Behavioural Surveillance Surveys - The Wisdom of Whores

Because BSS asks questions of just a
sample of the universe of possible respondents,
the indicator calculated from the replies of
respondents may not reflect the true proportion
of members of the sub-population falling into
the category in question (in other words,
those who have the behavior in question).
It is therefore useful to calculate confidence
intervals around the indicator. A confidence
interval is the range in which you can be
“confident” (within a specified degree) that the
proportion of people reporting the behavior is
accurate. In fact, it is important to remember
that the “true” values of proportions are never
known. Rather, on the basis of statistical
theory, we construct confidence intervals
which give us a range within which we
assume the true value lies.
Typically, the level of confidence used in
calculating this range is set at 95 percent.
For example, if a survey shows that 15 percent
of truck drivers have had sex with a sex
worker in the past year, and the 95 percent
confidence interval has been calculated to be
between 12 and 18 percent, then it means that
we are 95% confident that the true value lies
in between these values. This helps us to
understand the accuracy and precision of
the estimates we obtain.
As the sample size becomes larger, the
confidence intervals become narrower, and
we can be more confident about the precision
of our estimates.
Indicators based on simple proportions
are calculated simply by dividing those that
report the behavior by the total number of
respondents who were questioned about the
behavior. Where the indicator is expressed as
a percentage (which is often the case), the result
should be multiplied by 100. So if 700 high
school girls were asked whether they had ever
had sex, and 460 replied that they had, the
calculation for indicator: the percentage of girls
in high school ever having had sex would be:
Indicator 1 = 460/700 x 100 = 65.7%
In some cases, the denominator may not
be all respondents, but may be all respondents
reporting another behavior. For example,
an indicator may be calculated for the percent
of sexually active high school girls who have
ever used a condom. In this case, the question
would only be asked of the 460 girls who had
ever had sex. If 194 of them replied that they
had used a condom, the indicator would be
194/160 x 100 = 42.2%
The confidence interval is based on the
standard error of a percentage. The standard
error (SE) is calculated as follows:
SE = √(p (100-p))
√n
B EHAV I OR A L S U R V EI L L A NC E SURV EY S CHAPTER 7
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