Behavioural Surveillance Surveys - The Wisdom of Whores

Targeted (snowball) sampling
The rationale for preferring probability over
non-probability sampling methods for BSS was
outlined earlier in this section. Non-probability
sampling methods are a last resort. They are
used in situations where probability methods
are not feasible because it is not possible to
construct an adequate sampling frame of sites
or locations where members of a sub-population
of interest congregate. Groups for which
non-probability sampling methods may have
to be used include injecting drug users, some
types of sex workers, and possibly men-whohave-sex-with-men.
The basic form of non-probability sampling
recommended for BSS is a modified form of
snowball sampling referred to as targeted
sampling. The basic idea in snowball sampling
is to compensate for the lack of a sampling
frame by learning the identities of members of
a given “network” of persons who engage in
a given risk behavior through key informants
and respondent group members themselves.
Snowball sampling is an iterative process.
Typically, the data collection process begins
by interviewing key informants and subpopulation
members known to the researchers
in order to learn the identities of other group
members and to gather information on where
other members might be found. These persons
are then contacted, data are collected, and
these sub-population members are asked to
provide information on how and where
additional sub-population members might
be found. “Leads” from each wave of
referrals are followed-up until a sample of
pre-determined size has been reached.
An important limitation of snowball sampling
is that “lead” sub-population members are
more likely to provide information on other
group members who are in their own social,
economic, and/or sexual network. To the
extent that risk-taking and/or protective
behaviors differ across networks, this poses
a potential bias problem for sub-population
surveys. Research in the United States
(San Francisco), for example, revealed the
existence of different social networks in
terms of racial, ethnic and drug-type among
drug-users, even in relatively small geographic
areas. In order for the snowball sampling
approach to yield meaningful monitoring data,
it is therefore necessary to ensure individuals
from different networks are included in the
sample.
The targeted sampling approach extends
the ideas of snowball sampling to include
an initial ethnographic assessment aimed
at identifying the various networks or
sub-groups that might exist in a given setting.
The sub-groups so identified are then treated
as sampling strata, and quota samples are
chosen within each stratum using snowball
sampling techniques. More information about
targeted sampling can be found in a paper
by John Watters and Patrick Biernacki:
“Targeted sampling: options for the study of
hidden populations”, published in the journal
Social Problems (Vol.36, No.4, 1989).
B EHAV I OR A L S U R V EI L L A NC E SURV EY S CHAPTER 4
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