Quota Sampling
When researchers sample from strata without randomly selecting elements,
they are using quota sampling. Like stratified random sampling, researchers
select strata based on what is already known about the phenomenon being
studied. The desired number, or quota, needed to fill each stratum should be
proportionate to the population to obtain a representative sample.
The difference between quota sampling and stratified random sampling is the
use of random selection. In quota sampling, elements are conveniently selected
from each strata rather than being randomly selected. For example, in a study
about how age is related to success in nursing programs, a researcher could
stratify according to age by creating three age groups: 18–22, 23–30, and older
than 30. Suppose 80% of nursing students are ages 18–22, 15% are ages 23–30,
and 5% are older than age 30. If the researcher needs a sample size of 100, the
sampling would continue until 80 students between the ages of 18 and 22 years,
15 students between the ages of 23 and 30 years, and 5 students older than age 30
had been recruited. By establishing proportional quotas that mirror the nursing
student population, the researcher is more likely to obtain a representative sample.
Convenience and quota sampling are predominantly used in quantitative
research studies. They are frequently used because it is often difficult in nurs-
ing research to determine a sampling frame in advance of a study. They also
are less time consuming and less costly than probability sampling methods.
The ability to generalize results to the target population depends heavily on
the appropriateness of the sampling method used.
Purposive Sampling
Purposive sampling, which is a nonprobability sampling method, is used in
qualitative studies. Sampling methods used in qualitative research must be
appropriate to allow researchers to gain insight into the experience or event
being studied. There is less focus on the results of the study being generalized
to the target population, and more emphasis is placed on the interpretation
and understanding of the event or experience, including relevant contextual
factors being studied (Leeman & Sandelowski, 2012). Researchers use purposive
sampling in qualitative research to select a distinct group of individuals who
either have lived the experience or have expertise in the event or experience
being studied. In qualitative studies, individuals in the sample are referred to
as participants. For example, researchers investigating the lived experience of
women younger than the age of 25 years who have survived a liver transplant
would use purposive sampling. Because the focus of the study is so specific, the
researcher can hand select women who meet the inclusion criteria. Researchers
must carefully document the process used to select subjects.
KEY TERMS
quota sampling:
Nonprobability
sampling method
involving selection
of elements from
an accessible
population that has
been divided into
groups or strata
purposive
sampling:
Nonprobability
sampling method
used in qualitative
studies to select a
distinct group of
individuals who
either have lived the
experience or have
expertise in the
event or experience
being studied;
sampling method
to recruit specific
persons who could
provide inside
information11.2 Sampling Methods 297