Government Information Quarterly 26 (2009) 144–147

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Government Information Quarterly

j ourna l homepage: www.e lsev ie r.com/ locate /gov inf

Evaluating population estimates in the United States: Counting the populationbetween the censuses

Stephen WoodsPennsylvania State University, USA

E-mail address: swoods@psu.edu.1 The focus of this article is on population estimates an

of projections. There is a distinct difference betweeEstimates focus on the past and the present, whereas p

0740-624X/$ – see front matter © 2008 Elsevier Inc. Aldoi:10.1016/j.giq.2007.02.006

a b s t r a c t

a r t i c l e i n f o

Available online 26 October 2008

Keywords:

Population estimates play anneed to be proficient in evpopulation estimates as w

CensusPopulation estimatesStatisticsDemographics

ell as a discussion of general principles for evaluating and understandingpopulation estimates.

© 2008 Elsevier Inc. All rights reserved.

important role in shaping public policy and research. Librarians as well as usersaluating the limitations of the statistic. This article provides a brief history of

1. Introduction

The United States Census of Population and Housing is typicallyconsidered to be the most important source for population statisticsas well as information about a broad range of social and economiccharacteristics. The problem is that the survey only occurs every tenyears and often researchers and policy makers are interested in whatchanges have occurred in the time between the censuses. Forexample, the baby boom following the end of World War II made itnecessary for the government to consider and plan for the impactthis would have on the education, welfare and a myriad of othersocial issues.

An examination of the annual Statistical Abstract of the UnitedStates provides researchers and librarians with a yearly statisticalpopulation count from the U.S. Census Bureau known as thepopulation estimate.1 Often the statistic is used without anyconsideration for its limitations or knowledge of how it wascalculated. This problem is intensified when these statistics arepublished in secondary sources without any reference to the type ofstatistic that they represent.

2. Literature review

The literature on providing reference research support forpopulation estimates has mostly focused on brief descriptions of themajor statistical series published by the U.S. Government. Stratfordand Juri (1992) provides a succinct discussion of the statistical series

d does not include a discussionn estimates and projections.rojections focus on the future.

l rights reserved.

Population Estimates and Projections (P-25) and Local PopulationEstimates (P26) published by the U.S. Census Bureau. Sears andMarilyn (1994) also provide a description of both of these series intheir chapter on Population Statistics with a separate chapter devotedto projections. However, both of these publications erroneously claimthat the population estimates published by the U.S. Census Bureauwere calculated from the Current Population Survey.2 Moeller (2003)constructed an excellent index of the series and accurately states thatthe estimations are based on administrative records.

Statistical techniques for creating population estimates havebecome increasingly complex making it difficult for librarians andusers to evaluate the limitations of this unique type of statistic. Thepurpose of this article is to provide reference librarians and users withan overview of the history of population estimates as well as somegeneral principles to consider when using population estimates.3

3. Brief history

One of the justifications of instituting the permanent establishmentof the U.S Bureau of Census in 1902 was the mandate to publishaccurate annual population estimates for the nation, states, countiesand metropolitan areas.4 This became increasingly important as thenation grewand decisions needed to bemade for the annual allocationof Federal and State funds. Demands for administrative planning andmarketing guidance also played a key role in the development of thepopulation estimates program. Equally important was the emergence

2 The Current Population Survey is a sample survey taken by the U.S. Census Bureausince 1940 and later jointly with the Bureau of Labor Statistics.

3 These principles were taken from The Methods and Materials of Demography(volume 2) pages 728–729.

4 32 Stat. 51 Section 8.

Fig. 1. Estimate components.

145S. Woods / Government Information Quarterly 26 (2009) 144–147

of major government sample surveys that relied on accurate popula-tion estimates to provide benchmarks or survey controls.5

The earliest methods used by the Census Bureau for annualpopulation estimate were not very sophisticated. The Bureau beganissuing population estimates for cities, states and the nation in 1904for the years 1901 to 1903 making the assumption that annualincreases for years following 1900 were one-tenth of the decennialincreases from 1890 and 1900.6

In 1907, the Census Bureau used the returns from 14 state censusestaken in 1905 or 1904 to provide a benchmark for city, state andnational population estimates from 1904–1906.7 The Bureau revertedback to the one-tenthmethod for Reports issued by the Census Bureauin 1914,8 1916,9 191810 with one significant change. The enumerationfor the 1910 Census occurred on April 15 whereas the enumeration forthe 1900 Census occurred on June 1. Consequently, they divided thetotal increase by 118. 5 months andmultiplied by 12 to find the annualincrease. Population estimates published in 192311 also had to makeallowances for the fact that the 1920 Census was taken in January 1rather than April 15 meaning that the annual increase was calculatedby dividing the total increase by 116. 5 months and multiplying by 12.

