Behavioural Surveillance Surveys - The Wisdom of Whores

Table 4 : Selection of a systematic - random sample of primary sampling units with
equal probability - an example
No.
Selection
Primary sampling unit
001 Planned no. of clusters = 40
002 X Sampling interval = 170/40 = 4.25
003 Random start between 1 and 4.25 = 2
004 Clusters selected = 002, 006, 011, ....
005
006 X
007
008
009
010
011 X
-
-
-
170 (Last)
Note: in selecting sample clusters, it is important that the decimal points in the sampling
interval be retained. The rule to be followed is when the decimal part of the sample selection
number is less than.5, the lower numbered cluster is chosen, and when the decimal part of the
sample selection number is.5 or greater, the higher numbered cluster is chosen. In the above
example, the sample selection number for the third sample cluster was 10.5, and thus cluster
011 was chosen for the sample.
Selecting respondents within
sample PSUs for “fixed”
populations
In conventional two-stage sampling, where
the number of respondents is basically fixed
at each site, respondents are chosen from a
list of individuals associated with each of the
primary sampling units selected, using either
simple random or systematic sampling. A
discussion of the number of respondents to
include in each PSU can be found in the section
titled Number of PSUs and sample sizes from
each, later in this chapter. Examples of this
type of sampling are contained in Appendix 3.
Selecting primary sampling
units (time-location clusters)
for “floating” populations
When the population is a “floating” one,
with individuals not being associated with
the sites in any fixed manner (such as with
truckers at truck stops, or men who have sex
with men at pick-up points), then time-location
clusters should be used. As explained above,
there are two dimensions to a PSU, place and
time. This means that the first step in creating
a sampling frame - listing all possible PSUs,
can be somewhat complicated.
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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