The four-parameter logit life table system.

Summary Brass's model life table system, which is a two parameter system based on the logit transformation of survivorship values, has been widely and successfully used to describe age patterns of mortality in many populations. As more reliable information has become available for populations with mortality patterns which differ in important ways from the assumed standard pattern of mortality, a more flexible model system is needed. This paper shows how Brass's system can be expanded into a four-parameter model, and evaluates the performance of the new system by examining how well it can fit observed life table data.