Behavioural Surveillance Surveys - The Wisdom of Whores

a number of advantages for use as PSU’s in
household surveys, notably that (1) they
have already been mapped, (2) they have
population sizes associated with them that
can be used as measures of size during sample
selection, and (3) they are roughly the same
size in terms of population, making control
of the survey fieldwork somewhat easier.
Sample selection procedures
Sample PSU’s or clusters should be
chosen from a geographically ordered list
using systematic sampling with probabilityproportional-to-size
(i.e. proportional to the
total population or number of households
located in each CEA) as described Chapter 4.
The numbers of sample PSU’s/clusters and
youth per sample PSU will be determined by
the sample size for the population and the
expected “take” per household. For example,
if the sample size for a given male youth
population survey were set at n=500, and the
latest census data indicated that a male aged
15-19 would be found on average in one
in every four households, a total of 2,000
households would have to be contacted in
order to find the required number of eligible
respondents. If 30 clusters were to be used,
this would result in a per-cluster sample size
of 67 households (which would be expected
to yield a sample of approximately 18 male
youth per cluster).
How should youth within sample PSU’s or
clusters be chosen? The preferred procedure
is to first create a list or sampling frame of all
households containing one or more youth
located within each sample PSU, and then
choose a sample of units using either simple
random or systematic sampling. However,
because creating complete lists of households
tends to be costly and time consuming,
“short-cut” procedures are often used.
This section describes two alternative methods
that may be used to select samples of youth
within sample clusters.
Segmentation method
The basic approach of the so-called
“segmentation” method is to divide sample
clusters into smaller “segments” of approximately
equal size, choose one segment at
random from each cluster, and interview all
youth found in all households in the chosen
segment. The advantages of this approach are
two-fold : (1) it avoids the household listing
operation and (2) it results in a self-weighting
probability sample. The specific steps in using
the segmentation method are summarized in
Figure 6 and Figure 7 provides an illustrative
example of a sample cluster that has been
segmented following the prescribed procedures.
In an actual application, one of these segments
would be chosen randomly, all households
located in the segment contacted, and youth
found in those households interviewed.
Because all households in the chosen
segment are to be interviewed, the target
segment size under the segmentation method
should be the same as the target number of
sample households to be chosen per cluster.
For example, if it had been determined as in
the example above that 30 clusters will be
chosen for a given survey and 67 households
would be chosen per cluster, the target
segment size method might be 74 households
(67 plus a 10% allowance for non-response).
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