As EPs are often measured using sophisticated imaging, neurophysiological or neuropsychological measures, there is a tendency to assume that such ‘harder’ measures are, of necessity, more reliable than the ‘softer’ psychiatric diagnoses. That is, we commonly assume that λEP exceeds λPD. However, this assumption may be incorrect. Many putative EPs are measured over short time intervals and can be influenced by transient state effects such as ambient noise, temperature, time of day, or variations in machine functioning—as well as temporary changes in mental state of the participant due to stressors, or consumption of or withdrawal from nicotine, caffeine or alcohol. By contrast, some PDs are assessed using years of medical records and the recording of symptoms occurring over similar periods, or with carefully constructed psychological instruments. For example, Gur et al.12 report that the 1-year stability for a commonly used 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
