Thailand’s sero surveillance system became supplemented by a sentinel behavioral surveillance system in 1995 to provide a fuller assessment of current trends in the epidemic. The sentinel sites were set up in 20 of Thailand’s 76 provinces. The sentinel behavioral populations included army conscripts, male and female factory workers aged 15–29 years, male and female grade 11 students from secondary schools and pregnant women attending public antenatal clinics. The system annually collects data from 350 persons per sentinel population per province using non-probability sampling (quota samples collected through non-systematic selection of venues) and collection of data through self-administered questionnaires [4–5]. In practice, some sentinel sites do not use the assigned sampling methods and do not obtain sufficient sample sizes of some subpopulations [6]. Because of biases in the selection process and inadequate sample sizes, these surveillance results may deviate significantly from true prevalence and cannot accurately describe the magnitude of risk and trends in this risk over time. These limitations have been present in multiple target populations at both the provincial and national levels.