Behavioural Surveillance Surveys - The Wisdom of Whores
Behavioural Surveillance Surveys - The Wisdom of Whores
Behavioural Surveillance Surveys - The Wisdom of Whores
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Conventional clusters vs.<br />
time-location clusters<br />
Before determining how PSUs (clusters)<br />
will be selected, a key decision must be made<br />
about whether to use conventional clusters or<br />
time-location clusters. This decision is<br />
based on the nature <strong>of</strong> the population to be<br />
sampled. When members <strong>of</strong> the population<br />
are associated with sites in a “fixed” manner,<br />
then conventional clusters can be used.<br />
Examples <strong>of</strong> “fixed” populations would include<br />
brothel based sex workers who live at the<br />
brothel, police attached to a certain unit or<br />
division, or secondary school students at<br />
particular schools. However, frequently the<br />
populations included in surveillance are not<br />
associated with a site, but rather come and<br />
go freely from the site. Examples <strong>of</strong> these<br />
“floating” populations might include<br />
street-based sex workers, men who have sex<br />
with men in parks (or pick-up points), truck<br />
drivers at halt-points, or injecting drug users<br />
at shooting galleries. For these “floating”<br />
populations, the number you might find at a<br />
particular site will vary from day to day, and<br />
even by time <strong>of</strong> day or by the time <strong>of</strong> the<br />
month. This is not very important if the<br />
individuals at a PSU share the same patterns<br />
<strong>of</strong> behavior regardless <strong>of</strong> when they go to that<br />
site. But if people with different levels <strong>of</strong> risk<br />
visit a site at different times, then the time<br />
selected for sampling may affect the results <strong>of</strong><br />
survey and make it less representative <strong>of</strong> the<br />
population as a whole. An example might be<br />
men who have sex with men and gather at<br />
cruising sites. Men who are found at these<br />
sites mid-afternoon may be disproportionately<br />
more likely to be unemployed than men who<br />
come to the sites in the evenings, and may<br />
therefore be more likely to sell sex for income.<br />
So for this population, sampling mid-afternoon<br />
would probably give higher estimates <strong>of</strong> risk<br />
behavior than sampling in the evening.<br />
It may well give a doubly distorted picture,<br />
in fact, because more men may visit a cruising<br />
site in the evening (after work and a few<br />
drinks) than mid-afternoon. In this case, risk<br />
levels measured in the afternoon would be<br />
representative <strong>of</strong> a smaller proportion <strong>of</strong> the<br />
overall population than risk levels measured<br />
in the evening.<br />
Time location sampling provides a way<br />
around these difficulties. <strong>The</strong> same site may<br />
be included in the sampling frame more than<br />
once, at different times <strong>of</strong> the day or different<br />
days <strong>of</strong> the week, in order to give a more<br />
accurate picture <strong>of</strong> the different levels <strong>of</strong> risk<br />
associated with the site. In other words, the<br />
PSU is defined not as the site alone but as the<br />
site plus the time <strong>of</strong> the day/week/month at<br />
which sampling takes place. <strong>The</strong> same<br />
physical site can therefore become several<br />
PSUs. This system has the added advantage<br />
that it is not necessary to count the total<br />
number <strong>of</strong> individuals associated with a single<br />
location. Only the number <strong>of</strong> individuals in<br />
the sampling time interval need be discussed.<br />
B EHAV I OR A L S U R V EI L L A NC E SURV EY S CHAPTER 4<br />
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