The first British
National Survey of Sexual Attitudes and Lifestyles (NATSAL)
was conducted in 1990-1991 and the second in 1999-2001. When surveys are
repeated, the changes in population parameters are of interest and are
generally estimated from a comparison of the data between surveys. However,
since all surveys may be subject to bias, such comparisons may partly reflect
a change in bias. Typically limited external data are available to estimate
the change in bias directly. However, one approach, which is often possible,
is to define in each survey a sample of participants
who are eligible for both surveys, and then to compare the reporting
of selected events that occurred before the earlier survey time point.
A difference in reporting suggests a change in overall survey bias between
time points, although other explanations are possible. In NATSAL, changes
in bias are likely to be similar for groups of sexual experiences. The
grouping of experiences allows the information that is derived from the
selected events to be incorporated into inference concerning population
changes in other sexual experiences. We use generalized estimating equations,
which incorporate weighting for differential probabilities of sampling
and non-response in a relatively straightforward manner. The results, combined
with estimates of the change in reporting, are used to derive minimum established
population changes, based on NATSAL data. For some key population parameters,
the change in reporting is seen to be consistent with a change in bias
alone. Recommendations are made for the design of future surveys.
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