Fig. (1) Illustration of misclassification of HPV infection status in studies with long interval sampling. Four hypothetical women are represented from onset of sexual debut (diamond) through their lifetime of type-specific HPV infection. All women acquire HPV soon after sexual debut. Woman #1 then clears infection sometime between t2-t3, woman #2 has infrequent recurrent detection throughout her life, woman #3 has infrequent reactivation then reactivation with longer duration at later life, and woman #4 has an increase in reactivation frequency (but not duration) in later life. If all women were enrolled at t1, woman 1 & 2 would have no recurrent detection observed, despite periodic recurrence in woman 2. Woman 3 &4 would be found to have a single recurrence at time 5, and the multiple recurrent detection which occurs during the sampling interval would be missed. If all women entered the study at t3, the recurrent events in women 3 & 4 would be misclassified as 'incident' because the past HPV infection was never observed under study follow-up.