Population Estimates and Projections in the U. S.

1. Population Estimates and Projections in the U. S. John F. Long www.usademography.com

2. Population Estimates and Projections • Both Estimates and Projections • Calculate the size of the population in the absence of complete demographic data • May also calculate demographic characteristics • Are used for planning and policy decisions • Population Projections • Project trends beyond the time interval of the input data • Use mathematical models of varying sophistication • Often use models of components of demographic change: births, deaths, migration, and immigration • Population Estimates • Estimate population using data from the same time period • May use demographic data: births and deaths • May use indicator variables – housing, administrative records

3. Projection: Simple Extrapolation Px+1t+1 = (Pxt ) ((Pxt )/(Px-1t-1)) where, Pxt is the population age x in a given place at time t

4. U.S. Population: Census Counts from 1790 and Projections for 1860-1930 Source: U.S Census Bureau, Decennial Censuses 1790-1930 ; Abraham Lincoln, “Annual Message to Congress: December 1, 1862”.

5. Population Projections UsingComponents of Change • Begins with a measured base population (census, etc) • Calculates trends in births, death, migration • Usually based on cohort-component method • May used multi-regional projection matrix methods • Accepted method for official U.S. population projections

6. Births Migration Migration Deaths

7. Component-based Projections Px+1t+1 = Pxt + B (if x<0) - Dx + Ix - Ox where, Pxt is the population age x in a given place at time t B is the number of births in a given place between t and t+1 Dx is then number of deaths age x in a given place between t and t+1 Ix is the number of in-migrants age x to a given place between t and t+1 Ox is the number of out-migrants age x from a given place between t and t+1.

8. Development of Demographic Accounting Approach • Nationwide birth and death registration began in the early 1900’s • Estimates and projections for states, counties, and places developed in the last half of the 20th century • Demographic Analysis became a key tool to evaluate decennial census quality • “Integrated” accounting system implicit in the American Community Survey for the 21st century

9. Elements of US Demographic Accounting System • Master Address Files • Decennial Censuses • Birth and Death Registers (NCHS) • Immigration Data (INS & ACS) • Domestic Migration Data (IRS) • Adjustments and Corrections • American Community Survey

10. Master Address Files • Importance of tying population to geography • Address canvassing prior to census year • Corrections during census operation • Updates between censuses • Provides “frame” for census and survey operations • Issues with permits, construction, demolitions, and conversions • Addresses vs. physical location.

11. Decennial Census Counts • Base on which all of system is built • Provides ground truth check of accounting system • Key demographic data collected: location, relationship, sex, age, race, Hispanic origin

12. Measure Death and Birth Components • Birth Registration • Ties birth to place of residence of mother • Race data is for parents not child – and uses older classification system • Completeness of registration is assumed • Death registration • Ties death to place of residence or last address? • Race is not self reported • Completeness and speed of tabulation is an issue

13. Estimating International Migration Flows • Legal immigration • Difference between residence change and legal status change • Problems with place of intended residence • Completeness issues • Legal Emigration • Lack of data collection when leaving the country • Problems of using other countries data • Illegal immigration • Methods are often indirect • Question of census coverage • Alternative data from the American Community Survey

14. Measuring Domestic Migration • Use of matched IRS returns to provide county in and out migration • Coverage of IRS data • Assigning age, race, hispanic origin, and sex data • Timing of data collection and tabulation

15. Develop Yearly Population Estimates and Controls • Develop annual national, state, county, and place data for funding and other administrative uses • Provide county age, race, sex, Hispanic origin controls for the American Community Survey • Issues of timeliness and accuracy of input data

16. Problems with an Accounting System • Errors in specifying the system can multiply over time • Errors in data input can cause faulty results • Structure of the system may change over time

17. Reconcile with new decennial census data • Need to recalibrate estimates from demographic accounting with decennial census results • Census coverage error vs. population estimation error • Incorporating “error of closure” corrections into new estimates.

18. Reconciling Inventory and Accounting Systems • Audit of the current demographic accounts • Differences with a Population Register • Potential inconsistencies in the system • Future Hazards and Opportunities