Evaluation of Sub-National Population Projections: a Case Study for London and the Thames Valley

Abstract

Sub-national population projections help allocate national funding to local areas for planning local services. For example, water utilities prepare plans to meet future water demand over long-term horizons. Future demand depends on projected populations and households and forecasts of per household and per capita domestic water consumption in supply zones. This paper reports on population projections prepared for a water utility, Thames Water, which supplies water to over nine million people in London and the Thames Valley. Thames Water required an evaluation of the accuracy of the delivered projections against alternatives and estimates of uncertainty. The paper reviews how such evaluations have been made by researchers. The factors leading to variation in sub-national projections are identified. The methods, assumptions and results for English sub-national areas, used in five sets of projections, are compared. There is a consensus across projections about the future fertility and mortality but varying views about the future impact of internal and international migration flows. However, the greatest differences were between projections using ethnic populations and those using homogeneous populations. Areas with high populations of ethnic minorities were projected to grow faster when an ethnic-specific model was used. This result is important for assessing projections for countries housing diverse populations with different demographic profiles. Historic empirical prediction intervals are used to assess the uncertainty of the London and the Thames Valley projections. By 2101 the preferred projection suggests that the population of the Thames Water region will have grown by 85% within an 80% empirical prediction interval between 45 and 125%.

Household Forecasts for the Planning of Long‐Term Domestic Water Demand: Application to London and the Thames Valley

“…There are differences between the projections in input data, methods and assumptions which are difficult to disentangle for projections produced by different teams as explained in Rees, Clark, Wohland, and Kalamandeen (). So, the following interpretations are suggestive rather than definitive.…”

“…The projected population of the TW region in 2101 employed in this paper (WRMP2019‐D), using a 2014‐base is higher at 185%. Rees et al () argue that this is because this projection reflects the result of including ethnic heterogeneity in the method, which means higher growth for the London and Slough‐Wycombe‐Aylesbury water resource zones.…”

“…Uncertainty in forecasts can be assessed through formal error prediction methods or through variant projections. Elsewhere we apply historical errors in UK sub‐national projections developed by UKWIR () to the household forecasts (Rees et al, ). Table has presented results for a set of variants for each WRMP round.…”

Education and demography: a review of world population and human capital in the 21st century

“…Currently, comparisons of WIC model results with UN, World Bank, or US Census Bureau projections have focused on differences in the assumptions underlying the outputs (Abel et al 2016). Studies on country-based sub-national projections have explored the consequences of alternative model designs with the same assumptions (Rogers 1976;van Imhoff et al 1997;Wilson and Bell 2004;Wilson 2015), while another study has discussed the difficulties of making comparisons between published projections if model design differences are not considered (Rees et al 2018). A recent systematic review of this question with applications of models of different degrees of complexity and assumptions for India is given in KC et al (2018).…”

“…A recent systematic review of this question with applications of models of different degrees of complexity and assumptions for India is given in KC et al (2018). However, Rees et al (2018) has called such comparisons 'interpretative', as they are based on subjective judgements, rather than on 'controlled' experiments (e.g. Wilson and Bell 2004).…”