Behavioural Surveillance Surveys - The Wisdom of Whores

Figure 5 : sample size calculations to measure indicators with two dimensions
Example 1
Consider the indicator “proportion having used a condom during the last sexual encounter
with a non-regular partner, of those who have had sex with a non-spousal, non-cohabiting
partner in the last 12 months. Using the formula presented above, it might have been
calculated that a sample size of n=320 respondents was needed to register a change of a
specified size in condom use in irregular sex at the desired levels of power and significance.
Data from a recent survey suggest that about 20 percent of men and 5 percent of women
engaged in such encounters in an earlier 12-month period. The number of males and females
that would have to be contacted in order to obtain n=320 respondents would thus be
estimated as:
Males: n = 320 /.20 = 1,600
Females: n = 320 /.05 = 6,400
Example 2
For the indicator “proportion using a condom during their last sexual encounter with a sex
worker” among truck drivers, it is thought that most truck drivers in a given setting (75 percent)
have sex with sex workers during any given 12-month period. Thus, if the required sample
size (again calculated as described above) were to be n=275, the number of truck drivers that
would have to be contacted would be n = 275 /.75 = 367 per survey round.
In deciding what level of change to measure,
surveillance managers should ask themselves
the question: what level of change is worth
measuring? In other words, what level of
change might be deemed to reflect success in
achieving the aims of prevention efforts? And
what level of change might have an impact on
the risk of the spread of HIV? The doubling of
condom use during sex with sex workers might
make headlines, but if usage has doubled from
only five percent to 10 percent, it will have
virtually no impact on stemming the potential
spread of HIV. Given the very large sample
sizes required to measure small changes with
any degree of confidence, it may well not be
deemed worthwhile to register this change.
Some National AIDS Programs have stated
targets for various indicators. For example,
a country might, in its strategic plan, set the
goal of increasing condom use in extra-marital
sex by 25 percentage points over five years.
In cases where large changes in indicators are
expected, there is a temptation to set the
magnitude of change parameter (P 2
- P 1
) in the
sample size calculations at quite a high level,
thereby decreasing the sample size needed.
It should be recognized, however, that this will
jeopardize the ability to detect smaller changes
that may in fact be programmatically significant
in their own right. By the same token, the cost
of measuring smaller changes annually may be
prohibitive, because of the large sample sizes
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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