Behavioural Surveillance Surveys - The Wisdom of Whores

The information from the mapping exercise
suggests the following.
• The sampling frame should include both
daytime and night-time time-location
clusters, as well as clusters on both weekends
and weekdays. This is needed in
order to ensure a good distribution of sex
workers with different characteristics.
• A “take-all” approach would not be prudent,
since the possibility exists of selecting some
very large time-location clusters.
• The time interval should be two hours per
site, and the cluster size should be fixed at
10 women per site, since that is the average
number of floating sex workers to be found
in two hours.
• Some sites will inevitably not yield the
desired ten respondents, due to the fact that
some women will refuse, some will be
duplicates, and some clusters will have
fewer than ten women to begin with.
Therefore a few extra clusters should be
chosen to make up the shortfall. Note:
the fact that some sites will not yield ten
women is of no consequence, since this is
not a self-weighted design.
Step by step sampling procedure
Sampling approach: Time-location clusters
with fixed cluster size
Sampling frame: List of bars, hotels and street
corners frequented by floating sex workers
Time interval: Each selected cluster will be
included for two hours. This means that every
sex worker who comes in contact with the site
during the two-hour time selected for the
survey, is included in the measure of size for
the cluster. The measure of size is needed to
calculate the sampling probability.
Calendar timeframe: Each site should be
included in the sampling frame three times per
week (one weekday night, one weekday
afternoon and one weekend night) over a
three-week time-frame which spans a payday.
Sample size: 300
Cluster size: 10
Number of time-location clusters to be selected:
34... this includes 300/10=30 clusters plus 4
extra clusters to make up the shortfall from
clusters that end up with fewer than ten
respondents. So the total number of clusters
should be 34.
First stage sampling procedures
Figure 3 provides an example of a sampling
frame which includes 25 sites each listed three
times, once on weekday night, once on a
weekday afternoon and once on weekend
night. In reality, it is possible that not all
sites will be included at all three times in the
sampling frame. Sites should be included only
at times when they are known to be active,
based on what is learned in the mapping
exercise. Sites are numbered from one to 25,
with each number corresponding to a site
that has been mapped.
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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