Behavioural Surveillance Surveys - The Wisdom of Whores

Table 6 provides a “lookup” table based
upon the above formula to permit final sample
sizes to be determined without having to
perform calculations. The table provides
sample sizes needed to measure changes in
behavioral indicators of magnitude 10 and 15
percentage points for different initial values
of a given indicator, as well as for different
combinations of significance and power.
Note that some indicators include two
behavioral dimensions. For example the
proportion of all students who had sex with
a sex worker in the last year (dimension one)
who used a condom the last time they had sex
with a sex worker (dimension two). In these
cases a further step is needed in calculating
the sample size. In this case, the first step in
calculating the sample size required would be
to determine how many students would be
needed to measure a change in the proportion
who used a condom during an encounter with
a sex worker during the previous year. For
this step, proceed exactly as in the examples
given in Figure 4. Say the number comes to
200. Does that mean the final sample size will
be 200? No, because not all of the students
sampled will have had sex with a sex worker
in the last year, and this indicator takes only
the ones that have had sex with a sex worker
into account in the denominator. So it is
necessary to estimate how many students
would be needed in the overall sample, in
order to capture 200 who have had sex with
a sex worker in the last year.
Computationally, the procedure is simple -
one merely divides the required sample size
calculated as described above by the estimated
proportion of the sub-population with the
required “qualifying” behavior. For example,
if 40 percent of male vocational students in a
given setting are thought to have had sex with
a sex worker in the last year, it would be
necessary to interview n=500 (=200/.4) students
in order to find n=200 subjects who had had
sex with a sex worker in the last year, to allow
for the indicator of condom use to be calculated.
Examples of this type of calculation are given
in Figure 5.
The difficult part of this procedure is,
of course, knowing what proportion of the
total population engages in the behavior of
interest. Here, other surveys or anecdotal
information might be consulted for guidance.
If resources permit, a small pilot survey might
be conducted to better inform sample size
calculations for the main survey effort.
As there may be considerable uncertainty
concerning these parameters, the general
guidance is to err toward under-estimating
the proportion engaging in a given behavior,
as this will ensure a sufficient sample size for
the main survey effort. For example, if it were
thought that between 20% and 30% of students
typically engage in sex with sex workers on
an annual basis in a given setting, the 20%
figure should be used in determining sample
size requirements for BSS.
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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