Between 1923 and 1947, the Bureau sporadically published popula-tion estimates as part of a series titled Population, Special Reports usingsimilar techniques to adjust for the April 1st enumeration date for the1930 and 1940 Censuses.12 The completion of a nationwide death andbirth registration system in 1933 allowed the Census Bureau toexperiment with more sophisticated estimation techniques.13

The Census Bureau provided population estimates in a 1943publication, Vital Statistics Rates in the United States 1900 to 1940,based on the current technique called, Component Method. Thecomponents for measuring population estimates are on based fourmeasurements: total population counts from the decennial census

5 Surveys that rely on accurate population estimates include: Current PopulationSurvey, Survey of Income and Program Participation, Consumer Expenditure Survey,Hospital Discharge Survey, National Crime Victimization Survey, National HealthInterview Survey, and the National Hunting and Fishing and Wildlife AssociatedRecreation Survey.

6 This assumes that there is a uniform increase from year to year. See “Estimates ofpopulation of the larger cities of the United States for 1901, 1902, and 1903”, CensusBulletin C3.3:7.

7 “Estimates of population 1904, 1905, 1906” Census Bulletin 71 (C3.3:71).8 “Estimates of population 1910, 1911, 1912, 1913, 1914” Census Bulletin 122 (C

3.3:122).9 “Estimates of population of the United States 1910, 1911, 1912, 1913, 1914, 1915, and

1916” Census Bulletin 133 (C3.3:133).10 “Estimates of population of the United States 1910, 1911, 1912, 1913, 1914, 1915,1916 and 1917” Census Bulletin 138 (C3.3:138).11 Estimates of population of the United States: by states and cities, 1910 to 1923 andarea July 1, 1922. (C3.2:P 81/2).12 The SUDOC for this series is C 3.161.13 The reporting structures by states for births and deaths in the 19th century wereabysmal. Though Massachusetts and a few other states had begun building up deathand later birth registration systems it was not until 1933 that a nationwide cooperativesystem administered by the U.S. Bureau of Census was fully implemented.

(base data), adding births, subtracting deaths, and adding orsubtracting migration calculated from immigration and emigrationdata (see Fig. 1).14 The Bureau began regularly publishing populationestimates and projections in 1947 in a series popularly known as theCurrent Population Reports P-25 Series.15

4. Principles for evaluating population estimates

4.1. Quality and nature of the base data

The quality and nature of the base data of the population isextremely important for evaluating the accuracy of the estimates.Historically, the Census Bureau has used the decennial census as thebase data for its estimates. The one exception being 1907 as discussedearlier when they used individual state census data. However, thereare other ways to calculate the base data.

Some countries use a partial census or a sample survey as theirbased data. The Census Bureau is currently using the AmericanCommunity Survey (ACS) a sample survey to collect base data toprovide annual and multiyear estimates of the characteristics of thepopulation and housing.16 The Census Bureau will use the ACS toprovide demographic, housing, social and economic characteristicsevery year for states, cities, counties, metropolitan areas and cities.This is an enormous departure from historical methodology forgathering population estimates and there are arguments on each sidewhether the data will be as accurate. What is critical for the user tounderstand is that the method for collecting the base data greatlyaffects the accuracy of the ongoing collection of population estimates.

