title: Bar charts enhance Bland-Altman plots when value ranges are limited.
abstract: OBJECTIVE
  A common form of validation study compares alternative methods for collecting data. The Bland-Altman plot pairs observations across methods and plots their mean values vs. their difference. This method provides only limited information, however, when the range of observed values is small relative to the number of observations. This brief report shows how adding a simple bar chart to a Bland-Altman plot adds essential additional information.
  
  
  METHODS
  The methodological approach is illustrated using data from a randomized controlled clinical trial of patients in a U.S. county health system.
  
  
  RESULTS
  When the number of unique values is small, a Bland-Altman plot alone may provide inadequate information. Adding a bar chart yields new and essential information about agreement, bias, and heteroscedasticity.
  
  
  CONCLUSIONS
  Studies validating one data-collection method against another can be performed successfully even when the number of unique values is small.
authors.affiliation: Health Economics Resource Center, U.S. Department of Veterans Affairs, VA Palo Alto HCS, 795 Willow Road (152 MPD), Menlo Park, CA 94025, USA. mark.smith9@va.gov
authors.forename: Mark W
authors.initials: MW
authors.lastname: Smith
authors.forename: Jun
authors.initials: J
authors.lastname: Ma
authors.forename: Randall S
authors.initials: RS
authors.lastname: Stafford
mesh_terms: D015982:Bias; D003627:Data Interpretation, Statistical; D004636:Emergency Service, Hospital / Q000706:statistics & numerical data; D006760:Hospitalization / Q000706:statistics & numerical data; D006801:Humans; D016032:Randomized Controlled Trials as Topic / Q000379:methods; D015203:Reproducibility of Results; D012107:Research Design; D018709:Statistics, Nonparametric