Qualité des prévisions démographiques à court terme. Etude de l'extrapolation de la population totale des départements et villes de France 1821-1975

Henry Louis and Gutierrez Hector. The Quality of Short-Term Demo{p*aphic Forecasts; the Extrapolation-Study of the Total Population of the Departemente and Towns in France, 1821-1975. Demographic projections and forecasts are constructed from extrapolations of time series of various indices and the difference between the observed and expected population depends on the validity of these extrapolations. This article is devoted to the results to be expected from the extrapolation, over 5 and 10 years, of a particular index, namely the intercensal rate of growth, of the population of the départements and towns or conurbations in France since 1821 or 1831. The choice of this very summary index is imposed by the need to have long series, but the lessons to be drawn from the results extend beyond this particular case. As the extrapolated growth-rate is the one observed over the 5 or 10 previous years, we have led to compare the growth rates of two successive quinquennia or decennia and to study the central value and dispersion of the for of the second period for each value of the rate during the first.