Behavioural Surveillance Surveys - The Wisdom of Whores

Notes:
1. It may not always be possible to randomly
select respondents in this fashion, but one
should always try to find a way to get a
headcount of target group members present
at the site, and then systematically select
the number that are needed. It is never
advisable to take volunteers instead of
selecting respondents randomly.
2. If fewer than the required number of
respondents (in this case 15) are present
at the time of the survey team’s visit, then
it will be necessary to return to the site at
a later time to complete the cluster. If this
cannot be done, then it will be necessary
to record the number of respondents
interviewed so that the second stage
probability can be calculated at a later time.
(See chapter 5 for instructions on how to
calculate sampling probabilities and conduct
weighted analysis). (Note : although
returning to a site is sometimes necessary
for this design, where conventional clusters
are being used, it is not appropriate to
return to a site when time-location clusters
are being used).
3. If, at the time of the team’s visit, it is
discovered that the measure of size for
the site that was estimated in the sampling
frame is inaccurate, then it will be necessary
to obtain an estimate of the number of
target group members who really are
associated with the site, so that sampling
probabilities can be calculated.
Variation when measure of size for the cluster is
not available ahead of time
A variation on this design is one in which a
measure of size is not available ahead of time,
so it is not possible to select clusters by PPS.
In this case, clusters will need to be selected
with equal probability (see Figure 2 and Table
3 from chapter 4), and it will be necessary to
obtain an estimate of the number of people at
the site on the day the data is collected, so
that a first stage probability can be calculated.
There are two options for selecting respondents
at the second stage. One option is to select
the same number of respondents at each site.
This makes the field work less complicated,
but the resulting sample will not be selfweighted.
So it will be necessary to calculate
sampling probabilities, and perhaps conduct
a weighted analysis. In order to avoid this,
it is possible to decide on a sampling fraction
ahead of time to be applied at each site. For
example, it could be decided that at each site,
25% of respondents found there would be
interviewed. For example, if 80 people were
found at a site, then 20 people (80/4=20)
would be interviewed. And if 25 people were
found at the site, then 6 people (25/4= 6.25)
would be interviewed. This procedure is
slightly more complicated and one issue is that
you lose control over the total sample size.
It is difficult to define what the sampling
fraction should be, so that the final sample will
not overshoot or undershoot the desired total
sample size. However, the advantage of this
approach, is that if it is done at every site, then
the final sample is self-weighted. Whichever
option is chosen, it is still necessary to obtain
an estimate of the number of respondents
at the site, so that sampling probabilities can
be calculated.
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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