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Keywords:

  • Comorbidity;
  • co-occurrence;
  • disease;
  • covariation

Comorbidity, meaning the co-occurrence of two supposedly separate conditions at above chance levels, provides a most important research opportunity. It requires a rigorous examination of diagnostic concepts and a systematic critical testing of those concepts. Such testing may shed much needed light on risk mechanisms. Four issues that have given rise to misunderstanding in this connection are discussed: definitions of terms, diagnostic validity, reassessment and research strategies.