Calculation of the mean duration and age of onset of a chronic disease and application to dementia in Germany

This paper deals with analytical methods for calculating the mean age of onset and the mean disease duration in chronic diseases. For this calculation so-called IPM models are used. IPM stands for incidence, prevalence and mortality. First, the general IPM model is introduced. Then, formulas for the mean duration and the mean age of onset of the disease will be developed and applied to epidemiological data about dementia in Germany. Since we are interested in basic epidemiological parameters such as incidence, prevalence and mortality with respect to a disease, it has proven to be helpful to look at so-called state models (compartmental models). The model that will be used here consists of the three states Normal, Disease, Death and the transitions between the states.