The purpose of this paper is to test this relationship using the 1996 ACS test site data and the 1996 intercensal population estimates.
Race and Ethnicity Definitions
Finally, understanding the crossroads in terms of the definitions of racial and ethnic characteristics is necessary to the purpose of this research. Examination of both data sources reveal similarities as well as differences in definitions.
In general, the racial and ethnic definitions utilized by the Bureau of the Census reflect self-identification, and therefore cannot be assumed to represent any clear-cut scientific definition of biological stock. The data represent self-classification by people according to the categories with which they most closely identify. On the decennial census form, and during sample surveys, persons are instructed to select the one response category which best describes their racial identity and ethnic origin group. The Census Bureau recognizes that the categories include both racial and national origin or socio-cultural groups.
The racial and ethnic category classifications used by the Census Bureau generally adhere to the "Race and Ethnic Standards for Federal Statistics and Administrative Reporting" set forth in Statistical Policy Directive No. 15, issued by the Office of Management and Budget (OMB). This Directive provides common language, and promotes uniformity and comparability of race and ethnicity data. 12 The minimum race and ethnic categories designated by OMB are:
White includes persons who indicated their race as "White" or reported entries such as Canadian, German, Italian, Lebanese, Near Easterner, Arab, or Polish
Black includes persons who indicated their races as "Black or Negro" or reported entries such as African American, Afro-American, Black Puerto Rican, Jamaican, Nigerian, West Indian or Haitian.
American Indian, Eskimo or Aleut includes persons who classified themselves as:
Asian and Pacific Islander includes persons who reported in one of the Asian or Pacific Islander groups listed on the questionnaire or who provided write-in responses such as Thai, Nepali, or Tongan.
- Asian includes "Chinese," Filipino," "Japanese," Asian Indian," Korean," "Vietnamese," and "Other Asian."
- Chinese includes persons who indicated their race as "Chinese" or who identified themselves as Cantonese, Tibetan, or Chinese American. In standard census reports, persons who reported as "Taiwanese" or "Formosan" are included here with Chinese.
- Filipino includes persons who indicated their race as "Filipino" or reported entries such as Philipino, Philipine, or Filipino American.
- Japanese includes persons who indicated their race as "Japanese" and persons who identified themselves as Nipponese or Japanese American.
- Asian Indian includes persons who indicated their race as "Asian Indian" and persons who identified themselves as Bengalese, Bharat, Dravidian, East Indian or Goanese.
- Korean includes persons who indicated their race as "Korean" and persons who identified themselves as Korean American.
- Vietnamese includes persons who indicated their race as "Vietnamese" and persons who identified themselves as Vietnamese American.
- Other Asian includes persons who provided a write-in response such as Bangladeshi, Cambodian, Indonesian, Laotian, Pakistani, Sri Lankan, Amerasian, or Eurosian.
- Pacific Islander includes persons who indicated their race as "Pacific Islander" by classifying themselves into one of the following groups or identifying themselves as one of the Pacific Islander cultural groups of Polynesian, Micronesian, or Melanesian.
- Hawaiian includes persons who indicated their race as "Hawaiian" as well as persons who identified themselves as Part Hawaiian or Native Hawaiian.
- Samoan includes persons who indicated their race as "Samoan" or persons who identified themselves as Chamorro or Guam.
- Guamanian includes persons who identified their race as "Guamanian" or persons who identified themselves as Chamorro or Guam.
Other race includes all other persons not included in the "White," "Black," "American Indian, Eskimo, or Aleut," and the "Asian or Pacific Islander" race categories described above.
The above outlined race categories are mainly consistent across both the 1990 decennial census form and the questionnaire used in the four 1996 test sites. The ACS questionnaire had one important difference. In addition to the racial categories included in the cecennial census, respondents in the ACS were given the opportunity to designate themselves as "multiracial." Multiracial persons where also asked to supply a write-in response, similar to the procedure for choosing "other" race. 13
Although the intercensal estimates are strongly tied to the decennial data, for the intercensal population estimates program some modifications were introduced to the age and race data. The construction of the modified age, race, sex (MARS) file was necessary to ensure comparability to other data sources. From the decennial census data the race statistics were collapsed into four categories (white; black; American Indian, Eskimo and Aleut; and, Asian and Pacific Islander). The two ethnicity groups remain constant, Hispanic and non-Hispanic. The age data were modified to correspond with the April 1, 1990 census date. Overall the "modified" data counts remain consistent with the 1990 counts of the census as enumerated, and are used as the base to construct annual intercensal population estimates. The following paragraphs describe the construction and demand for this file.
In the 1990 census there were just under ten million reports of "other race" which needed to be assigned to one of the four race categories when collapsed in the MARS file. Indicating other race meant that these people were not included in one of the fifteen racial categories listed on the census form and therefore could not be collapsed into one of the four categories. The existence of this group is inconsistent with the race categories defined by OMB in Directive No. 15. Such "non-specified" race persons are not found in data sources other than the census. In order to serve the needs of some portions of the user community, and to construct intercensal estimates, it is necessary to assign each of these persons to a specified race. The methods for doing this in the MARS file are outlined below.
After evaluating many alternatives, the following race assignment rule was used, namely to assign each "other race" person to the specified race reported by a nearby person with an identical response to the Hispanic origin question. Specifications of this Race Assignment rule include:
- First, that the specific Hispanic origin of each "other race" person in the 1990 census was taken into account when assigning them to a specified race. This was considered appropriate because over ninety-five percent of the "other race" persons were of Hispanic origin. Their Hispanic origin response was used, whether or not it had been allocated, in order to preserve the race distribution within each type of origin. The specific Hispanic origin responses were "not Spanish/Hispanic," "Mexican," "Puerto Rican," "Cuban," and "other Spanish/Hispanic."
