The baseline cohorts used in the projections are single-year-of-age cohorts for males and females of Anglo, Black, Hispanic and Other racial/ethnic groups extracted from Summary Tape File 2 from the 1990 Census of Population and Housing. Population data for 1990 were used as the starting base because they provide the last complete count information available.

The baseline cohorts used for the Anglo, Black, Hispanic and Other populations were obtained by subtracting the number of Spanish-origin persons by age, sex and race from the total population by age, sex and race. The cohorts thus produced are Anglos (composed of White Non-Spanish-origin persons), Blacks (consisting of Black Non-Spanish-origin persons), Hispanics (Spanish-origin persons of all racial and ethnic groups), and Others (composed of persons of other racial and ethnic groups who are not of Spanish-origin). So constructed, the sum of populations in the four racial/ethnic groups of Anglo, Black, Hispanic, and Other are equal to the total population. All cohorts are for single years of age for ages 0-75+ for both sexes for each of these four racial/ethnic groups.

Although the 1990 Census Count provided useful baseline data for the projections, there is clear evidence of problems in the reporting of age and race/ethnicity in the 1990 Census. Particularly evident was a shifting of roughly 20 percent of those 0-1 years of age to older ages and the inclusion in the Other category of persons who were Anglo, Black, or Hispanic. Because of this problem, the U.S. Bureau of the Census found it necessary to adjust the age and race/ethnicity data for 1990 and provided modification ratios by age, sex, and race/ethnicity for each State. Analyses by the Bureau indicated that these ratios were similar across states and areas within states. These ratios were applied to all counties and controlled to the total State's modified population. Appendix A provides a description of the procedure used by the Census Bureau to complete the age and race/ethnicity adjustment. These modifications do not alter the total population values for counties or the State, but because the Census Bureau reconstructed the modified populations from individual census records which contain inferred characteristics, values are changed from the published 1990 counts for some age, sex, and race/ethnicity groups. For most areas, the modification results in a reduction in the 1990 Other population and an increase in Anglo, Black, and Hispanic racial/ethnic categories.

It was also necessary to adjust the base population for "special populations". Special populations are populations who reside in an area, usually in institutional settings, who do not generally experience the same demographic processes over time as the indigenous population in the area. Rather, they tend to come into and leave an area at fixed intervals. Examples of such populations are college populations, prison populations, military base populations, and other persons in institutional settings. Because their movement into and out of an area is a function of events (e.g., enrollment, graduation, incarceration) which are not determined by local socioeconomic conditions, special populations must be removed from the base populations of projection areas before birth, death and migration rates are applied to the base population. If special populations of substantial size are not removed, they will create distortions in age and other characteristics of the population that will remain in the population through the cohort aging process and create inaccuracies in the projections. Special populations are, therefore, generally removed from the cohort base, the base cohorts projected forward and a separate projection of the special population for the projection date is added to the projected base cohorts to obtain the projection of the total population.

In Texas, several continuing special population groups are especially large and must be removed from base populations. These are college and university populations, state prison populations, military populations, and populations in other State institutions. In the projections presented here, each of these groups was removed from the base population of the counties in which they are located by subtracting these special populations from the 1990 population reported in the Census (and modified as noted above) for these counties. Since these special populations must be subtracted from base populations that are age, sex and race/ethnicity specific, it was necessary to obtain age, sex and racial/ethnic detail for the special populations. This was done for the college populations by obtaining information on college enrollment for each public college and university in the State for 1990-1995 by age, sex and race/ethnicity from the Texas Higher Education Coordinating Board. For prisons, information on the age, sex and race/ethnicity of prisoners in each institution in 1990-95 was obtained from the Texas Department of Criminal Justice. For both college enrollments and prisons, projected values from the appropriate agencies (Texas Higher Education Coordinating Board and the Texas Department of Criminal Justice) for the periods after 1995 were incorporated in the projections. For other institutions, information on age, sex and race/ethnicity were obtained from the group quarters data from the 1990 Census STF2B file and updated with post- 1990 data.

For 1990, one additional special population group had to be removed from the 1990 base population before computing migration rates. This was the large population of illegal immigrants admitted under the amnesty provision of the Immigration Reform and Control Act (IRCA) of 1986. The group seeking amnesty in Texas was more than 400,000. To the extent that they were entrants into Texas since 1980 they are immigrants during 1980 to 1990, but assuming that IRCA's one time amnesty provision would be implemented periodically throughout the projection period was not feasible so that those who were not in Texas in 1980 or not counted in 1980 were removed from the base to eliminate an artificial inflation of migration rates. It is not clear what proportion of the IRCA admittees were counted in 1980 so it was uncertain how many should be subtracted from the base population. After extensive consultation with U.S. Bureau of the Census and U.S. Immigration and Naturalization Service demographers, it was decided that one-half would be assumed to have been counted in 1980 and one-half assumed to be one-time "special" event migrants and removed from the base before computing migration. However, because they are now permanent residents, unlike members of other special populations who are removed from the base population for purposes of computing future births, deaths and migration, IRCA admittees were used in the base population for computing births, deaths and migration. They are treated as special populations only for the purpose of computing the base migration rates for 1980 to 1990.

Given the distributions of the special populations by age, sex and race/ethnicity, it was then possible to subtract the special populations from the baseline 1980 and 1990 Census cohorts to obtain a baseline set of cohorts free from the influence of special populations. These procedures for baseline cohorts were completed for all counties in the State. However, following standard practice special populations were removed from the base population only when they made up five percent or more of the population of the area. For counties with special populations of sufficient size, the baseline cohorts without special populations are projected forward and projections of special populations for the projection years are added to the projections for the baseline cohorts to obtain projections of the total population.