Behavioural Surveillance Surveys - The Wisdom of Whores

What should be done if, even after an
allowance for surprises has been made, the
“accomplished” sample falls short of the target
sample size? The preferred course of action
would be to choose a supplementary sample
of PSU’s and include these in the survey.
The same procedures used in choosing the
original sample of PSU’s should be followed in
choosing a supplementary sample, except that
PSU’s chosen in the original sample should be
deleted from the sampling frame prior to
choosing the supplementary sample. If this
option is chosen, the sample selection process
needs to be carefully documented so that
sampling probabilities may be calculated.
A second, less desirable option would be to
return to the original sample of PSU’s for
additional intervals of data collection.
What should be done in cases where all
PSU’s are already included in the sample, and
it is thus impossible to choose more PSU’s?
In this situation the only alternative is to visit
sample sites/PSU’s for longer intervals than
had been originally planned.
What should be done if even after repeated
visits to all PSU’s, it is not possible to reach the
target sample size? The answer to this question
depends upon the underlying reason why it was
not possible to reach the target sample size.
One possible cause is that the sampling frame
was incomplete. In such a case, one option
would be to update the sampling frame and
choose a supplementary sample of PSUs of
sufficient size to enable the target sample size
to be reached. Alternatively, a lower-thanplanned
sample size could be accepted for
the current survey round, and more resources
could be put into sampling frame development
in subsequent survey rounds in which
larger sample sizes would be used. It will be
noted that larger sample size in subsequent
survey rounds can offset the effects of sample
size deficit in earlier rounds.
One approach to dealing with this problem
which was tried in Nepal was to inquire from
successfully interviewed establishment-based
sex workers about any friends who worked
at the same establishment but had not been
present during the times that data was being
collected for the sub-population survey.
These “leads” were then followed up and
included in the sample as having been
sampled from the “referring” establishment.
Such an approach should be used cautiously
and as a last resort, as it departs from a probability
sampling approach. Furthermore, it is
necessary to verify that such “leads” truly
belong to the sub-population being monitored.
In the Nepal case, for example, it was
found that a number of
the “leads” were in fact not sex workers,
but friends of the sex workers who were
“nominated” in order to collect the incentive
offered for identifying other sex workers.
In the final analysis, it is probably better to
accept a lower than expected sample size than
stretch the sampling methodology to the point
where the survey data cannot be justified as
having been produced using scientifically
defensible sampling methods.
A final note on the problem of insufficient
sample size is that in some instances, it may
be the case that there simply are not enough
members of the sub-population. In such
cases, the key issue is not the sampling
method used, but rather whether there is
sufficient justification for doing surveys for the
sub-population in question.
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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