A standard approach for analyzing time to first incident detection of HPV of a particular type, such as HPV16, is to use a Cox proportional hazards model. The Cox model is defined as follows: (1.7)λ(ti)=λ0(ti)eβZi(ti)+γWi(ti) where λ0(t) is the baseline hazard function and exp(β) is the hazard ratio of HPV16 infection between women in a certain CD4+ / HIV RNA stratum and HIV-negative women, while holding all other factors constant. Both the primary covariate Z and adjusting covariates W can be time-dependent, i.e., the values of Z and W are allowed to change during the follow-up.
