The Shelf Life of Official Sub-National Population Forecasts in England

Abstract

We measure the empirical distribution of the accuracy of projected population in sub-national areas of England, developing the concept of ‘shelf life’: the furthest horizon for which the subsequent best estimate of population is within 10% of the forecast, for at least 80% of areas projected. Since local government reorganisation in 1974, the official statistics agency has projected the population of each local government area in England: for 108 areas in nine forecasts up to the 1993-based, and for over 300 areas in 10 forecasts from the 1996-based to the 2014-based forecasts. By comparing the published forecast (we use this term rather than projection) with the post-census population estimates, the empirical distribution of errors has been described. It is particularly dependent on the forecast horizon and the type of local authority. For 10-year forecasts the median absolute percentage error has been 7% for London Boroughs and 3% for Shire Districts. Users of forecasts tend to have in mind a horizon and a required accuracy that is of relevance to their application. A shelf life of 10 years is not sufficient if the user required that accuracy of a forecast 15 years ahead. The relevant effective shelf life deducts the user’s horizon. We explore the empirical performance of official sub-national forecasts in this light. A five-year forecast for London Boroughs requiring 10% accuracy is already beyond its effective shelf life by the time it is published. Collaboration between forecasters and users of forecasts can develop information on uncertainty that is useful to planning.