Introduction to Tools for Demographic Estimation
Tools for Demographic Estimation is the result of a project, funded by the United Nations Population Fund (UNFPA) and run under the auspices of the International Union for the Scientific Study of Population (IUSSP), to bring together in one place, and in a user-friendly style, key methods used by demographers everywhere to measure demographic parameters from limited and defective data.
The idea for Tools for Demographic Estimation first arose at a joint IUSSP/UNFPA meeting on ‘Applied and Technical Demographic Training in Developing Countries’ held in The Hague in March 2009, where concern was expressed that the training of demographers in the use and application of indirect estimation techniques was waning at almost every academic institution around the globe.
Several factors have contributed to this state of affairs. First, changing global population priorities, notably the revised agenda adopted by the International Conference on Population and Development held in Cairo in 1994, had altered the funding landscape, with more resources being devoted to the emergent fields of reproductive and sexual health rather than the technical demography required to study patterns of growth and to manage population increase. Associated with this, the cohort of demographers who had been trained in the classical methods and techniques was ageing rapidly and few younger demographers were being trained in either the science or the craft of demographic estimation from limited and defective data.
Second, the Demographic and Health Surveys programme (DHS), associated with the collection of full birth histories and attendant direct estimation methods for fertility and mortality, has created the impression that the tools and techniques for estimating mortality and fertility from census or other survey data were no longer as important as they had been in the past. While there can be no doubt that the DHS has contributed enormously to, and helped reshape, the discipline of demography, the growing marginalization of demographic analysis of census data and other demographic materials limits our ability to understand demographic dynamics in developing countries. The role of the census in providing a sampling frame for demographic surveys is often forgotten. Moreover, the typical sample size of most DHS means that precise estimates from such surveys are seldom available at spatial resolutions smaller than regions or provinces, while the information collected on relatively rare events (such as adult deaths) is usually too sparse to permit the derivation of robust estimates.
Third, in most parts of the developing world (sub-Saharan Africa being the notable exception), improvements in systems of vital registration and the collection of demographic data in censuses mean that the existing techniques of demographic estimation from limited and defective data are regarded as obsolete. It is certainly the case that in countries with complete and accurate registration of vital events and a series of reliable censuses, direct and continuous estimation of demographic parameters becomes possible. In many low-income and middle-income countries, however, neither condition yet prevails and so it remains important to evaluate critically the quality of registration-based statistics and cross-check them against census-based questions on fertility and mortality.
A further reason for the decline in the priority accorded to the teaching of indirect techniques of demographic estimation is the natural evolution of populations where even in the poorest countries, fertility is falling after several decades of mortality improvement. The age distributions of these populations are thus far from the theoretical stable or even quasi-stable population model so that many of the techniques based on such models and first formally published by the UN Population Division (1967) are clearly outmoded. This demise of so-called stable population analysis led some analysts to prefer the DHS-style direct estimation methods over the whole suite of methods developed initially by Ansley J Coale and William Brass, authors of the early United Nations volumes.
In many instances, direct demographic estimation from census, survey, or vital registration data remains impossible or problematic. This implies a continuing need for census-based and other indirect estimates. The 2009 meeting further noted that the canonical manual for demographic estimation from census data, Manual X (UN Population Division 1983), was more than a quarter of a century old and that several new methods and techniques had been developed since its publication. Two other manuals have been prepared since, Estimating Demographic Parameters from Census Data (Sloggett, Brass, Eldridge et al. 1994) and Methods for Estimating Adult Mortality (UN Population Division 2002) but neither attempted a full and comprehensive revision and update of Manual X.
The meeting at The Hague therefore resolved that a project be initiated to revise and update Manual X. Following a competitive call for proposals evaluated by the IUSSP, a consortium of demographers based at the University of Cape Town and the London School of Hygiene & Tropical Medicine and independent demographers associated with Harvard University was awarded the contract to develop the material. Tools for Demographic Estimation is the result.
The material presented here follows in a direct line of descent from Manual X and the rationale underpinning the work is fundamentally the same – to set out the methods for estimating demographic parameters from limited or defective data. We therefore strongly urge users of Tools for Demographic Estimation to read the introductory chapter to Manual X (available on the UN Population Division website) both for its description of the need for and history of indirect estimation methods, and for its discussion of the limitations of reference works of this kind.
