Behavioural Surveillance Surveys - The Wisdom of Whores

Implications of alternative sampling strategies
for analysis
The choice of sampling method has important
implications for data analysis. More can be
done during analysis to compensate for
potential biases in survey data when probability
sampling methods have been used than when
non-probability methods have been used.
For example, the fact that some respondent
group members may have had a greater
chance of being included in a survey than
others can be taken into account in probability
samples by introducing sampling weights.
These procedures are described in Chapter 5.
Such adjustments are not possible with nonprobability
sampling schemes. The use of
conventional statistical procedures to determine
the statistical significance of observed changes
in indicators is also on more solid theoretical
grounds when probability sampling methods
are used.
Probability sampling methods often imply
a greater investment in developing sampling
frames, and complex fieldwork. The trade
off between complexity of fieldwork and
precision of results should be taken into
account when choosing a sampling approach.
In general, more reliable survey results are
more likely to lead to more appropriate
decisions about investment in effective HIV
prevention efforts, so where probability
designs are possible, they are usually worth
the effort.
Sample size requirements
One of the key design parameters for any
survey is, of course, the sample size needed to
satisfy the survey’s measurement objectives.
In this section, procedures are presented for
calculating sample size requirements for
repeated rounds of BSS. Several points should
be borne in mind when reviewing the material
below. First, the procedures presented are
intended for surveys whose primary objective
is to measure changes in selected behavioral
indicators over time. The sample sizes required
to measure changes in indicators over time
are larger than those required to measure a
variable or indicator at a single point in time,
and this must be taken into account in
order to ensure sufficient statistical power.
Secondly, sample size requirements are
addressed here with respect to indicators
measured as proportions. This is the type of
indictor most commonly used in behavioral
surveillance for HIV. Examples might include
the proportion of respondents who used a
condom the last time they had sex with a sex
worker, or the proportion of respondents who
shared injecting equipment the last time they
injected drugs.
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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