Behavioural Surveillance Surveys - The Wisdom of Whores

Changes in sampling methodology over
time are almost certain to result in selection
bias, making trends over time much more
difficult to interpret than when a consistent
methodology is used.
Even if sampling methodology remains the
same, changes in the political or social climate
may have important influences on who is
included in a survey. A crackdown on illegal
immigrants may completely change the profile
of the sex worker population between one
survey and the next, for example. Differences
in risk behavior over time may not reflect
behavior change in this case, but may simply
mean the population measured is no longer
the same. It should be noted that rapid
population turnover is typical of many of the
sub-populations of interest to HIV prevention
programmes. As long as the population
sampled relates in the same way to the wider
population to which results will be
extrapolated, this will not increase sampling
error. It will, however, affect the interpretation
of trends over time.
One important potential source of selection
bias is refusal bias. BSS asks people about
personal and often illegal behaviors. It is
absolutely essential, therefore, that the purpose
of the study be carefully explained to selected
respondents, and that their full consent to
participate be obtained before questioning
begins. Refusal bias arises when those who
refuse to participate have different behaviors
than those who agree. In the case of BSS,
refusal bias may well underestimate true levels
of risk behavior, because people may avoid
participating because they do not want to
admit to behaviors that they recognize are
risky. Survey reports should always state what
proportion of the selected sample refused
participation, and should give socio-demographic
profiles of refusers where available.
One way of attempting to gauge the
importance of selection bias is to collect some
basic socio-demographic variables from selected
participants, including those who refuse to
participate in the study. These variables can be
compared with the same variables in respondents
in earlier survey rounds (as well as in those who
refuse to take part) to see if any systematic
differences are discernable. Variables that will
help judge the likely magnitude of selection
bias will depend on the local situation but
may include nationality, province of origin,
length of time associated with the site, etc.
Measurement bias
Another source of potential error in surveys
is error in the measurement of the variables.
This can arise when survey forms are not
clear, for example when the meaning of a
question in the local language is open to
different interpretations.
Interviewer bias can also affect the correct
measurement of behavior. Some interviewers
are more judgmental than others, and people
may be more or less willing to report
stigmatized behaviors to an interviewer
depending on their attitude. Indeed people’s
unwillingness to tell the truth about their
sexual or drug-taking behavior may be the
most important source of measurement bias.
It is not clear to what extent this is likely to
change over time.
Where an interviewer has to turn a verbal
answer into one that fits in to a coded category,
they may also influence results, by consistently
preferring one category over another when
answers are ambiguous.
Many of the difficulties associated with
measurement bias can be minimized through
comprehensive training of interviewers and
survey staff, and active supervision.
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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