title: Why Are Medicare and commercial insurance spending weakly correlated?
abstract: OBJECTIVE
  To investigate the source of the weak correlation across geographic areas between Medicare and private insurance spending.
  
  
  METHODS
  Retrospective, descriptive analysis.
  
  
  METHODS
  We obtained Medicare spending data at the hospital referral region (HRR) level for 2007 from the Dartmouth Atlas, and commercial claims from large employers for 2007 from the Truven MarketScan Database. We constructed county-level data on hospital market structure from Medicare patient flows and obtained county-level data on the Medicare wage index from the Centers for Medicare & Medicaid Services website. We aggregated these sources to the HRR level. We decomposed Medicare and private spending into 2 components: price and volume. We also decomposed Medicare and private prices into 2 components: a common measure of cost and a sector-specific markup. We computed correlations between Medicare and private prices and volumes, and the correlation of each sector’s price and volume with cost and markup.
  
  
  RESULTS
  We found that Medicare and private prices are strongly positively correlated, largely because both are keyed off of common costs. Consistent with previous work, we found that Medicare and private volumes are strongly positively correlated as well.
  
  
  CONCLUSIONS
  The weak correlation between Medicare and private spending is consistent with these 2 empirical regularities. It is mathematically due to negative correlations between each sector’s price and the other sector’s volume. In particular, we found that private prices have important spillover effects on Medicare volume. Future research on the effects of competition should take account of this phenomenon.
authors.forename: Laurence C
authors.initials: LC
authors.lastname: Baker
authors.forename: M Kate
authors.initials: MK
authors.lastname: Bundorf
authors.forename: Daniel P
authors.initials: DP
authors.lastname: Kessler
mesh_terms: D003365:Costs and Cost Analysis; D005102:Health Expenditures; D006801:Humans; D007348:Insurance, Health / Q000191:economics*; D006278:Medicare / Q000191:economics*; D012189:Retrospective Studies; D014481:United States