Behavioural Surveillance Surveys - The Wisdom of Whores

The second stage sampling procedure will
involve taking a fixed number from each site.
The following describes how this was done
based on a design developed for use with
brothel-based sex workers in an Asian setting.
Some details and numbers have been modified
for the sake of clarity.
It was decided to use a self-weighted twostage
cluster design, selecting PSUs by PPS at
the first stage, and sampling an equal numbers
of respondents from each brothel at the
second stage. This was possible because
women lived at the brothels, so they were
associated with the sites in a fixed manner,
and because measures of size were available
for each brothel. The total sample size was
300. The number of women at each brothel
ranged from 10 to 30, so it was decided to
select 20 clusters of 15 sex workers each,
to reach the desired sample size.
Sampling approach : PPS sampling with
equal cluster sizes
Sampling frame : List of brothels with
MOS (# of women
living at the brothel
or connected to
the brothel)
Sample size : 300
Cluster size : 15
Number of clusters
required : 20
Second stage sampling procedure
Once the clusters had been selected,
the supervisor and interviewers visited the
brothels and the supervisor randomly selected
15 women to be interviewed from each one.
There are various ways of doing the random
selection. In this situation it was important
not to select volunteers, and not to let the
brothel owner select which women should be
interviewed. In order not to make the women
feel that they were being singled out, the
supervisor made the process into a game.
(Note that this might not be appropriate in
every setting). Each woman was given a piece
of paper with a number. Pieces of paper
with matching numbers were put into a hat.
One of the women from the brothel was then
asked to pick fifteen numbers from the hat,
and the women holding the papers with
the matching numbers were selected to be
interviewed. For each one who refused,
another number was drawn from the hat
to randomly replace the women who had
refused. Refusal rates were carefully recorded
by keeping track of the total number of
women who were invited to be interviewed,
and the total number who refused. For
record-keeping, it is good practice to have
well-designed forms to help field workers
keep track of information. Sample forms are
provided in Appendix 4 of this guide.
First stage sampling procedure
Following the procedures outlined in
Figure 1 and Table 2 in chapter 4 on sampling,
clusters were selected by PPS.
This process is illustrated in Figure 2.
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A PPEN DI X 3 B EHAV I OR A L SURV EI L L A NC E S U R V EY S