Behavioural Surveillance Surveys - The Wisdom of Whores

where n is the sample size in the denominator
of the indicator and p is the percentage
in which the outcome variable is observed
(i.e., the indicator itself)
The 95% confidence interval is calculated
as follows:
95% CI = p ± 1.96 (SE)
In comparing the same indicator in two
different populations, it is possible to be
confident that there is a real difference only
if there is little or no overlap in the confidence
intervals around the two indicators. Indicators
based on categorical variables and the
confidence intervals surrounding them are
often represented graphically using a bar chart
with lines denoting the confidence interval.
An example from Tamil Nadu, India
These data come from the first round of
behavioral surveillance in Tamil Nadu, India.
The data, presented in both table and graph
format, are for a key indicator: the percentage
of men who used a condom the last time they
had sex with a sex worker, among all men who
had sex with a sex worker in the past year.
In tabulating the data, it is helpful to give the
actual numbers in the numerator and the
denominator, as well as the percentage and the
95 percent confidence interval. It is especially
important to give numerators and denominators
when the denominator is not the total sample
size (as is the case in this indicator, where only
men who have had sex with a sex worker are
included in the denominator.
Table 1 : Percent of male target group reporting condom use during most recent
act of sexual intercourse with a commercial sex partner (Tamil Nadu)
Respondent group % N 95% confidence interval
Male STD patients 15 68/435 11.6, 18.4
Truck Drivers and Helpers 55 144/262 49.0, 61.0
Male Factory Workers 28 27/97 19.0, 37.0
Male Students 80 14/18 61.5, 98.5
Source: The AIDS Prevention and Control Project (APAC), Voluntary Health Services, Chennai,
Tamil Nadu, India, 1996.
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C H A PTER 7 B EHAV I OR A L S U R V EI L L A NC E S U R V EY S