title: Dichotomy - a forgotten ancient principle.
abstract: Dichotomy is an ancient principle of categorisation, where a class is divided into two jointly exhaustive and mutually disjoint categories. The principle as a general requirement was abandoned during the middle. The recent inquiry shows that studying this principle is still worthwhile and in some cases it can be used as a quality assessment tool. The paper presents algorithms that can transform any kind of categorial structures into dichotomy. The resulting representation sometimes can make apparent the problematic parts of the source. Problems often result from stating Is_a relations without differentiating criteria. A simple experiment of dichotomous transformation of the high level categories of the first chapter of ICD was carried out. The problem of "other" and "not elsewhere classified" categories is discussed.
  
  
  CONCLUSIONS
  we should not strive to build dichotomous structures but sometimes a dichotomous transformation of an existing structure can be helpful to detect critical parts of a system of categories.
authors.affiliation: National Institute for Strategic Health Research Budapest, Hungary. surjan.gyorgy@eski.hu
authors.forename: György
authors.initials: G
authors.lastname: Surján
mesh_terms: D000465:Algorithms; D002965:Classification; D006801:Humans; D019451:Information Management; D038801:International Classification of Diseases; D008128:Logic; D008492:Medical Informatics Computing; D010684:Philosophy; D011785:Quality Assurance, Health Care; D018875:Vocabulary, Controlled