The reviewed paper is by Sukanta Saha, Jan Barendregt and colleagues and focuses on the epidemiology of Schizophrenia. The authors conduct a systematic review of published studies on schizophrenia prevalence, incidence, remission and mortality as well as models of incidence and prevalence. The prevalence and incidence are linked with an equation which roughly speaking says that the prevalence of schizophrenia in the population is increased by new cases (i.e incidence) and decreased both by mortality and remission. This is a standard approach to modelling in Epidemiology. Indeed in epidemiological models the incidence is referred to as an in-flow variable, the prevalence as a stock variable and the remission and mortality as outflow variables.
The authors paired studies with published prevalence and incidence rates with published modelling of data. They then used a computer program to model the data – DisMod II – which is freely available. The software does require that when the input data is stratified into narrow age groups and authors report that such stratifications were not available for all of their studies. The authors have referenced lots of big prevalence and incidence studies as well as systematic reviews on prevalence and mortality which means that they should have a good data-set to use in their model. With regards to remission the authors only included studies with 5 years of follow-up.The authors then examined the effects of modifying some of the variables in the model.
The authors found a strong correlation between incidence and prevalence of schizophrenia in their selected studies (p=0.003). However they found that if the incidence was high in a given population and low in the modelled data, then it was paired with a prevalence rate that was lower than the modelled data. This suggested to the authors that the paired prevalence and incidence data were not internally consistent and they went on to discuss reasons why this might be so. In their ‘thought experiments’, by modifying variables in the model they found that reducing the incidence caused a marked drop in the prevalence whilst changes in the remission rate did little to effect the prevalence. A reduction in mortality of 25% had only a small effect on prevalence (7% in males and 5% in females).
This is a very important study with a very interesting methodology and I would recommend this paper to those who are intending to carry out modelling of prevalence/incidence data as the authors have explained their approach very clearly.
Saha S, Barendregt J et al. Modelling disease frequency measures in schizophrenia epidemiology. Schizophrenia Research. Vol 104. Issues 1-3. pp246-254.
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