The NIDI mortality model. A new parametric model to describe the age pattern of mortality

The NIDI mortality model. A new parametric model to describe the age pattern of mortality Joop de Beer and Fanny Janssen The NIDI mortality model. A new parametric model to describe the age pattern of mortality Abstract Parametric mortality models are aimed to describe the age pattern of mortality in terms of a mathematical function of age. We present a new parametric model to describe mortality for the entire age span: the NIDI mortality model. The model describes mortality in adulthood and advanced age by a mixture of two logistic models. The NIDI mortality model includes four interpretable time-varying parameters, reflecting (i) changes in the shape of the mortality age schedule in young and old age, resulting in compression of mortality, and (ii) a shift of the mortality age schedule to older ages. Fitting the NIDI model to probabilities of death in 1950 and 2009 for Japan, France, the USA and Denmark, showed a better fit than the well-known Heligman-Pollard model. The four time-varying parameters explain 99 per cent of the change in life expectancy at birth between 1950 and 2009. Shifts in the mortality age schedule explain two thirds of this change. The NIDI model, thus, is a valid instrument for describing the age pattern of mortality, for disentangling the effects of delay and compression of mortality on the increase in life expectancy, and can serve as a basis for the projection of mortality into the future.