Tools for Demographic Estimation differs from its precursors in several important respects. The differences stem in part from the enormous increases in computing power available to analysts since the time Manual X was published. They also reflect advances in approaches to demographic estimation, new methods, and the evolution of insights into how well different methods work, and under what conditions. Thus, the methods described in Tools for Demographic Estimation and the earlier manuals are not the same. A number of methods that have been developed since the publication of Manual X are presented here for the first time. Other methods that were presented in Manual X have been excluded on the grounds that they have since been found to work poorly or that more refined or newer methods render them obsolete.
Second, unlike its precursors, Tools for Demographic Estimation is primarily an electronic, web-based, resource. The print version represents the material on the project’s website (demographicestimation.iussp.org) at the date of printing. The website, however, is designed to be dynamic, updated and changing over time. It follows that, whenever possible, the reader’s primary point of reference should be the website, rather than the print version of the manual. The website, hosted by the IUSSP, is freely and readily accessible to anyone on registration.
Third, the website includes downloadable spreadsheets that implement the methods described, so as to facilitate their application and use. The decision to implement the methods using spreadsheets rather than in the form of downloadable executable programmes (such as, for example, MortPak) is intended to ensure a maximum degree of transparency. The formulae and calculations are visible to the end-user, and the spreadsheets can be modified by users if they do not exactly match the data available. The spreadsheets are in Microsoft Excel format but have been designed to be compatible with other open-source spreadsheet applications. Only in exceptional circumstances have Excel-specific facilities (such as Solver) been employed.
A fourth difference from earlier manuals on indirect estimation is that while Tools for Demographic Estimation adopts much the same approach as its precursors in providing step-by-step descriptions on how to apply the methods covered, a greater degree of emphasis has been placed on setting out the assumptions underlying each of the methods, as well as the situations and conditions under which the methods may be contra-indicated, or may produce unreliable results. To assist users interested in understanding how the methods work, we have endeavoured to present the mathematical derivation of the methods in as accessible a style as possible.
Fifth, Tools for Demographic Estimation incorporates material on the assessment and measurement of migration using census data, an area not covered at all in Manual X, and last described in a work of this kind in Manual VI (UN Population Division 1970).
Despite these advances, the present work suffers from many of the same limitations as its precursors. In presenting each method separately, the bigger picture associated with demographic estimation from limited and defective data is all too often lost. A significant component of this kind of demographic work lies in piecing together a puzzle composed of demographic parameters from multiple methods and sources into a coherent, internally-consistent whole. Demographic estimation of the kind presented here is, ultimately, as much a craft as it is a science. Where possible, we have sought to give a sense of the craft involved. To facilitate and encourage the careful application of the methods described here, the website also includes discussion forums, which we hope will provide a vehicle for discussion of the results from applications of the methods presented, for suggestions for modifications or corrections to existing methods, and for proposals for new approaches to demographic estimation from limited and defective data.
Tools for Demographic Estimation has been a work long in preparation. The editors record their gratitude to the many people and organisations that have helped bring the project to fruition. We note the contributions of Ralph Hakkert (UNFPA) and Mary Ellen Zuppan (IUSSP) in securing funding for and overseeing the project; of the anonymous reviewers appointed by the IUSSP who offered extensive and useful comments on the initial draft of the material; of the web designer (Charles Oertel) and book designer (Jo-Anne Friedlander); and of the proof-reader (Debbie Budlender). We are also exceedingly grateful to those responsible for the UN Manuals as well as the Statistical Institute for Asia and the Pacific for waiving copyright and allowing us to reproduce material from those resources where necessary.
Tom Moultrie, Rob Dorrington, Allan Hill,
Kenneth Hill, Ian Timæus and Basia Zaba
Cape Town, July 2013
Sloggett A, W Brass, SM Eldridge, IM Timæus, P Ward and B Zaba (eds). 1994. Estimation of Demographic Parameters from Census Data. Tokyo: Statistical Institute for Asia and the Pacific.
UN Population Division. 1967. Manual IV: Methods for Estimating Basic Demographic Measures from Incomplete Data. New York: United Nations, Department of Economic and Social Affairs, ST/SOA/Series A/42.
UN Population Division. 1970. Manual VI: Methods of Measuring Internal Migration. New York: United Nations, Department of Economic and Social Affairs, ST/SOA/Series A/47.
UN Population Division. 1983. Manual X: Indirect Techniques for Demographic Estimation. New York: United Nations, Department of Economic and Social Affairs, ST/ESA/SER.A/81.
UN Population Division. 2002. Methods for Estimating Adult Mortality. New York: United Nations, Department of Economic and Social Affairs, ESA/P/WP.175.