Pearl index is used to calculate: March, September 2009
**Core Concept**
The Pearl index is a measure of the effectiveness of a contraceptive method, specifically its failure rate per 100 woman-years of exposure. It is a statistical tool used to compare the efficacy of different contraceptive methods in preventing pregnancy.
**Why the Correct Answer is Right**
The Pearl index is calculated by taking the number of failures (pregnancies) and dividing it by the total number of women-months or woman-years of exposure, multiplied by 100. This gives a rate of failure per 100 woman-years, allowing for a standardized comparison of different contraceptive methods. The Pearl index is a widely used measure of contraceptive efficacy and is often used in clinical trials and real-world studies to evaluate the effectiveness of new contraceptive methods.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the Pearl index is not a measure of the time to pregnancy or the number of menstrual cycles required to achieve pregnancy.
**Option B:** This option is incorrect because the Pearl index is not a measure of the incidence of side effects or complications associated with a contraceptive method.
**Option C:** This option is incorrect because the Pearl index is not a measure of the cost-effectiveness or accessibility of a contraceptive method.
**Clinical Pearl / High-Yield Fact**
The Pearl index is often used in conjunction with the "number needed to treat" (NNT) to provide a comprehensive understanding of the effectiveness and safety of a contraceptive method.
**Correct Answer:** C. The Pearl index is used to calculate the failure rate of a contraceptive method per 100 woman-years of exposure.