9.1 IntroductionGeneral cognitive ability, or reasoning, has an important impact on everyday functioning. Reasoning skills can be impaired to a variable degree in psychotic illness. This impairment, further exacerbated by the symptoms of illness and medication side effects, may influence considerably a person's capacity to undertake normal activities.
9.2 Assessment of general cognitive abilityGeneral cognitive ability was assessed in the course of the survey in two ways.
To assess current general cognitive ability, a test of the speed at which individuals process information was used, namely the Digit Symbol Coding test from the Repeatable Battery for the Assessment of Neuropsychological Status.20 In this test, individuals fill in digits corresponding to shapes, as quickly as they can, on the basis of a coding key. The lower the test score, the poorer a person's speed-based performance.
To estimate general cognitive ability prior to the onset of illness, the National Adult Reading Test-Revised was used.14 This is a test in which individuals are asked to pronounce irregularly spelt words (for example, palm). The test is based on the assumption that verbal ability remains relatively unaffected despite the decline in cognitive ability that may follow illness onset. This test is used internationally as a reliable and widely accepted measure for estimating retrospectively IQ levels in people with psychotic disorders before they became ill.
9.3 General cognitive ability in people with psychosisCurrent cognitive ability was markedly impaired in people with psychosis compared to the general population. On average, people with psychosis scored 38.6, 1.6 standard deviations below the general population score of 54.2.21
People with psychosis had a moderately lower score compared to the general population on the test measuring IQ prior to illness onset. Their average estimated premorbid IQ score was lower but still within the normal range compared to the population norm (98.1 and 107.4 respectively).14
These data suggest that there is some cognitive impairment in people with psychosis prior to illness onset and that this is amplified in the years following diagnosis.