Interpreting the Volume–Outcome Relationship in the Context of Health Care Quality: Workshop Summary
The Institute of Medicine (IOM) held a workshop on May 11, 2000, to review the current understanding of the relationship between volume of health services and health-related outcomes; assess the potential health policy implications of the use of volume as an indicator of quality of care; and identify needed areas for future research relative to volume and quality of care. The workshop was jointly sponsored by IOM's Committee on Quality of Health Care in America and the National Cancer Policy Board, with financial support from the Agency for Healthcare Research and Quality. The workshop was structured around presentations of two background papers: a cross-disciplinary, systematic review of the published research literature conducted by Drs. Ethan Halm, Clara Lee, and Mark Chassin of the Mount Sinai School of Medicine and a synthesis of the literature relevant to policy issues by Dr. Adams Dudley and colleagues from the University of California, San Francisco.
A higher-volume, better-outcome association was observed in three-quarters of the studies reviewed. Volume is, however, an imprecise indicator of quality. Some low-volume providers have excellent outcomes, and conversely, some high-volume providers have poor outcomes. Volume per se does not lead to good outcomes in health care; it is instead a proxy measure for other factors that affect care. With few exceptions, however, the literature does not shed light on the structures or processes of care that underlie the apparent relationship.
Many workshop participants noted that volume may be the best available proxy indicator of quality for certain conditions, but efforts should be made to accompany volume information with other quality indicators and with explanatory information. Furthermore, several participants suggested that when research confirms a volume–outcome link, this information should be disclosed to the public to support health care decision-making. In making such disclosure, however, the limitations of the data and how to interpret the information must be made clear to the intended audience. Public release of data on volume may motivate providers to report process and outcome data, thereby laying the ground for the next generation of health care quality measures. When better measures of quality of care than volume are developed, they should replace the volume measures.
A wide-ranging set of research topics—from policy research and demonstration programs, to basic methodological research—was outlined by workshop participants to elucidate why the relationship between volume and outcome exists, and how best to implement policies to improve health care.
An association between higher volumes and better outcomes has been well documented for certain types of health care. Some health care purchasers are using these research findings to refer patients to higher-volume settings for selected procedures. The evidence prompted the Na-