Covariate
\koʊˈvɛər i ɪt\
A rating factor which is allowed to vary simultaneously in a longevity model, used to differentiate between individuals with different characteristics.
A rating factor could alternatively be treated as a stratifier, where separate models would be developed for each value of the factor.
Treating rating factors as covariates in Generalized Linear Models allows you to maximize the amount of data used to infer the impact of any particular rating factor.
An example of the use of covariates in longevity modeling is the use of pension bands, occupation, and geo-location (ZIP codes, postal codes or post codes depending on location) in the VitaCurves models.