4.2. Migration and geography

The population estimates of a nation are much more accurate thanthe population estimates of smaller geographic areas such as counties,states and cities. The principal reason for this problem has to do withthe inaccuracy of calculating migration figures for smaller geographicareas.17

Over the years the Census Bureau has used different methods forcalculating the internal migration between sub-national geographies.Before the 1960s the Census Bureau used a method that utilizedschool enrollment statistics to estimate migration. In the 1960s theCensus Bureau used amore sophisticatedmodel that used vital events,school enrollment, tax returns, votes, motor vehicle registration andbuilding permits to calculate migration flows.18

The General Revenue Sharing Act of 1971 created a demand formore accurate sub-county estimates. Consequently, the CensusBureau in cooperation with state government formed the Federal-State Cooperative Program for Local Population Estimates (FSCPE).19

The FSCPE provides the Census Bureau with local statistics such asschool enrollment, vital statistics, and college dorm or prisonspopulations. Federal agencies provide tax records, Medicare recordsand some vital statistics. Once these are gathered, the FSCPE andCensus Bureau use statistical models to produce population estimatesfor smaller geographic areas.

14 The reporting structures by states for births and deaths in the 19th century wereabysmal. Though Massachusetts and a few other states had begun building up deathand later birth registration systems it was not until 1933 that a nationwide cooperativesystem administered by the U.S. Bureau of Census was fully implemented.15 For a nice concise description of this series go to Sherry Moller's website: http://www.lib.ohio-state.edu/sites/reference/govdocs/CPR/cprP25-1.htm.16 For a description and discussion of the American Community Survey, see www.census.gov/acs/.17 Early on the Census Bureau used the 1/10 method for calculating populationestimates for smaller geographic areas.18 This is why you will often find reports for voting, schools, and vital statistics in theCurrent Population Reports P-25 Series.19 For more information about the FSCPE see http://www.census.gov/population/www/coop/history.html.

Fig. 2. Estimate methods.

146 S. Woods / Government Information Quarterly 26 (2009) 144–147

4.3. Demographic characteristics

Accuracy of the population estimate is reduced by breaking downthe population into demographic characteristics such as sex, age, raceand ethnicity. Although the estimates may be “good enough”, usersneed to be aware that accuracy has been sacrificed by addingdemographic variables. Why do demographic variables affect theaccuracy of population estimates?

First, demographics add a layer of complexity inwhich poor qualitydata is often all that is available. For example, a county may havereported accurately the total births that occurred in a year, butneglected to collect the sex of the infants. Consequently, statisticswere used to interpolate a “best guess” breakdown by sex for thatparticular year. This means that the total population estimate is muchmore accurate than the population estimate by sex.

Second, variables such as race and ethnicity have historically beendifficult to consistently classify particularly over time. Alice Robbin(1999) states that the “theoretical, conceptual, definitional andmeasurement problems inherent in the classification of racial andethnic groups … are significant”. Finally, often the statistics that arecollected from sub-national geographic areas have to be statisticallyadjusted. For example, a county may have collected age statistics forthe age range 17–21 and another county may have collected statisticsfor the age range 18–21.

4.4. Problem of time

The period of time which has elapsed since the collection of thebase datewill have a major effect on the accuracy of the final estimate.For example, a population estimates for 1946 will be more accuratethan a population estimate in 1947 if the base population for both isthe 1940 decennial Census.

To address this problem, some population estimates will use apre-census technique to recalculate historical population estimates(see Fig. 2). This method uses the base data from a decennialcensus and essentially works backwards (pre-census) and a forwardfrom the base data from a different decennial census (post-census).This technique reduces the time error to five years and increasesthe accuracy of the population estimates. It is essential for the userto know which technique has been used to produce the populationestimate.

4.5. Armed forces

It is important to be aware that often the armed forces are notincluded in the population estimates of the U.S. Census Bureau, butare maintained as a separate statistic. This was not a statisticallysignificant issue until the 1940s and the growing number of servicemen and women as a result of World War II. Following the war, theexpansion of the military and the growing occupation of militarybases around the world necessitated a better understanding of howthis effected population growth and related issues of federal andstate funding. Also, with the admission of Alaska on January 3, 1959

and Hawaii on August 21, 1959 population numbers had to beadjusted, because military personnel were no longer considered“overseas”.

The Census Bureau began publishing data on the size anddistribution of the armed forces from statistics obtained from theDepartment of National Defense until 1949 when the NationalMilitary Establishment briefly took over responsibility until theemergence of the Department of Defense in 1949.

