The Pearl Index, also called the Pearl rate, is the most common technique used in clinical trials for reporting the effectiveness of a birth control method. It is a very approximate measure of the number of unintended pregnancies in 100 woman-years of exposure that is simple to calculate, but has a number of methodological deficiencies. The index was introduced by Raymond Pearl in 1934. It has remained popular for over eighty years, in large part because of the simplicity of the calculation. Calculation Several kinds of information are needed to calculate a Pearl Index for a particular study: the total number of months or cycles of exposure by women in the study the number of pregnancies the reason for leaving the study (pregnancy or other reason) the number of children in a single pregnancy (twins or triplets can affect the final number) There are two calculation methods for determining the Pearl Index: in the first method, the relative number of pregnancies in the study is divided by the number of months of exposure, and then multiplied by 1200 in the second method, the number of pregnancies in the study is divided by the number of menstrual cycles experienced by women in the study, and then multiplied by 1300. 1300 instead of 1200 is used on the basis that the length of the average menstrual cycle is 28 days, or 13 cycles per year Usage The Pearl Index 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. Usually two Pearl Indexes are published from studies of birth control methods: the actual use Pearl Index, which includes all pregnancies in a study and all months (or cycles) of exposure the perfect use or method Pearl Index, which includes only pregnancies that resulted from correct and consistent use of the method, and only includes months or cycles in which the method was correctly and consistently used Criticisms Like all measures of birth control effectiveness, the Pearl Index is a calculation based on the observations of a given sample population. Thus studies of different populations using the same contraceptive will yield different values for the index. The culture and