Behavioural Surveillance Surveys - The Wisdom of Whores

Number of PSUs (clusters) and sample sizes
from each
Once sample size requirements have been
determined, the final step in developing the
sample size parameters for two-stage surveys
is determining how many PSUs should be
chosen, and how many individuals per PSU.
There are three primary considerations in
reaching decisions on these issues. The first
is the magnitude of the two-stage sampling
design effect (D). This is particularly important
with behaviors that are likely to be influenced
by association with a PSU itself. Frequency of
condom use among sex workers is more likely
to be associated with the PSU where the PSU
is a brothel (because brothel-owners often
have policies on condom use) than it is where
the PSU is a city block (because freelance sex
workers and their clients are not subject to the
rules of a brothel). In situations where there
is likely to be considerable homogeneity
among individuals in a PSU, a smaller sample
size per PSU is particularly desirable.
The second consideration is the actual size
of the PSUs. Obviously, the sample size per
PSU is limited by the number of individuals
associated with the PSU. So before deciding
on sample sizes per PSU, it is worth obtaining
measures of size for a handful of PSUs thought
to be roughly representative of the range
available. This information should guide
decision-making.
Thirdly, there is the issue of cost (in time,
money and effort). For the same overall
sample size, a design that includes fewer
respondents from a larger number of PSUs
will likely (although not necessarily) be more
costly than a design that interviews large
numbers of respondents from a smaller
number of sites.
As so often in sampling, there is clearly a
tradeoff between what is feasible and what
will produce the most reliable results, and
surveillance managers must weigh the costs
and benefits of various approaches. From a
sampling precision point of view, more PSUs
with a smaller number of respondents selected
from each is the best option. For a fixed
target sample size (e.g., 400 sex workers),
a design that selects 10 individuals from each
of 40 PSUs will give more reliable results than
one which selects 40 individuals from each of
10 PSUs. As a general rule, selecting no more
than 20-25 study subjects per PSU should be
relatively safe. As that number increases, the
reliability of the study results will decrease.
Sampling more than 40 individuals from each
PSU should be avoided.
Although the use of 30 PSUs has become
a standard of sorts in behavioral surveys,
there is in fact no statistical justification for 30
as a minimum or ideal number. There is,
however, a need to ensure that samples of
sub-population members are sufficiently
well “spread” across enough PSUs that survey
estimates are not unduly influenced by
behaviors practiced in only a handful of PSUs.
As a working guideline, a minimum of 20 PSUs
per respondent group is recommended, and
more is desirable when feasible.
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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