Pearl Index

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The Pearl Index, also called the Pearl rate, is a technique used in clinical trials for measuring the effectiveness of a birth control method. It is calculated by dividing the number of unintended pregnancies by the number of months of use of the particular method whose efficacy is being measured, and multiplying the result by 1200. In more accurate surveys, the number of cycles of use (rather than months) are measured, and the result is multiplied by 1300 (on the basis that the average cycle length is 28 days).

It is sometimes used as a statistical estimation of the number of unintended pregnancies in 100 woman-years of exposure (e.g. 100 women over one year of use, or 10 women over 10 years). It is also sometimes used to compare birth control methods, a lower Pearl index representing a lower chance of getting unintentionally pregnant.

The index is a calculation based on the observations of a given sample population. Thus different studies for the same contraceptive, will give different values for the index.

The index was introduced by Raymond Pearl in 1933.da:Pearl-index de:Pearl-Index fr:Indice de Pearl lt:Perlo indeksas nl:Pearl-Index pl:Wskaźnik_Pearla