measure of the Continuous Performance Test was ‘found to be 0.65 for schizophrenia patients and 0.72 for healthy subjects,’ whereas another such measure had stability over 2 years, which ranged from 0.56 to 0.73. The reliability of brain functional magnetic resonance imaging (fMRI) is quite variable and for some paradigms is under +0.30.13–15 However, other EPs, such as structural MRI, might be highly reliable as indicated by a recent report showing test–retest correlations > +0.95 for measures of cortical thickness.16 By contrast, in the Irish Study of High-Density Schizophrenia Families, which used both in-depth personal interviews and extensive reviews of hospital records, the diagnostic reliability, assessed using a weighted κ, was +0.94.17 Conversely, in the Virginia Adult Twin Study of Psychiatric and Substance Use Disorders, the long-term stability of an interview-based assessment of lifetime major depression was κ = +0.43,18 considerably lower than the reliability of the short form of the Eysenck’s neuroticism scale (r = +0.69) which has been proposed as an EP for MD—measured over a comparable time period. The major point here is that the relative ‘performance’ of EPs versus PD in assessing a ‘genetic signal’ cannot be divorced from the problems of measurement. It is perfectly
