Life table methods are often used to study birth control effectiveness.
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.
Two kinds of decrement tables are used to evaluate birth control methods.
Multiple-decrement (or competing) tables report net effectiveness rates.
Single-decrement (or noncompeting) tables report gross effectiveness rates, which can be used to accurately compare one study to another.