three-year time limit on the coverage of immunosuppressive drugs for transplant recipients. The committee also intended that its findings and conclusions should be credible to practicing clinicians, patients, and the public. Several principles guided the committee’s work within the limits of existing evidence, time, and resources:
Findings and conclusions should be consistent with available knowledge; apparent departures from the evidence should be explained.
Health outcomes meaningful to patients or consumers—not only changes in physiological measures—should be emphasized in assessments. Meaningfulness relates to the kinds of benefits and harms identified, the magnitude of the effect of an intervention on an outcome, and the preferences of individuals about different outcomes.
The quality, strength, and limits of the evidence for findings and conclusions should be assessed and described. Evidence about effectiveness (results in usual clinical practice) as well as efficacy (results under controlled research conditions) should be considered.
The role of expert judgment and experience in assessing evidence and making judgments about the effectiveness of services should be identified.
Key analytic choices—such as the specification of the health care intervention, the identification of target populations, and the selection of data and methods for cost analyses—should be explained.
The limitations of analytic methods should be described. In this report, for example, a notable limitation is a cost estimation strategy that (consistent with the committee’s charge) focused on costs to the Medicare program rather than costs or cost-effectiveness from a societal perspective.
The committee’s task was not to craft statements that were precise and detailed enough to serve as legislative or regulatory language or clinical practice guidelines. (See Eddy et al.  and IOM [1990a] for discussions of principles and criteria for development of practice guidelines.) While acknowledging their importance, the committee also did not examine the full range of ethical, economic, cultural, political, and other issues relevant to decisions about Medicare coverage policies or other options for achieving health goals.
For each intervention examined, the committee found it helpful to consider a version of the “evidence pyramid” that Figure 2–1 depicts for a generic health care intervention. In this pyramid, each lower tier represents a condition to be met before the next-higher tier is considered. This generic pyramid has been modified to fit the special characteristics of the interventions examined in the next three chapters.