Errors from past rounds of population projections can provide both diagnostic information to improve future projections as well as information for users on the likely uncertainty of current projections. This paper assesses the forecast accuracy of official Australian Bureau of Statistics (ABS) population projections for the states and territories of Australia and is the first major study to do so. For the states and territories, it is found that, after 10-year projection durations, absolute percentage errors lie between about 1% and 3% for the states and around 6% for the territories. Age-specific population projections are also assessed. It is shown that net interstate migration and net overseas migration are the demographic components of change which contributed most to forecast error. The paper also compares ABS projections of total population against simple linear extrapolation, finding that, overall, ABS projections just outperformed extrapolation. No identifiable trend in accuracy over time is detected. Under the assumption of temporal stability in the magnitude of error, empirical prediction intervals are created from past errors and applied to the current set of ABS projections. The paper concludes with a few ideas for future projection rounds. 1. Introduction Population projections for states, provinces, statistical regions, and other major subnational areas are produced periodically by government agencies for a variety of planning purposes. Ex-post evaluations of the forecast accuracy of such projections remain comparatively rare, even though there are good reasons for undertaking such work. For example, they may reveal hitherto concealed deficiencies in methods, assumption setting, or data which can be resolved in future projection rounds. In addition, past errors can provide users with guidance on the approximate magnitude of error which current forecasts might be subject to. Ideally, past errors would be used to create prediction intervals for current forecasts, either in the form of empirical prediction intervals based directly on the observed error distributions [1, 2] or in the form of fully probabilistic forecasts in which error distributions for demographic rates are represented by time series models [3, 4]. Population projections for Australia’s states and territories have been published on a regular basis by the Australian Bureau of Statistics (ABS) since the late 1970s. They constitute the official population projections for Australia, are generally highly regarded, and are widely used and cited by government, researchers, business,
References
[1]
M. A. Stoto, “The accuracy of population projections.,” Journal of the American Statistical Association, vol. 78, no. 381, pp. 13–20, 1983.
[2]
S. Smith and S. Rayer, “Projections of Florida population by county, 2009–2035,” Florida Population Studies 156, Bureau of Economic and Business Research, University of Florida, Gainesville, Fla, USA, 2010, Vol. 34, http://www.news-press.com/assets/pdf/A415294132.PDF.
[3]
N. Keilman, D. Q. Pham, and A. Hetland, “Why population forecasts should be probabilistic-Illustrated by the case of Norway,” Demographic Research, vol. 6, pp. 409–453, 2002.
[4]
M. Bell, T. Wilson, and E. Charles-Edwards, “Australia's population future: probabilistic forecasts incorporating expert judgement,” Geographical Research, vol. 49, no. 3, pp. 261–275, 2011.
[5]
A. Y. Adam, “The abs population projections: overview and evaluation,” Journal of the Australian Population Association, vol. 9, no. 2, pp. 109–130, 1992.
[6]
Australian Bureau of Statistics (ABS), “Past ABS projections: how well have they matched reality,” in Population Projections, Australia: 1999 to 2101. Catalogue No. 3222.0. ABS, pp. 147–152, ABS, Canberra, 2000.
[7]
T. Wilson, “The forecast accuracy of Australian Bureau of Statistics national population projections,” Journal of Population Research, vol. 24, no. 1, pp. 91–117, 2007.
[8]
M. Bell and J. Skinner, “Forecast accuracy of australian subnational population projections,” Journal of the Australian Population Association, vol. 9, no. 2, pp. 207–235, 1992.
[9]
Statistics New Zealand, How Accurate are Population Projections? An Evaluation of Statistics New Zealand Population Projections, 1991–2006, Statistics New Zealand, Wellington, New Zealand, 2008.
[10]
Office for National Statistics (ONS), “Subnational population projections accuracy report,” Tech. Rep., ONS, London, UK, 2008, http://www.statistics.gov.uk/downloads/theme_population/SNPP_Accuracy_Report.pdf.
[11]
P. Rees, M. Kupiszewski, H. Eyre, T. Wilson, and H. Durham, “The evaluation of regional population projections for the European Union,” ERDF Study 97/00/74/018, 1999.
