Does specifi cation matter? Experiments with simple multiregional probabilistic population projections

Population projection models that introduce uncertainty are a growing subset of projection models in general. In this paper we focus on the importance of decisions made with regard to the model specifi cations adopted. We compare the forecasts and prediction intervals associated with four simple regional population projection models: an overall growth rate model, a component model with net migration, a component model with in-migration and out-migration rates, and a multiregional model with destinationspecifi c out-migration rates. Vector autoregressive models are used to forecast future rates of growth, birth, death, net migration, in-migration and out-migration, and destinationspecifi c out-migration for the North, Midlands, and South regions in England. They are also used to forecast diff erent international migration measures. The base data represent a time series of annual data provided by the Offi ce for National Statistics from 1976 to 2008. The results illustrate how both the forecasted subpopulation totals and the corresponding prediction intervals diff er for the multiregional model in comparison to other simpler models, as well as for diff erent assumptions about international migration. The paper ends with a discussion of our results and possible directions for future research.