Behavioural Surveillance Surveys - The Wisdom of Whores

In-class sampling of school youth
The logistically simplest approach to
carrying out school surveys of youth is to
have student’s complete self-administered
questionnaires during class sessions. This
approach is not only logistically simpler than
trying to interview students outside of class,
but because of the low cost of self-administered
questionnaires, data can be obtained for larger
samples of students than will generally be
feasible when personal interviews are used to
collect the data. Provided that confidentiality
can be ensured, it is also possible that more
candid responses to sensitive questions might
be obtained through the use of self-administered
questionnaires.
When “in-class” data collection is possible,
a two-stage cluster sample design similar to
that used in household surveys of youth will
likely satisfy most sub-population survey
needs. Under this design, sample students
would be chosen by first selecting a sample
of schools, then selecting a sample of classes
from sample schools at the second stage of
selection, and gathering data from all students
in sample classes. Since measures of size
(i.e., number of school enrollees) are likely to
be available prior to sample selection in most
settings, sample schools should be chosen
using systematic sampling with probabilityproportional-to-size
(PPS). The steps involved
were outlined in Chapter 4.
The number of schools and classes/sections
to be chosen should be determined as follows.
First, divide the target sample size for the
survey by the average class/section size in the
schools in the survey universe. For example,
suppose the intended sample size for a
sub-population survey was n=800 male students,
and that classes/sections in secondary schools
in the setting in question averaged 25 male.
A minimum of 32 classes/sections would thus
be needed (32=800/25). As protection against
non-response, it is recommended that the
number of sample classes/sections to be chosen
be increased by 10 percent or so (e.g., to 35
classes/sections).
Next, the number of schools to be included
in the survey needs to be determined. As was
discussed in Chapter 4, it is preferable to take
larger rather than smaller numbers of “clusters”
in cluster surveys. Thus, the number of
schools to be included in a given survey effort
should be as large as resources will permit.
Ideally, 30 or more schools would be included
in a school survey. Where this is not feasible,
a smaller number of sample schools may be
used, but it is recommended that the number
of sample schools chosen not fall below 10-15.
In the above example, the sample size for
schools might be set at n=20, and two (2)
classes/sections chosen per school, yielding
a total of n=40 classes/sections. Because the
relative cost of collecting data “in-class” using
self-administered questionnaires is low,
rounding up the number of classes/sections
to be chosen will increase survey costs only
slightly. Note that to insure that the proposed
sampling scheme results in a self-weighting
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A PPEN DI X 3 B EHAV I OR A L SURV EI L L A NC E S U R V EY S