MEASURING CONTRACEPTIVE EFFECTIVENESS - A CONCEPTUAL-FRAMEWORK

We present a conceptual model that outlines the four measures of how w ell a contraceptive method works: 1) efficacy, 2) effectiveness, 3) pe rfect-use pregnancy rate, and 4) typical-use pregnancy rate. Moreover, we illustrate how four variables influence these measures: 1) capacit y to conceive, 2) frequency and timing of intercourse, 3) degree of co mpliance, and 4) inherent protection of the method. Because of inter-i ndividual as well as intra-individual variability of the first three v ariables, generalizing results from a contraceptive clinical trial to other populations is problematic. There is a hierarchy of generalizabi lity of the four outcome measures, with the typical-use pregnancy rate the least generalizable but the easiest to measure, and efficacy the most generalizable but the most difficult to measure. These four varia bles should be considered in the design and analysis of future contrac eptive clinical trials. Finally, this article illustrates why the term s ''pregnancy rate'' and ''failure rate'' are not synonymous and why w e recommend that the latter term not be used.