Evaluation of Alternative Cohort-Component Models for Local Area Population Forecasts

Abstract

It is generally accepted by demographers that cohort-component projection models which incorporate directional migration are conceptually preferable to those using net migration. Yet net migration cohort-component models, and other simplified variations, remain in common use by both academics and practitioners because of their simplicity and low data requirements. While many arguments have been presented in favour of using one or other type of model, surprisingly little analysis has been undertaken to assess which tend to give the most accurate forecasts. This paper evaluates five cohort-component models which differ in the way they handle migration, four of which are well known, with one-a composite net migration model-being proposed here for the first time. The paper evaluates the performance of these five models in their unconstrained form, and then in a constrained form in which age-sex-specific forecasts are constrained to independent total populations from an extrapolative model shown to produce accurate forecasts in earlier research. Retrospective forecasts for 67 local government areas of New South Wales were produced for the period 1991-2011 and then compared to population estimates. Assessments of both total and age-specific population forecasts were made. The results demonstrate the superior performance of the forecasts constrained to total populations from the extrapolative model, with the constrained bi-regional model giving the lowest errors. The findings should be of use to practitioners in selecting appropriate models for local area population forecasts.

Spatial demography: A unifying core and agenda for further research

“…Unfortunately, these data are not easy to obtain for many countries because of the reluctance of official statistics agencies to publish very detailed data matrices due to the risk of disclosure of individuals, and sampled data are usually not of sufficient size to capture the relatively small numbers of people moving between regions. (1976), Rees (1997), Wilson and Bell (2004), and Wilson (2016).…”

“…Fortunately, there are some examples of very large and detailed population projections from which we can draw experiences from, notably Wilson () for local areas in New South Wales, Australia, Rees, Wohland, Norman, and Boden () for ethnic projections across local authorities in the United Kingdom, and Lutz, Butz, and KC () for human capital projections for all countries in the world. See also Rogers (), Rees (), Wilson and Bell (), and Wilson ().…”

Population projections for all U.S. counties by age, sex, and race controlled to the Shared Socioeconomic Pathways

“…However, for areas experiencing population declines, CCDs have the potential of creating impossible negative populations through linear decline. In this work, I use a blended approach in which CCDs in areas projected to increase and CCRs in areas projected to decrease create more utility in the projections, preventing impossible negative populations and explosive population growth, and previous research has shown blended linear/exponential population projections outperform both linear and exponential models, respectively (Wilson 2016).…”

“…Blended linear/exponential demographic projections tend to outperform both linear and exponential models, respectively (Wilson 2016). This technique has all of the advantages of CCRs by remaining just as simple and parsimonious with minimal data requirements while producing projected populations without impossibly explosive growth.…”

“…Out-of-sample validation reveals errors on par with or better than cohort-component population projection models undertaken at the national and sub-national scale (Raftery et al 2012;Rayer 2008;Smith and Tayman 2003;Sprague 2012;Wilson 2016) (Table 5 summarizes several projection evaluations for comparison purposes).…”