2.2. IPM model

For IPM models, when the duration of the disease is not known, the rate of remission can be used instead. Furthermore, the mortality of patients with a disease is equal to the mortality of the population, plus the increased mortality associated with the disease (Capocaccia, 1993).

Saha et al.'s (2008) study was used to model prevalence from incidence, mortality, and remission rates. Indeed, we used the same standardised mortality ratio (SMR) that was calculated in a systematic review of mortality studies in schizophrenia (SMR = 2.58; Saha, Chant, & McGrath, 2007). To calculate the mortality of subjects with psychotic disorders, the increased mortality associated with the disease (calculated with the SMR) was added to the mortality of the general population of the catchment area. Similarly, we used the annualised remission rate (ARR) calculated by Saha et al. (2008), based on data from 12 studies of remission in schizophrenia (ARR = 1.37%/year).

As in Saha et al.'s study, all calculations were done using DisMod II, a computer program developed for the World Health Organization Global Burden of Diseases studies (Barendregt, van Oortmarssen, Vos, & Murray, 2003; Murray & Lopez, 1996). The model uses a set of linear differential equations that describe the transitions between different status states (healthy, diseased, and dead) using the different rates (incidence, SMR, and ARR).