Foulds' hierarchical model of psychiatric illness in a Dutch cohort: a re-evaluation.

In order to evaluate Foulds' hierarchical model of psychiatric illness, a study was carried out using data from a 3-year follow-up of patients suffering from functional non-affective psychoses. Of the 177 Present State Examinations carried out 86% yielded symptom patterns compatible with the model. It was shown that failures to fit the hierarchy can be explained either by measurement error or by the masking of major symptoms by the simultaneous presence of minor ones. A one-dimensional scale to measure the severity of psychiatric illness was constructed. Patients' scores were compared with ratings on the Index of Definition and the diagnosis of current mental state. The results of these comparisons were considered to be supportive validatory evidence. Some of the implications of the model and the data are discussed.