Behavioural Surveillance Surveys - The Wisdom of Whores

Analysis of trends in behavior over time
Just as we want to be confident that
differences in behavior observed between
populations or population sub-groups at one
point in time are real and not the result of
sampling error or confounding, so we want
to be sure that trends in behavior observed
over time are real. For these reasons, statistical
significance tests are also performed for trends
over time.
For analyzing trends in behavioral
surveillance, the most common test used is
the Chi-square test for trend. The purpose
of using this test is to compare trends between
survey rounds in an indicator that is measured
in terms of a binary variable. For example,
has condom use risen among truck drivers
over the last three survey rounds in a way
that is statistically significant and not just due
to chance? Statistical packages calculate the
chi-square test for trend.
An example from India
Figure 1 shows trends in reported condom
use by two male sub-population groups surveyed
as part of behavioral surveillance in Tamil Nadu:
truckers and factory workers. The graph
suggests that condom use during last sex with
a sex worker has increased in both groups.
However, this can only be established by
performing statistical tests for trend.
Figure 1 : Percent of males reporting condom use during
last sex with a female sex worker in Tamil Nadu
Source: The AIDS Prevention and Control Project (APAC), Voluntary Health Services, Chennai,
Tamil Nadu, India, 1998.
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