title: Major depressive disorder as a nonlinear dynamic system: bimodality in the frequency distribution of depressive symptoms over time.
abstract: BACKGROUND
  A defining characteristic of Major Depressive Disorder (MDD) is its episodic course, which might indicate that MDD is a nonlinear dynamic phenomenon with two discrete states. We investigated this hypothesis using the symptom time series of individual patients.
  
  
  METHODS
  In 178 primary care patients with MDD, the presence of the nine DSM-IV symptoms of depression was recorded weekly for two years. For each patient, the time-series plots as well as the frequency distributions of the symptoms over 104 weeks were inspected. Furthermore, two indicators of bimodality were obtained: the bimodality coefficient (BC) and the fit of a 1- and a 2-state Hidden Markov Model (HMM).
  
  
  RESULTS
  In 66% of the sample, high bimodality coefficients (BC>.55) were found. These corresponded to relatively sudden jumps in the symptom curves and to highly skewed or bimodal frequency distributions. The results of the HMM analyses classified 90% of the symptom distributions as bimodal.
  
  
  CONCLUSIONS
  A two-state pattern can be used to describe the course of depression symptoms in many patients. The BC seems useful in differentiating between subgroups of MDD patients based on their life course data.
authors.affiliation: University of Groningen, University Medical Center Groningen, Interdisciplinary Center Psychopathology and Emotion regulation (ICPE), PO Box 30.001, 9700 RB, Groningen, The Netherlands. b.hosenfeld@umcg.nl.
authors.forename: Bettina
authors.initials: B
authors.lastname: Hosenfeld
authors.affiliation: University of Groningen, University Medical Center Groningen, Interdisciplinary Center Psychopathology and Emotion regulation (ICPE), PO Box 30.001, 9700 RB, Groningen, The Netherlands. elske.bos@med.umcg.nl.
authors.forename: Elisabeth H
authors.initials: EH
authors.lastname: Bos
authors.affiliation: University of Groningen, University Medical Center Groningen, Interdisciplinary Center Psychopathology and Emotion regulation (ICPE), PO Box 30.001, 9700 RB, Groningen, The Netherlands. k.j.wardenaar@umcg.nl.
authors.forename: Klaas J
authors.initials: KJ
authors.lastname: Wardenaar
authors.affiliation: University of Amsterdam, Department of Psychology, Clinical Psychology, Amsterdam, The Netherlands. H.J.Conradi@uva.nl.
authors.forename: Henk Jan
authors.initials: HJ
authors.lastname: Conradi
authors.affiliation: University of Amsterdam, Department of Psychology, Psychological Methods, Amsterdam, The Netherlands. H.L.J.vanderMaas@uva.nl.
authors.forename: Han L J
authors.initials: HL
authors.lastname: van der Maas
authors.affiliation: University of Amsterdam, Department of Psychology, Developmental Psychology, Amsterdam, The Netherlands. i.visser@uva.nl.
authors.forename: Ingmar
authors.initials: I
authors.lastname: Visser
authors.affiliation: University of Groningen, University Medical Center Groningen, Interdisciplinary Center Psychopathology and Emotion regulation (ICPE), PO Box 30.001, 9700 RB, Groningen, The Netherlands. Peter.de.Jonge@umcg.nl.
authors.forename: Peter
authors.initials: P
authors.lastname: de Jonge
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D003865:Major Depressive Disorder / Q000175:diagnosis / Q000453:epidemiology* / Q000523:psychology; D039721:Diagnostic and Statistical Manual of Mental Disorders; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009426:Netherlands / Q000453:epidemiology; D017711:Nonlinear Dynamics; D010358:Patient Participation; D011569:Psychiatric Status Rating Scales; D016032:Randomized Controlled Trials as Topic; D055815:Young Adult