Although the Ebbinghaus illusion is commonly used as an example of a simple size-contrast effect, previous studies have emphasised its complexity by identifying many factors that potentially influence the magnitude of the illusion. Here, in a series of three experiments, we attempt to simplify this complexity. In each trial, subjects saw a display comprising, on one side, a target stimulus surrounded by inducers and, on the other, an isolated probe stimulus. Their task was to indicate whether the probe appeared larger or smaller than the target. Probe size was adjusted with a one-up, one-down staircase procedure to find the point of subjective equality between probe and target. From these experiments, we argue that the apparent effects of inducer size are often confounded by the relative completeness of the inducing surround and that factors such as the similarity of the inducers and target are secondary. We suggest a simple model that can explain most of the data in terms of just two primary and independent factors: the relative size of the inducers and target, and the distance between the inducers and the target. The balance between these two factors determines whether the size of the target is underestimated or overestimated.

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Attempts were made to identify, and include in a two-year follow-up study, every patient living in the catchment area of the Mapperley group of psychiatric hospitals in Nottingham (population 390,000) who made their first-ever contact with the psychiatric services for a potentially schizophrenic illness during a two-year period (1 August 1978 to 31 July 1980). Screening was based upon symptoms rather than diagnosis, covering both in-patient and out-patient services; a consensus diagnosis using ICD-9 was made by the project team. The Nottingham Psychiatric Case Register was used in a retrospective Leakage Study which added nine cases to the 99 identified by the screening procedures. Incidence rates are given for both broad and narrow concepts of schizophrenia, and for DSM-III diagnosis. The Nottingham incidence rates are similar to those reported from other UK centers, and are near the middle of the range found in the other collaborating centres in the WHO study on Determinants of Outcome of Severe Mental Disorders. At entry to the study, 27 patients were out-patients, and 11 were never admitted to hospital at any time in the two-year follow-up period. Reasons for believing that the Nottingham administrative incidence may be close to the incidence in the community are discussed.

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