The PHREG procedure fits the proportional hazards model of Cox (1972, 1975) to survival data that might be right censored. The Cox model is a semiparametric model in which the hazard function of the survival time is given by
![\[ \lambda (t;\mb {x}) = \lambda _0(t) \mr {e}^{\bbeta ^{\prime }\mb {x}(t)} \]](images/statug_introsurv0006.png){kind=small}
where 
is an unspecified baseline hazard function, 
is a vector of covariate values (possibly time-dependent), and 
is a vector of unknown regression parameters. The model is referred to as a semiparametric model because part of the model
involves the unspecified baseline function over time (which has an infinite dimension) and the other part involves a finite
number of regression parameters. Several texts that discuss the Cox regression models are Collett (1994); Cox and Oakes (1984); Kalbfleisch and Prentice (1980); Lawless (1982). Extensions of the Cox model are discussed in Therneau and Grambsch (2000); Andersen et al. (1992); Fleming and Harrington (1991). For more information about PROC PHREG, see Chapter 67: The PHREG Procedure.