A recent study of urbanisation and incidence of psychosis was based on data from the excellent Swedish population registers (Reference Sundquist, Frank and SundquistSundquist et al, 2004). Unfortunately, there are methodological problems with the study.

Sundquist et al argue that the prospective nature of their data allowed them to calculate incidence rates of psychoses rather than prevalence rates. However, their study confuses readmission and incidence rates. In incidence studies, the event of interest is new occurrence of a specified disease (see e.g. Reference Clayton and HillsClayton & Hills, 1993). Sundquist et al's cohort consists of people aged 25-64 years in 1996 (i.e. born 1932-1971). They analyse the data as incident data although they do not have this information. For persons who enter the study in old age, they claim that when admitted during the study period 1997-1999 this is the first admission ever of these people. They do, however, exclude people admitted 1992-1996, but this only partly solves this problem (e.g. for people born in 1932 in their study, the earliest possible age at onset of psychosis is 60 years). Let us say the earliest age at onset of psychosis is 25 years, the authors lack information on first admission for psychosis among people born 1966 or earlier. Roughly, they do not have information on first admission for psychosis among people born 1932-1966, about 85% of the study population. If the earliest age of first admission for psychosis is 15 years (Reference Pedersen and MortensenPedersen & Mortensen, 2001), the authors lack information on first admission for all people. This severely violates the assumptions of an incidence study (Reference Breslow and DayBreslow & Day, 1987), and the results may be extremely biased.

Sundquist et al measure urbanisation as the place where cohort members lived around 1996. Using this measure of urbanisation - in combination with the problems mentioned above - implies problems with causality; is the disease a result of the urbanisation or is the urbanisation a result of the disease?

Based on these methodological errors, it is difficult to interpret their findings. However, some possible biases include selective migration of people with psychosis after disease onset, and urban-rural differences in the readmission rates for people with psychosis.