- Second, virtually every person who reported both a specified race and an origin was included in the "donor pool" of eligible persons. The sole exception was the exclusion of several non-specific American Indian codes from the donor pool since: (1) preliminary 1990 research suggested questionable reporting in the American Indian category; and, (2) previous research showed that a high proportion of such persons were much less likely to be American Indians than those who actually provided a specific tribe response as instructed on the census form. These codes were: 548-American White; 549-American Black; 597 American Indian (no tribe reported); 598-American Indian (tribal responses not elsewhere classified), and 973-FOSDIC circle with no write-in response. These were excluded because of evidence from the 1980 census that misreporting of race was much higher in these codes than it was in codes representing specific American Indian tribes. Consistent with advisory committee recommendations, any person assigned to the American Indian race through allocation was give code 973 rather than a specific tribal code.
- Third, the assignment of a specified race was made on an individual basis. That is, no effort was made to minimize racial heterogeneity within households. Any such attempt would have made it difficult to assign race in a manner which approximated the specified-race distribution reported by persons with the same Hispanic origin response.
- Fourth, the race, origin, or sex of some persons also changed as a result of the assignment of a different age to them during the application of the age modification procedures. Their changed age sometimes caused the person to be allocated a different relationship and/or sex which resulted in the person receiving their race or origin from a different person in the household, since those items were allocated according to a hierarchy of relationships.
- Fifth, the results of the race modification procedures were overridden in four counties where the American Indian population grew by more than 100 percent and also became at least one percentage point more of the county's population: Adams county, WA; Harmon county, OK; Clark county, and Washington county, ID.
- Finally, in most census allocations procedures, acceptable data from eligible persons (donors) are far more common than are the cases where the value is assigned to persons without the characteristics (the donees). This means information from any given donor is rarely used more than once. Such large donor-to-donee ratios were not unusual here. However, there were a number of occasions where those needing a specified race outnumbered those who reported the same origin as well as a specified race. 14
As mentioned above, the ACS racial and ethnic definitions match those used in the decennial census, except for the addition of a "multiracial" category. Particularly since the 1990 census, the OMB standards have come under increasing criticism from those who believe that the minimum racial and ethnic categories set forth in Directive No. 15 do not reflect the increasing diversity of our Nation's population resulting primarily from growth in immigration and interracial marriages. Four years ago OMB initiated a thorough review of Directive No. 15 for possible revisions. As part of the review, the Bureau of the Census tested a "multiracial" race category in several surveys, including the ACS. Thus, race categories in the final ACS products are slightly different than those in the decennial census and include the general categories "White," "Black," "American Indian, Eskimo, and Aleut," and the "Asian and Pacific Islander," as well as, persons who identified themselves as "some other race" or "multiracial." Persons who identified themselves as "some other race" or "multiracial" were provided with a write-in area where they could be more specific. Written entries were reviewed, edited, and coded by subject matter specialists. 15
In terms of comparability then, the 1990 decennial census had a response category termed "other race." In the ACS this category is instead termed "some other race." As part of the effort to provide research useful to the OMB's review of Statistical Directive No. 15, the ACS also included a "multiracial" category on the race question. For weighting purposes we collapsed the full ACS race categories into the common four categories, white, black, AIEA and API to be consistent with the MARS file. For simplicity, for weighting purposes only, we chose to collapse the "some other race" and "multiracial" categories, in the ACS, with the white category. Results may show that this technique will need to be explored more fully in the future.
In sum, it is clear that there are important methodological differences between the American Community Survey, as implemented in the four 1996 test sites, and the intercensal population estimates which rely largely on the 1990 decennial census. In terms of temporal and residence rules the Census Bureau maintains that few substantial variations should appear in the two data sources as a result of these concept variations.
In addition to the differences in residence, temporal and race/ethnicity concepts we need to remember that the ACS results are products of sample surveys. Using sampling methodology the data in the ACS products are estimates of the actual figures that would have been obtained by interviewing the entire population. The estimates from the chosen sample also differ from other samples of housing units and persons within those housing units. The possibility of sampling errors arise due to the use of probability sampling, which is necessary to ensure the integrity and representativeness of sample survey results.
In addition to sampling error, other types of errors may appear during any of the various complex operations used to collect and process survey data. For example, operations such as editing, reviewing, or keying data from questionnaires may introduce error into the estimates. These and other sources of error contribute to the nonsampling error component of the total error of survey estimates. Nonsampling errors may affect the data in two ways. Errors that are introduced randomly increase the variability of the data. Systematic errors which are consistent in one direction introduce bias into the results of a sample survey. The Census Bureau protects against the effect of systematic errors on survey estimates by conducting extensive research and evaluation programs on sampling techniques, questionnaire design, and data collection and processing procedures. In addition, an important goal of the American Community Survey is to minimize the amount of nonsampling error through nonresponse for sample housing units. One way of accomplishing this is by following-up on mail nonrespondents during the CATI and CAPI phases.
Standard error is a measure of the deviation of a sample estimate from the average of all possible samples. Sampling errors and some types of nonsampling errors are estimated by the standard error. The sample estimate and its estimated standard error permit the construction of interval estimates within a prescribed confidence that the interval includes the average result of all possible samples. Direct estimates of the standard errors were calculated for all estimates which will be provided below. The standard errors, in most cases, are calculated using standard variance estimates software using a methodology that takes into account the sample design and estimation procedures.
In the next section the question of comparability will be raised; first stage weighted ACS results (after household weighting but before person weighting) for the four 1996 test sites will be compared to corresponding intercensal population estimates. Age, sex, race, and Hispanic origin characteristics will be explored to understand differences.