3. The Composite method is an estimating technique in which estimates of various age groups are derived separately and are then summed to secure a total for all ages. Death statistics are used to estimate the population in ages 45 and over, birth statistics are used to estimate the population in ages 18 to 44 and under 5, and the Census Bureau's Component Method II is used to estimate the population in ages 5 to 17 years. In the case of birth and death statistics, an arithmetic average of two years of data is used for both the base year (1970) and the estimate year (1971). The two year average is used to smooth out random fluctuations in the data which distort the estimates.

visional July 1, 1972, estimates for nonmetropolitan counties were developed by adding the change between 1971 and 1972 Component Method II estimates to the 1971 estimates. All counties were subsequently adjusted to agree with the provisional July 1, 1972, State estimate published in Current Population Reports, Series P-25, No. 488.

Corresponding estimates for other States in the program will be published as they become available. The jointly prepared estimates for counties shown here are presented by the Bureau of the Census in lieu of preparing competing estimates. These estimates for counties are consistent with independent State population totals regularly prepared by the Bureau of the Census and published in the P-25 series of reports.

County populations for 1970 reflect corrections in the census count made since tabulations of the census. The official 1970 resident State total shown on the table may differ slightly from the sum of the counties because of corrections made subsequent to the release of the State figure.

The estimates presented in the table have been rounded to the nearest hundred without being adjusted to the State total, which was independently rounded to the nearest thousand. Percentages are based on unrounded numbers.

This series of reports presents population estimates prepared under the auspices of the FederalState Cooperative Program for Local Population Estimates. The objective of this program is the development and publication of State-prepared estimates of the population of counties using uniform procedures largely standardized for data input and methodology. The methods used have been mutually agreed upon by the individual States and the Bureau of the Census and were selected on the basis of the results of an extensive test of methods against the 1970 census conducted in late 1971 and early 1972. 1

The estimates for July 1, 1971, and the provisional estimates for July 1, 1972, shown here for Illinois counties were prepared by the Bureau of General Administration, Illinois Department of Public Health. This agency was designated by the Governor to work with the Bureau of the Census in implementing and carrying out the FederalState Cooperative Program.

For a more detailed description of the program, see Meyer Zitter, "Federal-State Cooperative Program for Local Population Estimates, The Registrar and Statistician, U.S. Department of Health, Education, and Welfare, January 1968, and "Federal-State Cooperative Program for Local Population Estimates: Status Report, January 1971, The Registrar and Statistician, U.S. Department of Health, Education, and Welfare, April 1971. detailed analysis of the test results, see Current Population Reports, Series P-26, No. 21, "FederalState Cooperative Program for Local Population Estimates: Test Results--April 1, 1970.

For a

The estimates shown for July 1, 1971, are based on an average of the following methods:

1. The Regression (ratio-correlation) method is an estimating technique in which a multiple regression equation is used to relate changes in a number of different data series to change in population distribution. The series of data used in the Regression method for Illinois are resident births (X1), school enrollment (X2), and passenger car registration (X3). The prediction equation for Illinois for the 1970's is given by

= 0.0890+ 0.1297X+0.2507X2 +0.5461X3

2. The Census Bureau's Component Method II is an estimating technique which employs vital statistics to measure natural increase and school enrollment as a basis for measuring net migration. The estimates made by the Census Bureau's Component Method II are specific to the civilian population under 65, with Medicare statistics used to estimate the resident population ages 65 and over. The total resident population is derived by adding estimates of the military station strength in each county to the estimates of the civilian resident population.

The provisional July 1, 1972, estimates for the metropolitan counties were developed by adding the average change between 1971 and 1972 Housing Unit method and Component Method II estimates to the 1971 estimate. (The Housing Unit method is an estimating technique in which estimates of the population are based on estimates of housing units. The latter, in turn, are based on data on

building permits issued, or on data on electric, gas, or water meter connections.) The provisional July 1, 1972, estimates for the nonmetropolitan counties were developed by adding the change between the 1971 and 1972 Component Method II estimates to the 1971 estimates. All counties were subsequently adjusted to agree with the provisional July 1, 1972, State estimate published in Current Population Reports, Series P-25, No. 488.

Corresponding estimates for other States in the program will be published as they become available. The jointly prepared estimates for counties shown here are presented by the Bureau of the Census in lieu of preparing competing estimates. These estimates for counties are consistent with

independent State population totals regularly prepared by the Bureau of the Census and published in the P-25 series of reports.

County populations for 1970 reflect corrections in the census count made since tabulations of the census. Counties with corrections of more than 500 are Cook, DuPage, and Will. The official 1970 resident State total shown on the table may differ slightly from the sum of the counties because of corrections made subsequent to the release of the State figure.

The estimates presented in the table have been rounded to the nearest hundred without being adjusted to the State total, which was independently rounded to the nearest thousand. Percentages

are based on unrounded numbers.