Behavioural Surveillance Surveys - The Wisdom of Whores

The list of commercial sex establishments
is used to construct a sampling frame,
consisting of the name of the establishment
(or some other identifying information about
the establishment), the measure of size
(number of sex workers working at each
establishment) and the cumulative measure
of size. Once this sampling frame has been
created, then the procedures in Figure 1 and
Table 2 in Chapter 4 for the basic two-stage
cluster design can be followed.
Two dimensions that should be accounted
for in constructing the sampling frame are
geographic location within the area covered by
a survey and type of establishment. What is
typically done to ensure an adequate “spread”
of sample PSU’s with respect to such characteristics
is to arrange or order the sampling
frame on the basis of location prior to sample
selection. For example, commercial sex
establishments in a city might be ordered by
first listing establishments (or more precisely,
the PSU’s associated with them) located in the
northwest quadrant of the city, followed by
establishments in the southwest quadrant, the
southeast quadrant, and finally the northeast
quadrant. Within each quadrant, establishments
/PSU’s would be ordered by each of the
different types of establishments that are
considered as part of the sampling frame for
that target group ; for example, brothels first,
followed by massage parlors, karaoke lounges,
etc. When dealing with sex workers with
no fixed worksite (as described in the section
on “Selecting Primary Sampling Units (timelocation
clusters) for Floating Populations”
in Chapter 4), the type of site might include
street corners, parks, or other pickup points
(restaurants, bars, hotels, etc.)
Step-by-step sampling procedures
Once the sampling frame has been
developed, a sample of PSU’s can be chosen
either with probability proportional to size
(PPS) or with equal probability, as was
described in Chapter 4. PPS selection should
be used when establishments vary significantly
in terms of numbers of sex workers associated
with them (e.g., when there are larger
establishments with 3-5 times as many sex
workers as smaller establishments) and when
it is possible to obtain measures of size prior
to sample selection. Where the numbers of
sex workers associated with establishments
are roughly comparable (i.e., vary in size by a
factor of less than three), sampling with equal
probability will suffice.
The number of PSUs or clusters to be
chosen will depend upon the number of sex
workers expected to be found at establishments.
For example, if the target sample size for a
survey were n=300 sex workers and it is
thought that on average 10 sex workers may
be found per site, then 30 clusters might
be chosen. As noted earlier, however, it is
important that the sample be spread out across
as many clusters as is feasible for a given
survey effort. For example, if it were anticipated
that an average of 30 sex workers could be
found per site, although 10 clusters would give
you the 300 women needed for the sample,
having only 10 clusters would be insufficient
in terms of spread. In this event, the preferred
course of action would be to choose 20 sites
of 15 women each.
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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