Behavioural Surveillance Surveys - The Wisdom of Whores

ut to construct a new sampling frame and
select a new sample of sites in each survey
round. In addition, retaining the same sites
over an extended period of time does not
allow for new sites or “pockets” of risk
behavior to be reflected in the behavioral
monitoring data. Accordingly, there will be a
need to periodically update sampling frames
and allow new PSUs to have a chance to enter
the sample even if the strategy of maintaining
the same sample of sites is adopted.
A compromise strategy that is often used in
repeated surveys is to retain a fixed proportion
of sites between any two successive survey
rounds and replace the remaining sample of
sites with a new sample. For example, if a
50% retention rule were adopted, 50% of the
sites covered in the first survey round would
also be covered in the second round, 50% of
the sites covered in the second survey round
would also be covered in the third round,
and so on. The site rotation schedule could
be set up such that no site is visited more than
twice if it is felt that more than two visits to
each site were to be problematic in a given
setting. Under this strategy, retaining a fixed
proportion of sites between any two survey
rounds serves to dampen sampling variability
in the measurement of change, while at the
same time avoiding some of the negative
consequences of following the same set of
sites continuously over time.
In the particular context of BSS, the disadvantages
of retaining the same sites over time
in sub-population surveys seem to outweigh
the advantages. Therefore, the general
recommendation for BSS is to choose a new
sample of PSUs in each survey round.
While this will entail field costs in updating
the sampling frame, the costs of updating
are likely to be substantially lower than those
incurred in developing the sampling frame
for the initial round of surveys.
Duplicate observations
Irrespective of which sampling method
is used, one problem that will need to be
addressed in carrying out BSS is how to
handle duplicate observations of the same
individual. Duplicate observations are likely
to arise because some respondent group
members may be associated with more than
one of the sites or locations from which
respondents are to be recruited for the survey.
For example, sex workers may work at more
than one location or truck drivers may use
more than one truck stop during the period
when a survey is being carried out, and they
may therefore be sampled more than once.
One way to deal with this problem would
be to adjust the sampling weights to be
applied to the survey data at the analysis stage
in order to account for the fact that individuals
might be included in the survey sample more
than once. This requires rather complex
record keeping and data analysis techniques,
and is not recommended for BSS.
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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