The effectiveness of a family planning method can be measured by the number of unwanted pregnancies that occur when the method is used.

This failure rate is expressed as the so-called Pearl index (called after the statistician who introduced the formula for its calculation). The index gives the number of unwanted pregnancies that occur when 100 couples use a method for 12 months. Here is an example:

If 100 couples use condoms for contraception for one year and during this time 3 pregnancies occur, the condom has a failure rate (Pearl index) of 3. So the lower the Pearl index, the more effective the method.

However, the Pearl index can only be used as a general guide because it does not take into account some important factors. For example, it does not only count the pregnancies that result from 'failure' of the method but also those that result from incorrect use of the method.

When assessing the effectiveness of a contraceptive method it is therefore better to distinguish between

	pregnancies that occur when the method is used correctly during every act of intercourse, i.e. when the user has kept exactly to the instructions in the pack insert or the instructions for use, and
	pregnancies that occur on typical use of the method, i.e. taking into account all possible interfering factors (e.g. missed pills, incorrect use of a barrier method, frequency of sexual activity, interactions with other drugs).

Scientists and statisticians often use completely different methods to assess the effectiveness of a method. This explains why data on the effectiveness of the different contraceptives often vary.

However, the data do show consistently that contraceptives such as the pill or the intrauterine system are the most effective reversible methods of contraception.