Since they were first introduced in 1971, three terms have become established in the lexicon of health services research. The concepts of efficacy, effectiveness, and efficiency are now widely accepted as essential measures for assessing health care programs and practices. Efficacy is concerned with "establishing objectively that a new form of preventive, diagnostic, curative or restorative intervention is more useful and beneficial than it is harmful or useless for the purposes for which it is advocated." Effectiveness describes "the extent to which an efficacious form of intervention can be shown to have been made available or applied to all those in a defined population who could benefit from it." Efficiency, in turn, is concerned with "measuring the extent to which a stated level of effectiveness can be achieved with the least expenditures of personel, resources and money."1