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Although the DHSs for Uganda provide evidence of an upward trend in age at first sex between 1995 and 2000, they also provide evidence of bias in either response or sample membership sufficient to offset all evidence of trend for men and much of the evidence for women. These data therefore do not provide evidence that Ugandans are responding to the HIV epidemic by prolonging abstinence.
The Abstain-Be-Faithful-Condoms (ABC) approach to prevent HIV emphasises the delaying of age at first sex. To investigate adoption of the ABC approach, Zaba et al1 estimated the median age at first sex from the DHSs. They inferred that this median has increased. Gersovitz,2 however, showed that the bias in response could account for the deferral of first sex in several DHSs. This paper scrutinises two DHSs for Uganda, an oft-cited example of the success of ABC,3 and argues against the findings of Zaba et al.
METHODS AND DATA
With two surveys for one country at different times and questions about age at first sex, one can estimate trends in age at first sex and bias in respondents’ answers over time. The trend can be calculated as the difference in proportions in two surveys of men or women whose age falls in a fixed interval, and who report being virgins before a particular age. To exclude people with the same year of birth (same cohort) from both proportions and to avoid attenuating any trend, the age range should not exceed the difference in time between surveys.
As the interest is about populations and different respondents were selected with different probabilities, proportions and significance tests should use sample weights. As sampling used clusters, significance tests should adjust for correlation among the answers of respondents from the same cluster (SVYMEAN command of the STATA program).
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