Figure 1

Two-stage stratified random sampling scheme detailing selection of 500 women from the control (HPV-unvaccinated) arm of the Costa Rica Vaccine Trial (CVT). Legend: Stage 1 selected 388 women who were HPV16 DNA-negative at the cervix at enrollment, with sampling stratified by HPV16 serostatus at enrollment and HPV16 incident infection status over 4 years of follow-up. Stage 2 augmented the sample using analogous variables for HPV18, selecting 112 women who were HPV18 DNA-negative at enrollment. Twelve women for whom an assay failed were then excluded for analysis. The combined (Stage 1 + Stage 2) sample was weighted to represent all women in the CVT control arm who were HPV16 DNA-negative, HPV18 DNA-negative, or both at enrollment. *For Sampling Stage 2, the women selected in Stage 1 were not eligible to be selected again. Thus, 369 women were removed from the pool of eligible women in Stage 2; this group includes all women selected in Stage 1 who were not HPV18 DNA positive at enrollment (N = 388 selected in Stage 1 – 19 HPV18 DNA-positives = 369).