In this role, they are an alternative to the Pearl Index.

Most often, the Pearl Index is used to calculate effectiveness rates.

Failure rates may be calculated by either the Pearl Index or a life table method.

Three kinds of information are needed to calculate a Pearl Index for a particular study:

There are two calculation methods for determining the Pearl Index:

Usually two Pearl Indexes are published from studies of birth control methods:

It is also sometimes used to compare birth control methods, a lower Pearl index representing a lower chance of getting unintentionally pregnant.

Actual use Pearl Index, which includes all pregnancies in a study and all months (or cycles) of exposure.

The Pearl Index has unique shortcomings, however.

In 1966, two birth control statisticians advocated abandonment of the Pearl Index:

Like all measures of birth control effectiveness, the Pearl Index is a calculation based on the observations of a given sample population.

The Pearl Index also provides no information on factors other than accidental pregnancy which may influence effectiveness calculations, such as:

The Pearl Index is only accurate as a statistical estimation of per-year risk of pregnancy if the pregnancy rate in the study was very low.

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.

So the longer the study length, the lower the Pearl Index will be - and comparisons of Pearl Indexes from studies of different lengths cannot be accurate.

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.

Use of life table methods eliminates time-related biases (i.e. the most fertile couples getting pregnant and dropping out of the study early, and couples becoming more skilled at using the method as time goes on), and in this way is superior to the Pearl Index.