[12]
S. K. Smith and S. Rayer, An evaluation of population forecast errors for Florida and its counties, 1980–2010, Bureau of Economic and Business Research, University of Florida, Gainesville, Fla, USA, 2011.
[13]
C. Wang, “Evaluation of census bureau's 1995–2025 state population projections,” Working Paper 67, US Census Bureau, Washington DC, USA, 2002.
[14]
Australian Bureau of Statistics (ABS), “Projections of the population of the States and Territories of Australia, 1978–2011,” Catalogue Number 3214.0, ABS, Canberra, Australia, 1979.
[15]
Australian Bureau of Statistics (ABS), “Projections of the population of the States and Territories of Australia, 1981–2021,” Catalogue Number 3214.0, ABS, Canberra, Australia, 1983.
[16]
Australian Bureau of Statistics (ABS), “Projections of the population of Australia, States and Territories, 1984 to 2021,” Catalogue Number 3222.0, ABS, Canberra, Australia, 1985.
[17]
Australian Bureau of Statistics (ABS), “Projections of the population of Australia, States and Territories, 1987 to 2031,” Catalogue Number 3222.0, ABS, Canberra, Australia, 1989.
[18]
Australian Bureau of Statistics (ABS), “Projections of the population of Australia, States and Territories, 1989 to 2031,” Catalogue Number 3222.0, ABS, Canberra, Australia, 1990.
[19]
Australian Bureau of Statistics (ABS), “Projections of the population of Australia, States and Territories, 1993 to 2041,” Catalogue Number 3222.0, ABS, Canberra, Australia, 1994.
[20]
Australian Bureau of Statistics (ABS), “Projections of the population of Australia, States and Territories, 1995 to 2051,” Catalogue Number 3222.0, ABS, Canberra, Australia, 1996.
[21]
Australian Bureau of Statistics (ABS), “Population Projections, 1997 to 2051,” Catalogue Number 3222.0, ABS, Canberra, Australia, 1998.
[22]
Australian Bureau of Statistics (ABS), “Population projections, Australia, 1999 to 2101,” Catalogue Number 3222.0, ABS, Canberra, Australia, 2000.
[23]
Australian Bureau of Statistics (ABS), “Population Projections, Australia, 2002 to 2101,” Catalogue Number 3222.0, ABS, Canberra, Australia, 2003.
[24]
Australian Bureau of Statistics (ABS), “Population projections, Australia, 2004 to 2101,” Catalogue Number 3222.0, ABS, Canberra, Australia, 2005.
[25]
Australian Bureau of Statistics (ABS), “Population projections, Australia, 2006 to 2101,” Catalogue 3222.0, ABS, Canberra, Australia, 2008.
[26]
S. K. Smith, “Tests of forecast accuracy and bias for county population projections.,” Journal of the American Statistical Association, vol. 82, no. 400, pp. 991–012, 1987.
[27]
S. Openshaw and G. A. Van Der Knaap, “Small area population forecasting: some experience with British models.,” Tijdschrift voor Economische en Sociale Geografie, vol. 74, no. 4, pp. 291–304, 1983.
[28]
S. K. Smith and M. Shahidullah, “An evaluation of population projection errors for census tracts.,” Journal of the American Statistical Association, vol. 90, no. 429, pp. 64–71, 1995.
[29]
Australian Bureau of Statistics (ABS), “Population estimates: concepts, sources and methods,” Catalogue Number 3228.0.55.001, ABS, Canberra, Australia, 2009.
[30]
Australian Bureau of Statistics (ABS), “Demographic estimates and projections: concepts, sources and methods,” Catalogue Number 3228.0, ABS, Canberra, Australia, 1999.
[31]
N. Keilman, “How accurate are the United Nations world population projections,” in Frontiers of Population Forecasting, W. Lutz, J. W. Vaupel, and D. A. Ahlburg, Eds., vol. 24 of Population and Development Review, pp. 15–41, Population Council, New York, NY, USA, 1999.
