volume of some cardiovascular procedures has tripled in less than a decade). Consequently, volume may be most pertinent when a new technology is beginning to diffuse into general practice.

Volume per se does not lead to good outcomes in health care. It is a proxy measure for other factors that affect care. In one of the studies, about one-third of the survival benefit among patients with acute myocardial infarction that was attributed to high-volume hospitals could be explained by the use of appropriate processes of care, for example, use of beta-blockers, aspirin, thrombolytics, and revascularization procedures. Unfortunately, most of the studies reviewed did not include information on processes or systems of care that might explain the underlying reasons for the volume–outcome relationship.

It is difficult to summarize the findings by procedure or condition because of study differences in the definition of low- and high-volume, in analytic techniques, and in adjustment methodologies. Even so, the volume–outcome relationship appears to be particularly dramatic for certain low-frequency, high-risk, surgical procedures such as surgery for cancer of the pancreas and esophagus. For these procedures, rates of short-term mortality are 2 to 3 times greater in low- versus high-volume hospitals. For other procedures or conditions under review, the volume effect was not as great or as consistent (see Appendix C for procedure- and condition-specific findings). Statistically significant differences in outcomes by volume may not always be of sufficient magnitude to be clinically significant; furthermore, other considerations can mitigate the importance of such differences. For example, if the difference between high- and low-volume hospitals' surgical mortality rates was only 1 or 2 percentage points, and the underlying condition for which the surgery was performed had a poor long-term prognosis, it may not be prudent to implement policy based only on a statistically significant difference in short-term mortality.

The authors caution that they may not have identified all relevant studies of the relationship between volume and outcome and that many studies, particularly those involving cardiovascular procedures, were conducted in New York and thus may not generalize to states with less regulation of such procedures. Negative publication bias (researchers' failing to report the absence of a relationship between volume and outcome or journals' failing to publish negative results) could also overestimate the positive nature of the volume–outcome relationship.

Drs. Halm and colleagues judged the methodological strength of the research using an explicit quality scoring method⁴ and concluded that the methodological rigor of the studies was modest, limiting generalizability and leaving many unanswered questions about the nature and causes of the observed association:

• It is unclear whether volume thresholds exist above which outcomes are better. For example, is it only the busiest providers that have the best outcomes, or do all but the lowest-volume providers have better outcomes?

• It is likely that the effects of physician and hospital volume combine or interact. One study suggests a multiplicative effect leading to much better outcomes among high-volume physicians practicing in high-volume hospitals. The relative contributions of physician and hospital volume to outcomes, however, have been examined in only a few studies.

• Studies generally do not illuminate how experience with procedures (or conditions) that are closely related to the procedure under study affects outcomes. Are cognitive and technical

Front Matter (R1-R6)

Contents (R7-R8)

Abstract (1-1)

Introduction (1-2)

Why Does the Volume-Outcome Relationship Matter? Who Cares and How Could it Be Used? (3-3)

How is Volume Related to Quality in Health Care? (4-9)

What Are the Policy Implications of a Volume-Outcome Relationship? (10-16)

Where Do We Go From Here? (17-18)

Appendix A: Workshop Agenda and Participants (19-22)

Appendix B: Members of the Committee on Quality of Health Care in America and the National Cancer Policy Board (23-26)

Appendix C: How is Volume Related to Quality in Health Care? A Systematic Review of the Research Literature? (27-102)

Appendix D: When and How Should Purchasers Seek to Selectively Refer Patients to High-Quality Hospitals? (103-119)