A stochastic population projection from the perspective of a national statistical office

Gianni Corsetti, Istituto Nazionale di Statistica (ISTAT)
Marco Marsili, Istituto Nazionale di Statistica (ISTAT)

In this paper we expose the first attempts to produce stochastic population projections for Italy in addition to the official deterministic population projections currently released by Istat. The latest (2011-based) official projections have been developed on a set of assumptions about future – until 2065 - levels of fertility, mortality and migration, according to the so called “scenario” approach: a main variant assumed as best performance of the future demographic trends, integrated with two variants, namely the high and low scenario, having the task of defining, albeit in a deterministic approach, the framework for the future uncertainty. Stochastic population forecasts have been produced to overcome the limitations associated with the traditional methodology: the lack of probability attached to the projections and therefore the absence of information about the uncertainty associated with them. To that purpose, two methods have been implemented: a forecast based on the Conditional Expert Opinions (Billari et al., 2010) and a second one on the Scaled Model of Error (Alho and Spencer, 1997). The first method, falling within the so called “random scenario” approach, is built on the use of expert opinions in the definition of probability distributions for terminal vital rates. The second one is based on the extrapolation of empirical errors, where assessment of uncertainty is modelled according to the analysis of past projections errors. In the paper we discuss data and implemented procedures; furthermore, we analyze the results arising from the two stochastic methods; finally we make a comparison between the stochastic and the deterministic approaches.