Behavioural Surveillance Surveys - The Wisdom of Whores

The take-all approach
The first option is to use a “take-all”
approach, whereby every population member
who appears at the site during a fixed time
interval is included in the sample, irrespective
of how many that turns out to be. This
approach has the advantage of resulting in
a self-weighted sample. However it is often
not feasible to implement, especially where
the number of population members who come
into contact with the site is expected to be large.
This approach also has the disadvantage of
resulting in an overall sample size that may
be different from that expected.
A rule of thumb is that the “take-all” method
should not be attempted unless the average
number of individuals per time-location cluster
is expected to be small (i.e. 15 or fewer).
A rapid mapping exercise will be needed in
advance of the survey to determine the typical
or average number of individuals frequenting
sites at various time intervals.
Fixed number of respondents per cluster
The more commonly used approach for
selecting respondents at the second stage will
be to select a fixed number from each selected
PSU (time-location cluster). A discussion of
the number of respondents to include in each
PSU can be found in the section titled Number
of PSUs and sample sizes from each, later in
this chapter. Although this will not result in
a self-weighted sample, it is likely to be more
feasible to implement and it will result in a
predictable final sample size. When this
approach is used, it will be necessary to
estimate the number of population members
who appear at the site during the fixed time
interval, so that it will be possible to calculate
the probability that the population members
sampled are representative of the wider
population. This will require that someone
be stationed at the site throughout the time
interval to count population members who
appear at the site, even if the fixed sample size
of respondents is achieved before the end of
the specified time interval.
When fixed sample sizes are used with
time-location clusters, it will not be possible to
list all of the individuals who will appear at
the site ahead of time. Therefore, an approach
which is as systematic as possible will have to
be devised for randomly selecting respondents
at the site. How to do this will depend on the
number of people present at the site at the
beginning of the time interval. For example,
if the fixed number of respondents to be
sampled from each time-location cluster is 7,
then two scenarios are possible. If the
interviewing team arrives at the site and finds
fewer that 7 respondents, they can select all
those who are present at the time of arrival,
and then select the remainder consecutively,
in the order in which the respondents appear
at the site. If, on the other hand, the team
arrives at the site to find a number greater
than the required 7, they must then find a way
to randomly select 7 respondents. This could
be done by rapidly listing the respondents
(by some visible characteristic such as “man in
red shirt” or “woman with big gold earrings”
rather than by name) and then selecting every
ith respondent, using as the sampling interval
the total number of respondents present,
divided by the required sample size of 7.
Examples of how this has sometimes been
done are included in Appendix 3.
Figure 3 summarizes the discussion above,
and provides a “decision tree” to guide
selection of first- and second-stage sampling
approaches for cluster surveys.
B EHAV I OR A L S U R V EI L L A NC E SURV EY S CHAPTER 4
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