title: Lifetime prevalence and pseudocomorbidity in psychiatric research.
abstract: BACKGROUND
  Comorbidity is the rule rather than the exception with psychiatric disorders and is consequently of great interest to both researchers and clinicians. However, many studies of psychiatric comorbidity have been based on lifetime prevalence with mixed-age samples, a practice that (1) biases the assessment of epidemiologic comorbidity and (2) creates the appearance of comorbidity even when disorders are randomly associated. This bias is what we refer to as pseudocomorbidity.
  
  
  OBJECTIVE
  To clarify the source of the problem and to discuss strategies that might be adopted to deal hereafter with lifetime prevalence data.
  
  
  METHODS
  A simulated example is presented to show that even when there is only random association between disorders, there will appear to be nonrandom comorbidity when lifetime prevalence is used with mixed-age samples. An actual example relating psychosis to phobia is presented to show the bias that can result and to illustrate one way of dealing with lifetime prevalence data.
  
  
  CONCLUSIONS
  Use of lifetime prevalence with mixed-age samples, used almost exclusively in psychiatric research, generates problematic results, especially when used for assessment of comorbidity, and should be viewed with some skepticism. Hereafter, we recommend that any future use of lifetime prevalence should require determination of the age of onset, even if only by retrospective report. Comorbidity then should be reported by age.
authors.affiliation: Department of Psychiatry and Behavioral Sciences, Stanford University School of Medicine, Stanford, Calif 94305, USA. hck@stanford.edu
authors.forename: Helena Chmura
authors.initials: HC
authors.lastname: Kraemer
authors.forename: Kimberly A
authors.initials: KA
authors.lastname: Wilson
authors.forename: Chris
authors.initials: C
authors.lastname: Hayward
mesh_terms: D000293:Adolescent; D000328:Adult; D017677:Age Distribution; D000367:Age Factors; D017668:Age of Onset; D000368:Aged; D015982:Bias; D015897:Comorbidity; D003430:Cross-Sectional Studies; D006306:Health Surveys; D006801:Humans; D015994:Incidence; D001523:Mental Disorders / Q000453:epidemiology*; D010698:Phobic Disorders / Q000453:epidemiology; D015995:Prevalence; D011618:Psychotic Disorders / Q000453:epidemiology; D012107:Research Design / Q000592:standards; D012189:Retrospective Studies; D012494:Sampling Studies; D015983:Selection Bias; D017678:Sex Distribution; D013223:Statistics as Topic / Q000379:methods