QUALITATIVE AND QUANTITATIVE SAMPLINGsingle-parent families and tend to be racial minori-
ties. The family environment is one in which par-
ents and/or siblings have low education or are
themselves dropouts. In addition, many dropouts
engage in illegal behavior. We might seek dropouts
who are members of the majority racial group, who
have no record of illegal activities, and who are from
stable two-parent, upper-middle-income families.
By looking at atypical dropouts we might learn
more about the reasons for dropping out.
Sequential Sampling
Sequential samplingis also similar to purposive
sampling. We use purposive sampling to try to
locate as many relevant cases as possible. Sequen-
tial sampling differs because we continue to gather
cases until the amount of new information ends or
a certain diversity of cases is reached. The principle
is to gather cases until we reach a saturation point.
In economic terms, information is gathered until the
marginal utility, or incremental benefit for addi-
tional cases, levels off or drops significantly. It
requires that we continuously evaluate all collected
cases. For example, we locate and plan in-depth
interviews of sixty widows over 70 years of age who
have been living without a spouse for 10 or more
years. Depending on our purposes, getting an addi-
tional twenty widows whose life experiences, social
backgrounds, and worldviews differ little from the
first sixty may be unnecessary.Theoretical Sampling
In theoretical sampling, what we sample (e.g.,
people, situations, events, time periods) comes from
grounded theory. A growing theoretical interest
guides the selection of sample cases. The researcher
selects cases based on new insights that the sample
could provide. For example, a field researcher could
be observing a site and a group of people during
weekdays. Theoretically, the researcher may ques-
tion whether the people act the same at other times
or aspects of the site change. He or she could then
sample other time periods (e.g., nights and week-
ends) to have a fuller picture and learn whether
important conditions are the same.Adaptive Sampling and Hidden Populations
In contrast to sampling the general population or
visible and accessible people, sampling hidden
populations(i.e., people who engage in clandes-
tine or concealed activities) is a recurrent issue in
the studies of deviant or stigmatized behavior (such
as victims of sexual violence, illegal drug users).
This method illustrates the creative application of
sampling principles, mixing qualitative and quanti-
tative styles of research and combining probability
with nonprobability techniques.
Adaptive samplingis a design that adjusts
based on early observations.^15 For example, we ask
illegal drug users to refer other drug users as in
snowball sampling. However, we adjust the way
that we trace through the network based on our
research topic. We might identify a geographic area,
divide it into sections randomly, and then select
participants in that area through strategies such as
random-digit dialing or by posting recruitment
fliers. Once we identify members of the targeted
hidden population, we use them in a snowball tech-
nique to find others. AIDS researchers or studies
of illegal drug users that have sampled “hidden
populations” are instructive, often relying on mod-
ified snowball techniques. (See Example Box 8,
Hidden Populations).Sequential sampling A nonrandom sample in which
a researcher tries to find as many relevant cases as pos-
sible until time, financial resources, or his or her energy
is exhausted or until there is no new information or
diversity from the cases.
Theoretical sampling A nonrandom sample in
which the researcher selects specific times, locations,
or events to observe in order to develop a social the-
ory or evaluate theoretical ideas.
Hidden population A population of people who
engage in clandestine, socially disapproved of, or con-
cealed activities and who are difficult to locate and
study.
Adaptive sampling A nonprobability sampling tech-
nique used for hidden populations in which several
approaches to identify and recruit, including a snowball
or referral method, may be used.