title: Beta-binomial regression and bimodal utilization.
abstract: OBJECTIVE
  To illustrate how the analysis of bimodal U-shaped distributed utilization can be modeled with beta-binomial regression, which is rarely used in health services research.
  
  
  METHODS
  Veterans Affairs (VA) administrative data and Medicare claims in 2001-2004 for 11,123 Medicare-eligible VA primary care users in 2000.
  
  
  METHODS
  We compared means and distributions of VA reliance (the proportion of all VA/Medicare primary care visits occurring in VA) predicted from beta-binomial, binomial, and ordinary least-squares (OLS) models.
  
  
  RESULTS
  Beta-binomial model fits the bimodal distribution of VA reliance better than binomial and OLS models due to the nondependence on normality and the greater flexibility in shape parameters.
  
  
  CONCLUSIONS
  Increased awareness of beta-binomial regression may help analysts apply appropriate methods to outcomes with bimodal or U-shaped distributions.
authors.affiliation: Northwest Center for Outcomes Research in Older Adults at the VA Puget Sound Health Care System, 1660 S. Columbian Way, Seattle, WA, 98108.
authors.forename: Chuan-Fen
authors.initials: CF
authors.lastname: Liu
authors.forename: James F
authors.initials: JF
authors.lastname: Burgess
authors.forename: Willard G
authors.initials: WG
authors.lastname: Manning
authors.forename: Matthew L
authors.initials: ML
authors.lastname: Maciejewski
mesh_terms: D000368:Aged; D005260:Female; D006302:Health Services Research / Q000379:methods*; D006801:Humans; D008297:Male; D006278:Medicare / Q000706:statistics & numerical data*; D008875:Middle Aged; D012044:Regression Analysis; D014481:United States; D014493:United States Department of Veterans Affairs
keywords: Beta-binomial; Medicare; bimodal; primary care; utilization; veterans