van Baal et al. BMC Public Health 2011, 11:163 http://www.biomedcentral.com/1471-2458/11/163 Page 8 of 11 Prevalent cases per 1,000 persons Multiple sclerosis Parkinson Schizophrenia Macular degeneration Inflammatory bowel diseases Rheumatoid arthritis Dementia Glaucoma Epilepsy Heart failure Osteoporosis Stroke COPD Cataracts Arthritis Diabetes Hearing impairments Coronary heart disease 0.5 1.0 2.0 5.0 10.0 20.0 50.0 Figure 3 Prevalence per 1,000 (men plus women) in the Netherlands 1 January 2007. The black lines indicate prediction intervals using the fixed effects estimates only while the gray lines broken lines include the between GPRN variance. was considerable. This could suggest that GPRNs might differ in the distinction between ‘new’ and ‘old’ cases. There are several ways to estimate morbidity rates in a population in a cross-sectional manner [28]. Examples are health interview surveys and health examination sur- veys. It is obvious that health interview surveys have inherent limitations as they rely on self-report. Hospital- based data, although readily available in many countries, have the important disadvantage of selection bias as these include only those cases that lead to hospitaliza- tion. Information gathered in general practices, on the other hand, does not have these drawbacks. In countries in which the majority of the population is registered with a GP, such as the UK and the Netherlands, a source is available that could, in principle, be used to derive reliable, and representative estimates of descriptors of national health [30-32]. In the best of all possible worlds, all GPs would diagnose and document disease (episodes) in a uniform manner, reporting them compre- hensively to a central databank. However, in reality there are important obstacles to drawing from this well in a sound manner. Firstly, there is the limited availability of