Behavioural Surveillance Surveys - The Wisdom of Whores
Behavioural Surveillance Surveys - The Wisdom of Whores Behavioural Surveillance Surveys - The Wisdom of Whores
4 Sampling approaches This section of the BSS guidelines covers the topics of survey measurement and sampling issues specific to behavioral surveillance of HIVrelated high-risk behaviors. It is designed for use with behavioral surveys in general, and for defining sampling strategies for hard-to-reach, hidden and mobile populations in particular. The importance of sampling Sampling is of crucial importance in measuring trends over time. Sampling strategies should be systematic and replicable over time. If they are not, then any changes observed over time may simply be the result of different sampling strategies between sample rounds. A great deal of work has been done over the years in developing sampling methods that provide representative samples for the general population. Large international survey programs such as the Demographic and Health Survey series have perfected the art of household sampling. Sampling from established institutions such as schools is also a well-developed practice. These sampling methods are extensively described in many publications. While examples of these methods are given in Appendix 3, they are not covered in detail in this chapter. B EHAV I OR A L S U R V EI L L A NC E SURV EY S CHAPTER 4 29
The majority of sub-populations of particular interest to BSS are precisely those that are not easily accessible through conventional household or institutional sampling techniques. The primary challenge to conducting meaningful BSS is to devise sampling plans that are both feasible and capable of producing unbiased estimates (or, more realistically, estimates with minimal levels of bias) for population sub-groups that are not easily “captured” in household surveys. As a practical matter, this will require (1) the use of conventional, probability sampling approaches in non-conventional ways, (2) the use of different sampling strategies for different sub-populations, and (3) the occasional use of non-probability sampling methods in situations where probability methods are not feasible. Devising the sampling plan Once the step of selecting the sub-populations has been completed, a sampling plan must be devised before the actual mapping and development of the sampling frame can take place. During the process of mapping, the original sampling plan is usually revised several times, as the survey managers begin to understand the patterns of the population, and the effects of these patterns on operationalizing the design. This is particularly true for hardto-reach populations, such as sex workers, injecting drug users, men who have sex with men, and mobile populations, among whom BSS is frequently conducted. The first step in devising a sampling plan is to select a sampling approach. In this chapter, two sampling approaches are proposed for hard-to-reach populations: multi-stage sampling and a modified form of snowball sampling known as “targeted” sampling. Next, sample size requirements are considered. The final section discusses a number of key design issues and parameters for the sampling strategies proposed. Probability and non-probability sampling methods: issues and challenges Sampling procedures may be thought of as falling into two broad classes: formal or probability methods, and informal or non-probability methods. In a probability sample, every person in the defined universe may be selected into the sample, with a known (non-zero) probability. Probability sampling tends in practice to be characterized by the use of lists or sampling frames to select the sample, and by clearly defined sample selection procedures. With a probability sample, it is possible to use the data themselves to estimate the sampling error, or the effect of random fluctuations in sample selection on the accuracy of the observed results. Estimates of population characteristics derived from surveys based upon probability sampling methods may be expected to approximate the “true” population value (i.e. proportion or mean) within a specified margin of error with a known probability. 30 C H A PTER 4 B EHAV I OR A L S U R V EI L L A NC E S U R V EY S
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4<br />
Sampling<br />
approaches<br />
This section <strong>of</strong> the BSS guidelines covers the topics <strong>of</strong> survey measurement<br />
and sampling issues specific to behavioral surveillance <strong>of</strong> HIVrelated<br />
high-risk behaviors. It is designed for use with behavioral surveys<br />
in general, and for defining sampling strategies for hard-to-reach, hidden<br />
and mobile populations in particular.<br />
<strong>The</strong> importance <strong>of</strong> sampling<br />
Sampling is <strong>of</strong> crucial importance in<br />
measuring trends over time. Sampling strategies<br />
should be systematic and replicable over time.<br />
If they are not, then any changes observed<br />
over time may simply be the result <strong>of</strong> different<br />
sampling strategies between sample rounds.<br />
A great deal <strong>of</strong> work has been done over<br />
the years in developing sampling methods<br />
that provide representative samples for the<br />
general population. Large international survey<br />
programs such as the Demographic and<br />
Health Survey series have perfected the art<br />
<strong>of</strong> household sampling. Sampling from<br />
established institutions such as schools is also<br />
a well-developed practice. <strong>The</strong>se sampling<br />
methods are extensively described in many<br />
publications. While examples <strong>of</strong> these<br />
methods are given in Appendix 3, they are not<br />
covered in detail in this chapter.<br />
B EHAV I OR A L S U R V EI L L A NC E SURV EY S CHAPTER 4<br />
29
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