Behavioural Surveillance Surveys - The Wisdom of Whores

One thing to keep in mind with truck
drivers, is that they are moving all the time.
So depending on the distances traveled,
sampling from several different locations
might tend to result in a lot of duplication.
Therefore, if there is a single location (such as
a bridge or weighing station), that all drivers
must pass through to get where they are
going, it is sometimes a better idea for the
interviewing team to remain stationary.
The strategy would be to set up multiple
time-location units at the same spot, but on
different days of the week, and times of the
day, over a period of several weeks.
By repeatedly sampling from a few sites,
it might be possible to more efficiently
“capture” the population as they pass through.
Guidance on selected sampling
problems
Despite careful up-front research and
planning, surprises often occur during the
course of survey fieldwork. Guidance on
what should be done when several samplingrelated
surprises arise is provided below.
When insufficient numbers of the survey
population members are found at sample sites
In many of the sampling schemes described
above, the number of sites to be chosen will
be based upon an “expected” number of
sub-population members to be found per site
during a specified time interval. What should
be done if it is found during the course
of fieldwork that the actual number of
sub-population members found is substantially
lower than expected ?
The preferred course of action is to have
anticipated such a possibility when determining
survey sample size requirements, and to
have adjusted the target sample size upward
as a precaution against “surprises” in the field.
Such allowances for non-response are typically
made in sample size calculations for household
surveys. Given that the number of sub-populations
members expected to be found per site
will often be no more than an informed guess,
a strong case may be made for following the
same practice in all sub-population surveys.
The recommended procedure is to increase
the number of sites to be included in the
sample by 10% or so. So, for example, if it
were determined that 30 PSU’s would be
covered in a given survey, the recommended
course of action would be to increase this
to 33, 35, or even 40 if resources permit.
Although returning to sample sites for an
additional period of data collection is an
option, this option is less desirable than
increasing the number of sample PSU’s chosen
for two reasons. First, if the reason that
insufficient numbers of sub-population members
were found is that the estimates of the expected
daily volume of sub-population members were
faulty (i.e., too low), returning to the same
sites will be a relatively inefficient way of
increasing the sample size. Secondly, sampling
additional PSU’s will serve to increase the
precision of survey estimates more so than
sampling additional cases per PSU. Returning
to sample PSU’s different numbers of times
also complicates the calculation of sampling
probabilities. If this option is chosen, the
number of return visits made to each site
needs to be carefully documented.
B EHAV I OR A L S U R V EI L L A NC E SURV EY S APPEN DI X 3
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