The Pearl Index, also called the Pearl rate, is the most common technique used in
clinical trials
Clinical trials are prospective biomedical or behavioral research studies on human participants designed to answer specific questions about biomedical or behavioral interventions, including new treatments (such as novel vaccines, drugs, dietar ...
for reporting the effectiveness of a
birth control
Birth control, also known as contraception, anticonception, and fertility control, is the use of methods or devices to prevent unwanted pregnancy. Birth control has been used since ancient times, but effective and safe methods of birth contr ...
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
Raymond Pearl (June 3, 1879 – November 17, 1940) was an American biologist, regarded as one of the founders of biogerontology. He spent most of his career at Johns Hopkins University in Baltimore. Pearl was a prolific writer of academic books, ...
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 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 cycle
The menstrual cycle is a series of natural changes in hormone production and the structures of the uterus and ovaries of the female reproductive system that make pregnancy possible. The ovarian cycle controls the production and release of eggs a ...
s 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
Unintended pregnancies are pregnancies that are mistimed, unplanned or unwanted at the time of conception.
Sexual activity without the use of effective contraception through choice or coercion is the predominant cause of unintended pregnancy. W ...
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 demographics of the population being studied, and the instruction technique used to teach the method, have significant effects on its failure rate.
The Pearl Index has unique shortcomings, however. It assumes a constant failure rate over time. That is an incorrect assumption for two reasons: first, the most fertile couples will get pregnant first. Couples remaining later in the study are, on average, of lower fertility. Second, most birth control methods have better effectiveness in more experienced users. The longer a couple is in the study, the better they are at using the 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.
The Pearl Index also provides no information on factors other than accidental pregnancy which may influence effectiveness calculations, such as:
* dissatisfaction with the method
* trying to achieve pregnancy
* medical side effects
* being lost to follow-up
A common misperception is that the highest possible Pearl Index is 100 – i.e. 100% of women in the study conceive in the first year. However, if all the women in the study conceived in the first month, the study would yield a Pearl Index of 1200 or 1300. 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.
In 1966, two birth control statisticians advocated abandonment of the Pearl Index:
See also
*
Comparison of birth control methods
There are many methods of birth control (or contraception) that vary in requirements, side effects, and effectiveness. As technology, education, and awareness about contraception has evolved, new contraception methods have been theorized and p ...
*
Decrement table Decrement tables, also called life table methods, are used to calculate the probability of certain events.
Birth control
Life table methods are often used to study birth control effectiveness. In this role, they are an alternative to the Pearl Ind ...
*
Life table
In actuarial science and demography, a life table (also called a mortality table or actuarial table) is a table which shows, for each age, what the probability is that a person of that age will die before their next birthday ("probability of deat ...
Footnotes
{{Birth control methods
Birth control
Methods of birth control
Clinical research