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Developing strategies for deriving small population fertility rates

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Abstract

Many agencies require population estimates and projections by ethnic group. These projections need ethnic-specific, age-specific fertility rates (ASFRs) but their inclusion is challenging since ethnicity is not recorded at birth registration. In this paper maternity data are used in a case study of electoral wards in Bradford, West Yorkshire, to develop fertility rates for small populations for a 1991 based projection. The challenge is to capture local variations in fertility by ethnic group when data are sparse. Small areas were grouped together using cluster analysis to define combinations with similar sociodemographic and fertility experiences so that sparse data could be aggregated to estimate reliable ethnic-specific fertility rates. For comparison, the data were aggregated into the 1991 Office for National Statistics area type classification. Fertility rates by single year of age for all area types were smoothed using the Hadwiger function. For the White ethnic group there were sufficient births to create ethnic-specific, ward-level ASFRs. For other ethnicities grouping of areas was necessary. The accuracy of the ASFRs in predicting births was assessed using mean absolute percentage error. Results show that for some minority groups district-level ethnic-specific fertility rates produced the most accurate birth estimates even though they were based on a larger area. This implies that rates created may be informative about the local area for White ethnic type but not in the same way for smaller ethnic groups. In terms of grouping strategies we recommend that existing classifications are assessed to determine how well variations in rates are stratified before embarking on a custom scheme. Where population sub-groups are small in some areas, it may be more reliable to use rates derived for larger areas and apply these to local populations. Inevitably, the rates used in a projection are a compromise but hopefully will still capture important dimensions of population change.

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Acknowledgments

This work is based on Lee Williamson’s Ph.D thesis (Williamson 2007), a CASE studentship (S42200124001) funded by ESRC and Bradford Metropolitan District Council. Paul Norman’s research is funded by ESRC grants RES-163-25-0032 and RES-189-25-0162. The authors would like to thank Bradford Metropolitan District Council for allowing access to the Bradford Birth Statistics Database and the forecasting software ‘POPGROUP’ that enabled this research. 1991 Census data were obtained through the MIMAS CASWEB facility and the GIS boundary data through EDINA’s UKBORDERS facility, academic services supported by ESRC and JISC. The Census, official Mid-Year Estimates and Vital Statistics have been provided by ONS and the digital boundary data by OSGB. These data are Crown copyright and are reproduced with permission of the Office of Public Sector Information. The authors are very grateful to two anonymous referees whose useful comments helped to improve this work.

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  1. School of Geography & Geosciences, University of St Andrews, North Street, St Andrews, KY16 9AL, Scotland, UK

    Lee Williamson

  2. School of Geography, University of Leeds, Leeds, LS2 9JT, UK

    Paul Norman

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Williamson, L., Norman, P. Developing strategies for deriving small population fertility rates. J Pop Research 28, 129–148 (2011). https://doi.org/10.1007/s12546-011-9059-0

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