Behavioural Surveillance Surveys - The Wisdom of Whores

Table 7 : Summary of analysis procedures needed for different sampling approaches
Prototype Sampling Design
Weighted analysis required? Cluster analysis required?
1. PPS with equal number No Yes, if design effect is ≠ 1
of respondents selected
from each cluster (includes
segmentation method)
2. PPS with unequal number of Yes, if sampling Yes, if design effect is ≠ 1
respondents selected
weights differ by a
from each cluster
factor of 3 or more
3. EP with a fixed number Yes, if sampling Yes, if design effect is ≠ 1
of respondents selected
weights differ by a
from each cluster
factor of 3 or more
4. EP with “take-all” No Yes, if design effect is ≠ 1
respondents during equal
time periods for each cluster
PPS = Probability proportional to size
EP = Equal Probability
Another issue that comes up when cluster
designs are used for sampling is how to deal
with design effects in the analysis. When there
is a design effect, it affects the standard error
of the estimates, so normally a cluster analysis
must performed. This issue is discussed
further later in this Chapter. Table 7
summarizes the different sampling approaches
outlined in this guide, and categorizes them
in terms of the need for weighted analysis
and cluster analysis.
Weighting the data
To perform a weighted analysis, one must
begin by calculating sampling probabilities for
each sample cluster (each element in a given
cluster will have the same probability for
selection). Figure 6 provides the formulae
for making these calculations. It includes
some designs that were not included in
Chapter 4, but which are described in
appendix 3. Once you have calculated the
sampling probability, the sampling weight can
easily be obtained, since it is just the inverse
of the sampling probability (i.e. one divided
by the sampling probability). As will be
discussed later in the chapter, it is best to also
calculate standardized weights to be used
during analysis, in order to avoid incorrect
statistical results. Standard software packages
(such as SPSS or STATA) can then be used to
perform weighted analysis. It will be noted
that as sampling probabilities cannot be
calculated when snowball sampling is used,
formulae are not provided for data obtained
using this sampling approach.
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C H A PTER 5 B EHAV I OR A L S U R V EI L L A NC E S U R V EY S