Behavioural Surveillance Surveys - The Wisdom of Whores

certain age will be six months older than that
age. (For example those who say they are
15 will range from those who turned 15 on
the day of the survey to those who will turn
16 the following day. Assuming an even age
distribution, they will be on average 15.5).
Half a year should therefore be added to the
exact ages used in the calculation of the
median age at first sex.
Strengths and limitations
Because this indicator is constructed from
a question about current virginity status, it is
sensitive to recent changes in the age at first
sex. The indicator itself does not, however,
give any idea of the full distribution of ages
at sexual initiation. In some circumstances,
such as when a significant proportion of
girls are exposed to sex at very young ages,
it may be the tails rather than the middle of
the age curve which interest those designing
prevention programs.
Most questionnaires also include questions
such as “How old were you when you first
had sex?”. These data are NOT used in the
construction of this indicator. This is because
they exclude people who have not yet had
sex, and therefore tend to bias the median age
downwards. Retrospective data can be used
from age cohorts at which virtually everyone is
already sexually active. However an indicator
constructed in this way is not sensitive to recent
changes in the age at first sex, and it is these
recent trends that are of interest in monitoring
the success of HIV prevention programs.
The indicator is most useful where the
median is rather young — between 15 and
19 years. Where the median age at first sex is
over 19 for both men and women, promoting
abstinence among adolescents may be replaced
by other priority interventions within the
program and this indicator will diminish in
importance and may not even be measured.
To allow for the construction of a robust
indicator using this “current status” methodology,
reasonable sample sizes are needed at each
single year of age (preferably at least 100
respondents of each sex in single years,
especially the single years at which the median
age is expected).
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Chapter H A PTER 9 B EHAV I OR A L S U R V EI L L A NC E S U R V EY S