Guidelines on surveillance among populations most at risk for HIV

2.3.6 Convenience samplingDescriptionA convenience sample includes persons in the sample who are easy to reach or convenient to sample. Anexample might be testing and interviewing all sex workers attending an STI clinic.AdvantagesAlthough convenience sampling is not a form of probability sampling, data from convenience samples maybe the only information available on populations most at risk for HIV transmission.LimitationsIn convenience sampling, no rules are used for sample selection. There is no way to estimate error or biasin this sampling scheme.2.3.7 Time–location samplingDescriptionMany of the sampling methods discussed above rely on non-probability methods or tend to overestimatepeople who are more networked. Time–location sampling methods can avoid these limitations.This is a probability-based surveillance method that relies on a sampling frame derived from the timeswhen and locations where members of a target population congregate rather than where they live (44).To conduct time–location sampling: List all the venues where the population congregates as the primary sampling unit. For example, thepopulation may meet in certain clubs. Randomly select specific times, days and venues. The selected venues are visited during the day and time specified. Subjects are systematically approached and asked to participate.This represents a probability-based sample if: Every member of the target population has an equal probability of being at the venue at any given timeor day. Every person selected agrees to participate. Everyone gives truthful responses.Time–location sampling requires good-quality formative research to ensure that no bias is entered intoselection of the venue, day and time.AdvantagesA time–location sample can be derived from key informant interviews which describe where members ofthe population congregate. Once a full census of these sites has been identified, a random selection ofthese sites can be chosen for inclusion in the sample. This information will also be useful for preventionactivities.LimitationsA number of limitations exist. The venues selected might not necessarily be frequented by all of the target population. It is difficult to estimate the probability of missing someone who does not attend any of the venues. Some venues offer little privacy for disclosure of sensitive information. The accuracy of self-reported data given in a public setting is questionable. Identifying venues that most-at-risk populations frequent could be exposing them to unwanted attention.Although it is time- and labour-intensive, time–location sampling does have utility for surveillance amonggroups at higher risk for HIV infection as a result of high-risk behaviour (45).More guidance on how to conduct a time–location survey can be found at: http://globalhealthsciences.ucsf.edu/PPHG/assets/docs/tls-res-guide.pdf.20