An alternative to the WLW method is to use frailty models to analyze multiple incident HPV infections(27). A frailty model introduces a frailty term, νi, to the Cox model as follows (1.10)λk(tik)=λ0(tik)vieθk+δzi(tik)+ηWi(tik) Where νi, treated as a random effect, represents the ith woman’s unobserved individual susceptibility to develop an oncogenic HPV infection relative to the average level in the population. The coefficient δ is interpreted as the log of the hazard ratio for any incident oncogenic HPV detection in a woman at a certain CD4+ / HIV RNA stratum compared with her risk if she were HIV-negative while holding all other factors constant. Frailty models are a form of mixed effects model, and the parameter estimate δ has a subject-specific interpretation. Model (1.10) can also be extended to assess HPV type-specific associations (not shown).
