title: Relationship between measurement invariance and age-related differences in the prevalence of generalized anxiety disorder.
abstract: BACKGROUND
  Generalized anxiety disorder (GAD) peaks in prevalence in middle age and declines in prevalence into old age. Some commentators have suggested that this is not a meaningful epidemiological trend rather a methodological artifact. This study tested this hypothesis by matching respondents of different ages based on their severity and examining whether age influences the probability of endorsing GAD criteria and the prevalence of GAD.
  
  
  METHODS
  Self-reported worriers from a nationally representative survey of non-institutionalized adults were selected for investigation (n=1738). All respondents were interviewed using the World Mental Health version of the Composite International Diagnostic Interview. Age-related differences in the prevalence and invariance of the GAD criteria were examined between 16 and 29 years, 30 and 44 years, 45 and 59 years and 60 years+ age groups. Effect sizes were used to assess the impact of statistically significant criterion level non-invariance on the prevalence of GAD.
  
  
  RESULTS
  Respondents aged 60 years or more were less likely than younger adults to endorse most of the GAD criteria. Significant non-invariance was identified in each of the age group analyses. Nonetheless these differences had no impact on the prevalence of GAD in three group comparisons. In the other three group comparisons, the impact was minimal (d ≤ 0.3).
  
  
  CONCLUSIONS
  Our results support age-related differences in the prevalence of GAD but we are limited to the extent to which we can say why these differences occur.
  
  
  CONCLUSIONS
  Age-related prevalence differences in GAD are meaningful epidemiological trends.
authors.affiliation: Clinical Research Unit for Anxiety and Depression, School of Psychiatry, University of New South Wales, NSW, Australia. Electronic address: m.hobbs@unsw.edu.au.
authors.forename: Megan J
authors.initials: MJ
authors.lastname: Hobbs
authors.forename: Tracy M
authors.initials: TM
authors.lastname: Anderson
authors.forename: Tim
authors.initials: T
authors.lastname: Slade
authors.forename: Gavin
authors.initials: G
authors.lastname: Andrews
mesh_terms: D000293:Adolescent; D000328:Adult; D000367:Age Factors; D000375:Aging / Q000523:psychology*; D001008:Anxiety Disorders / Q000175:diagnosis / Q000453:epidemiology* / Q000523:psychology; D005260:Female; D006801:Humans; D007406:Interview, Psychological; D008297:Male; D008875:Middle Aged; D015995:Prevalence; D055815:Young Adult
keywords: Classification; DIF; Epidemiology; Invariance; Item response; Latent variable