Behavioural Surveillance Surveys - The Wisdom of Whores

Procedure for second stage sampling
For fixed number from each cluster.
The procedure for second stage sampling
is described in detail in Chapter 4. It involves
selecting respondents from among members
of the sup-population at the site in a
systematic way. In this example, 10 women
need to be randomly selected from each timelocation
cluster.
It is important that the team keep track of
the information in Table 1 (below) for each
cluster. A prototype cluster information form
is provided in Appendix 4 to help the survey
manager organize the fieldwork. It is important
to track this information separately for each
cluster, and for each cluster to have a unique
identifying number which is recorded on the
sheet. If this information is not tracked, it will
not be possible to calculate the sampling
probabilities.
Table 1 : Information to record
at each cluster
• cluster number and location
• number of respondents invited to
participate in the survey
• number of completed interviews
• number of refusals
• number of duplicates (if the person was
previously interviewed as part of this
round of the BSS in this sub-population
on another day or at another site)
• the total measure of size for the cluster
(number of respondents who come in
contact with the site during the specified
time interval)
Variation when “take-all” strategy is used
The second option that was described for
second stage sampling was the “take-all”
strategy, which has the advantage of resulting
in a self-weighted sample. The procedure
used for first stage sampling is exactly the
same as the one described above for floating
sex workers. The only difference is that
during the second stage of sampling, instead
of sub-sampling a fixed number from among
all women who appear at the site during a
fixed interval, all women who appear at the
site during the specified time frame must be
invited to participate. For this reason, it is not
recommended that this strategy be used unless
the expected number of respondents is small
for all clusters. Note that use of this strategy
will result in a final sample size that is
unpredictable. Therefore, this method should
not be used unless there are very good
estimates available from the mapping exercise
about the average number of respondents
who can be expected at a site during a given
time interval. It is also very important to pilot
test the process, so that all the assumptions
can be verified.
The importance of pilot testing
When using the “take-all” design, it is a
very good idea to pilot test the assumptions
used in calculating the inflated samples sizes
(i.e. refusal and duplication factor), and also
the number of clusters needed to get the
desired number of respondents (i.e. average
“sample take” per cluster. If this is not done,
then it is very easy to “over” or “undershoot”
the desired sample size.
B EHAV I OR A L S U R V EI L L A NC E SURV EY S APPEN DI X 3
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