The development of a series of instruments that has so far culminated in SCAN version 2.0 began in the late 1950s because of the need for better tools for clinical measurement (including categorization and diagnosis). The basic principles of the PSE had long been decided by the time of first publication in 1974 and have remained the same since. Based on the glossary of differential definitions, the aim is to provide comprehensive, accurate and technically specifiable means of describing and classifying clinical phenomena in order to make comparisons. Insofar as it fulfils this aim, SCAN can be used to enhance clinical work and education and advance knowledge through its use in biomedical, epidemiological and psychosocial research.