What is the effect of competition on quality of cardiovascular care in California between 1992-1998? The researchers used 3M’s All Patient Refined-Diagnosis Related Groups (APR-DRG) severity index as the main risk adjuster. They measured competition using an index based on the idea that hospitals compete with each other to the extent their patient pools overlap. They used three-level (patient, hospital, county) hierarchical generalized linear models to test the hypotheses for each outcome variable. Additionally, they conducted a series of in-depth case studies of patient flow in local markets to examine how selective referral to high volume hospitals can affect the accessibility, cost, and outcomes in markets with different numbers of hospitals. The objective of the study was to test three pairs of theory- and policy- relevant hypotheses: 1) Interaction between competition and volume; 2) Two different effects of competition on quality of care; 3) Competition has a stronger effect in more mature managed care markets. The goal of the study was to provide public and private decision makers with information about how competition affects the quality of care.