Background The nosologic structure of psychotic illness, still influenced as much by
historical as empirical perspectives, remains controversial. Methods Latent class analysis
was applied to detailed symptomatic and outcome assessments of probands (n= 343) with
broadly defined schizophrenia and affective illness ascertained from a population-based
psychiatric registry in Roscommon County, Ireland. First-degree relatives (n= 942) were
assessed by personal interview and/or review of hospital record. Results Six classes were …