THEORY AND METHODS

Evaluation of methodologies for small area life expectancy estimation

D Eayres¹ and E S Williams²

¹ National Centre for Health Outcome Development, London School of Hygiene and Tropical Medicine, London, UK
² Adur, Arun and Worthing Primary Care Trust, Goring by Sea, Worthing, West Sussex, UK

Correspondence to:
Correspondence to:
Mr D P Eayres
National Centre for Health Outcome Development, London School of Hygiene and Tropical Medicine, 99 Gower Street, London WC1E 7HT, UK: daniel.eayres{at}lshtm.ac.uk

Study objective: To evaluate methods for calculating life expectancy in small areas, for example, English electoral wards.

Design: The Monte Carlo method was used to simulate the distribution of life expectancy (and its standard error) estimates for 10 alternative life table models. The models were combinations of Chiang or Silcocks methodology, 5 or 10 year age intervals, and a final age interval of 85+, 90+, or 95+.

Setting: A hypothetical small area experiencing the population age structure and age specific mortality rates of English men 1998–2000.

Participants: Routine mortality and population statistics for England.

Main results: Silcocks and Chiang based models gave similar estimates of life expectancy and its standard error. For all models, life expectancy was increasingly overestimated as the simulated population size decreased. The degree of overestimation depended largely on the final age interval chosen. Life expectancy estimates of small populations are normally distributed. The standard error estimates are normally distributed for large populations but become increasingly skewed as the population size decreases. Substitution methods to compensate for the effect of zero death counts on the standard error estimate did not improve the estimate.

Conclusions: It is recommended that a population years at risk of 5000 is a reasonable point above which life expectancy calculations can be performed with reasonable confidence. Implications are discussed. Within the UK, the Chiang methodology and a five year life table to 85+ is recommended, with no adjustments to age specific death counts of zero.

Keywords: life expectancy; small areas; Monte Carlo method

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