Behavioural Surveillance Surveys - The Wisdom of Whores

Other measurement issues
for BSS
The primary objective of undertaking BSS
is to provide a basis for tracking or monitoring
changes in selected risk-taking or protective
behaviors among population sub-groups
that are strategically important to the AIDS
epidemic. These measurement objectives
have a number of important implications for
the design of surveys.
Should PSUs be retained or replaced in
each survey round?
One of the key design issues in multi-round
surveys whose primary objective is to measure
change is whether to retain the same PSUs or
to choose a new sample of PSUs in each
survey round.
There are two advantages in retaining
the same sample of PSUs. The first is that
it reduces the sampling frame development
work that needs to be done at the beginning
of each survey round. The second is that
it increases the confidence with which it can
be concluded that observed changes over time
are not due to random changes in sample
selection. This is because the background
characteristics and behaviors of individuals
associated with particular sites are likely to be
correlated over time. So for example men at
a particular gay bar may report high numbers
of partners in the last 12 months, because that
bar is known as a pick-up joint. If men at the
same bar next year report lower numbers of
partners in the last 12 months, it is likely to
mean that there has been an overall tendency
towards less risk behavior, even at the highrisk
end of the spectrum. However if the
following year men are sampled from a
different bar and lower partner numbers are
observed, the difference may well be related
to differences in the clientele. The second
bar may be favored more by couples who
have fewer non-regular partners, while levels
of risk behavior among men at the “pick-up”
bar may be as high as ever.
The effect of this correlation on statistical
precision, is to reduce the standard error of
survey estimates of change by a factor equal
to (1 - PR). P is defined as the proportion of
sample overlap between the two survey
rounds and R is the correlation (or more
precisely, the covariance) between indicators
for the same site across survey rounds.
However there are also important disadvantages
to retaining the same sample of PSUs
across survey rounds. The “gatekeepers”
who control access to PSUs (brothel owners,
for example) sometimes object to repeated
survey rounds in their establishments.
The response of individuals who are selected
for more than one survey round may be
influenced by having participated in a previous
round, and this is much ore likely to be the
case where PSUs are retained over time than
when new samples are drawn. And in the
fluid worlds of commercial sex, drug injection,
migrant labor etc, high proportions of PSUs
may simply not be there at a subsequent
round. In some settings the sites at which
sub-populations congregate might change
so rapidly over time that there is no choice
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C H A PTER 4 B EHAV I OR A L S U R V EI L L A NC E S U R V EY S