Behavioural Surveillance Surveys - The Wisdom of Whores

Figure 8 : Example of household survey of youth using cluster sampling
The modified cluster sampling method sometimes referred to as the “segmentation” method was
recently applied in a survey of youth residing within the catchment areas of health facilities
participating in a program to improve the “youth friendliness” of health services in Lusaka,
Zambia. First, the Census Enumeration Areas, EA’s, falling within the geographic catchment
areas of each facility were defined, and a sampling frame of such EA’s was developed for each
facility. A sample of five (5) EA’s was then chosen from each list with PPS. The total population
enumerated in the last population census was used as the measure of size.
Based upon data from the most recent census, it was estimated that each household would
contain on average just over one person aged 15-24 years, the age range defined as the universe
for the survey. Thus, in order to obtain interviews from 25 adolescents and young adults per
EA, the standard segment size was set at 30 (25 households to obtain interviews with 25 eligible
respondents, plus a cushion for non-response). (Note: A rapid field test should be conducted to
verify the number of households that should be in a standard segment . Failure to do so could
end up in serious over- or under-estimation of the number of households needed per segment
to reach the desired sample size).
Each sample EA was divided into segments of approximate size 30 households, and then one of
the segments was randomly chosen to be the sample segment. All households were contacted
during the course of the survey fieldwork, and all eligible respondents were interviewed. Two
repeat visits were made to each household in the event that eligible respondents were not at
home during the first visits.
Note: in this example, the segmentation method was applied only to areas surrounding certain
health facilities. The method could be applied in the same fashion to larger populations,
including national populations if necessary.
Modified random walk method
The other “short-cut” alternative is known as
the random walk method. It will be familiar
to many readers in that it is the method used
in Expanded Programme on Immunization
(EPI) Cluster Surveys. The method entails
(1) randomly choosing a starting point and
direction of travel within a sample cluster,
(2) conducting an interview in the nearest
household, and (3) continuously choosing
the next nearest household and conducting
interviews until the target number of interviews
has been obtained.
The variant of the random walk method
proposed for sub-population surveys differs
from the manner in which the method is often
applied in several respects. First, in order to
ensure that the households and youth chosen
for surveys are truly chosen randomly, maps
are used to indicate several possible starting
points at easily identifiable locations in the
cluster. One of these points is then randomly
chosen as the starting point for the survey.
Maps are used to avoid the common practice
of always starting in the center of the
cluster, as this procedure can result in bias.
Figure 9. illustrates how one might construct a
map of random starting points.
B EHAV I OR A L S U R V EI L L A NC E SURV EY S APPEN DI X 3
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