[32]
S. Kolassa and W. Schutz, “Advantages of the MAD/mean ratio over the MAPE,” Foresight, no. 6, pp. 40–43, 2007.
[33]
S. H. Murdock, F. L. Leistritz, R. R. Hamm, S. S. Hwang, and B. Parpia, “An assessment of the accuracy of a regional economic-demographic projection model,” Demography, vol. 21, no. 3, pp. 383–404, 1984.
[34]
“Evaluating forecasting methods,” in Principles of Forecasting: A Handbook for Researchers and Practitioners, J. S. Armstrong and J. S. Armstrong, Eds., pp. 443–472, Kluwer Academic Publishers, Norwell, Mass, USA, 2001.
[35]
D. A. Swanson and J. Tayman, “Between a rock and a hard place: the evaluation of demographic forecasts,” Population Research and Policy Review, vol. 14, no. 2, pp. 233–249, 1995.
[36]
T. Wilson and F. Rowe, “The forecast accuracy of local government area population projections: a case study of Queensland,” Australasian Journal of Regional Studies, vol. 17, no. 2, pp. 204–243, 2011.
[37]
W. Lutz, W. Sanderson, and S. Scherbov, “The end of world population growth,” Nature, vol. 412, no. 6846, pp. 543–545, 2001.
[38]
J. Bongaarts and R. A. Bulatao, Eds., Beyond Six Billion, National Academy Press, Washington, DC, USA, 2000.
[39]
R. A. Bulatao, “Visible and invisible sources of error in world population projections,” in Proceedings of the IUSSP Conference, pp. 18–24, Salvador, Brazil, August 2001.
[40]
N. Keilman, “Ex-post errors in official population forecasts in industrialized countries,” Journal of Official Statistics, vol. 13, pp. 245–277, 1997.
[41]
S. K. Smith and J. Tayman, “An evaluation of population projections by age,” Demography, vol. 40, no. 4, pp. 741–757, 2003.
[42]
Australian Bureau of Statistics (ABS), “Migration, Australia, 2009-10,” Catalogue Number 3412.0,, ABS, Canberra, Australia, 2011.
[43]
N. Keilman, “Erroneous population forecasts,” in Perspectives on Mortality Forecasting. II. Probabilistic Methods, N. Keilman, Ed., pp. 7–26, Swedish Insurance Agency, Stockholm, Sweden, 2005.
[44]
C. Shaw, “Fifty years of United Kingdom national population projections: how accurate have they been?” Population trends, no. 128, pp. 8–23, 2007.
[45]
N. Keilman, “European demographic forecasts have not become more accurate over the past 25 years,” Population and Development Review, vol. 34, no. 1, pp. 137–153, 2008.
[46]
N. Keyfitz, “The limits of population forecasting,” Population and Development Review, vol. 7, no. 4, pp. 579–593, 1981.
[47]
S. Rayer, S. K. Smith, and J. Tayman, “Empirical prediction intervals for county population forecasts,” Population Research and Policy Review, vol. 28, no. 6, pp. 773–793, 2009.
[48]
S. K. Smith and T. Sincich, “Stability over time in the distribution of population forecast errors,” Demography, vol. 25, no. 3, pp. 461–474, 1988.
[49]
J. Tayman, “Assessing uncertainty in small area forecasts: state of the practice and implementation strategy,” Population Research and Policy Review, pp. 1–20, 2011.
[50]
P. McDonald and R. Kippen, “Forecasting births,” ABS Australian Census Analytic Program, Catalogue Number 2051.0, ABS, Canberra, Australia, 2011.
[51]
Department of Immigration and Citizenship (DIAC), The Outlook for Net Overseas Migration, DIAC, Canberra, Australia, 2011.
[52]
M. Bell, Understanding Internal Migration, Australian Government Publishing Service, Canberra, Australia, 1996.
[53]
G. Hugo, K. Harris, E. Bamford, and J. Nottage, Internal Migration in Australia, GISCA-The National Centre for Social Applications of GIS, The University of Adelaide, Adelaide, Australia, 2005.