The collection of military population statistics was extremelycomplicate and took years to standardize. This is due to thetransitionary nature of the military personnel. For example, theBureau had to standardize how reserve forces were counted. They alsoneed to take into account how to consider crews of naval vessels inAmerican ports, foreign ports and those on the high seas. Thesestatistics and a discussion of their limitations are published in anannual publication of the P-25 series called “Components of Change”.

5. Conclusion

Population estimates from the U.S. Census Bureau can be anextremely useful statistic for researchers particularly those who arewanting to look at trends over time. It is important however thatlibrarians as well as researchers understand the limitations of thistype of statistic and how it might affect their outcomes. For example, ifa researcher is interested in the effect of perceived population on aparticular policy then it would be important to find the populationestimate reported by the Census Bureau at that time using a post-census methodology. It would be an egregious mistake to consider thepopulation estimate statistic reported by the Census Bureau laterusing a pre-census methodology.

References

Moeller, S.E. (2003). Current population reports and other population reports index.Retrieved September 27, 2006, from http://www.lib.ohio-state.edu/sites/reference/govdocs/CPR/CPRindex.html

Robbin, A. (1999). The problematic status of U.S. statistics on race and ethnicity: an“imperfect representation of reality”. Journal of Government Information, 26(5),467−483.

Sears, J., & Marilyn, M. (1994). Using government information sources: Print and electronic,2nd Phoenix Arizona: Oryx Press.

Stratford, J., & Juri, S. (1992). Major U.S. statistical series: Definitions, publications,limitations. Chicago and London: American Library Association.

Further reading

Bryan, T. (2000). U.S. Census Bureau population estimates and evaluation with loss offunctions. Statistics in Transitions, 4(4), 537−548.

Christenson, M. (2006). The official U.S. Census Bureau population estimates bydemographic characteristics: Requirements, evaluation, and future directions.Paper presented at the Population Association Annual Meeting March 30 throughApril 1, 2006, Los Angeles, California. Retrieved September 27, 2006, from http://paa2006.princeton.edu/download.aspx?submissionId=60206

Coale, A., & Zelink, M. (1963). New estimates of fertility and population in the UnitedStates: A study of annual white births from 1855 to 1960 and of completeness ofenumeration in the censuses from 1880 to 1960. Princeton: New Jersey: PrincetonUniversity Press.

Duncan, J. and William, S. (1978). Revolution in United States Government Statistics1926–1976. U.S. Department of Commerce (Office of Federal Statistical Policy andStandards).

Government Accounting Office (2004). American Community Survey: Key UnresolvedIssues Congressional Requesters United States Government Accountability Office.(GAO-05-82). Retrieved September 27, 2006, from http://www.gao.gov/new.items/d0582.pdf

Government Accounting Office (2006). Federal assistance: Illustrative simulations ofusing statistical population estimates for reallocating certain federal funding. (GAO-06-567). Retrieved September 27, 2006, from http://www.gao.gov/new.items/d06567.pdf

Judson, D. H. (2001). An evaluation of the accuracy of U.S. Census Bureau countypopulation estimates. Statistics in Transitions, 5(2), 205−235.

Long, J. (1993). Postcensal population estimates: States, counties, and places.(Population Division Working Paper No. 3). Retrieved September 27, 2006, fromhttp://www.census.gov/population/www/documentation/twps0003.html

147S. Woods / Government Information Quarterly 26 (2009) 144–147

Office of Federal Statistical Policy and Standards (1978). A framework for planning U.S.Federal Statistics for the 1980s. U.S. Department of Commerce.

Shryock, H. (1980). Population methods. In Henry Shryock, Jacob Siegal, & ElizabethLarmon (Eds.), The methods and materials of demography (pp. 725−770). U.S.Department of Commerce.

Smith, A. S. (1998). The American community survey and intercensal populationestimates: Where are the crossroad? (Population Division Technical Working Paper

NO. 31). Retrieved September 27, 2006, from http://www.census.gov/population/www/documentation/twps0031/twps0031.html

Stephen Woods has worked as a Social Science Librarian specializing in data andgovernment information at The Pennsylvania State University Libraries and Govern-ment Information Librarian at Idaho State University.