Behavioural Surveillance Surveys - The Wisdom of Whores

If there is no reason to believe that the
behavior of individuals at a given site varies
according to the time they visit the site (and it
is only the volume of individuals that is likely
to change) then it does not matter when
sampling at that site takes place. For example,
injecting drug users who visit a shooting
gallery in the afternoon may be unlikely to
have different injecting behaviors than
injecting drug users who visit a shooting
gallery in the morning or the evening. In this
case, the site can be included only once as a
PSU, and the time can be randomly selected
from a calendar listing.
If, on the other hand, work in the rapid
assessment phase shows that behaviors of
interest vary according to when a person
visits a site, then more work must be done
to characterize these differences. The day
(or week, or month) must be divided into
discrete time intervals according to variations
in respondent behavioral type. The PSU in
this case becomes the site during each of the
different risk periods specified, and each of
these “PSUs” is entered separately into the
listing of PSUs for the sampling frame.
For example, if some sex workers are known
to sell sex during periods of high demand
following payday but not during the rest of
the month, a single block of a red light district,
say Block A, might be entered three times as
follows: “Block A, week before payday”
“Block A, week after payday” and “Block A,
rest of the month. Other examples are
provided in Appendix 3.
It should be noted that in time-location
sampling where the number of sites is small,
one site may be entered in the sampling frame
on more than one occasion simply to reach
sample size requirements. Where there is no
variation expected in behavior with time,
several calendar time periods can be randomly
selected for the same site to create multiple
PSUs for inclusion in the sampling frame.
Where the same individuals visit the same
locations on a fairly regular basis, this may
result in rather high duplication of respondents.
Measures to eliminate duplicate respondents
are discussed in the section titled duplicate
observations later in this chapter. Duplication
may also occur between sites, since members
of “floating” populations may visit several
different sites associated with risk behavior
for HIV and selected as PSUs.
Once this list of PSUs has been created,
with both single and multiple entries as
appropriate, PSUs should be selected with
equal probability, using the same process
outlined in Figure 2 and Table 4.
Selecting respondents within
sample PSUs for “floating”
populations
There are two options available for selecting
respondents at the second stage when timelocation
sampling is used. These are discussed
below. The single most important aspect of
sampling at this stage is to ensure that sampling
takes place over a fixed time interval, which
is the same for every PSU selected. This
interval may vary according to the average
volume of members of a sub-population
visiting sites in a country. Typically it will be
one hour. The essential thing is that, once
selected, the same time interval for sampling
be applied to every selected PSU.
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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