In relation to longevity statistics, a covariate is a potential differentiating factor that might explain some of the variation in mortality rates and thereby increase the extent to which assumed mortality rates and survival curves can be tailored for a particular pension scheme. Examples of covariates that are normally capable of analysis in studies of longevity include age, gender, income level, reason for retirement, age of retirement, duration since retirement, location and socio-economic indicators derived from the pensioner’s location.