RESEARCH METHOD COHEN ok

4 Sampling
Introduction
The quality of a piece of research stands or falls
not only by the appropriateness of methodology
and instrumentation but also by the suitability
of the sampling strategy that has been adopted
(see also Morrison 1993: 112–17). Questions of
sampling arise directly out of the issue of defining
the population on which the research will focus.
Researchers must take sampling decisions early in
the overall planning of a piece of research. Factors
such as expense, time, accessibility frequently
prevent researchers from gaining information from
the whole population. Therefore they often need
to be able to obtain data from a smaller group or
subset of the total population in such a way that
the knowledge gained is representative of the total
population (however defined) under study. This
smaller group or subset is the sample. Experienced
researchers start with the total population and
work down to the sample. By contrast, less
experienced researchers often work from the
bottom up, that is, they determine the minimum
number of respondents needed to conduct the
research (Bailey 1978). However, unless they
identify the total population in advance, it
is virtually impossible for them to assess how
representative the sample is that they have drawn.
Suppose that a class teacher has been released
from her teaching commitments for one month in
order to conduct some research into the abilities of
13-year-old students to undertake a set of science
experiments; that the research is to draw on
three secondary schools which contain 300 such
students each, a total of 900 students, and that
the method that the teacher has been asked to use
for data collection is a semi-structured interview.
Because of the time available to the teacher it
would be impossible for her to interview all 900
students (the total population being all the cases).
Therefore she has to be selective and to interview
fewer than all 900 students. How will she decide
that selection; how will she select which students
to interview
If she were to interview 200 of the students,
would that be too many If she were to interview
just 20 of the students would that be too few If she
were to interview just the males or just the females,
would that give her a fair picture If she were to
interview just those students whom the science
teachers had decided were ‘good at science’, would
that yield a true picture of the total population of
900 students Perhaps it would be better for her to
interview those students who were experiencing
difficulty in science and who did not enjoy science,
as well as those who were ‘good at science’. Suppose
that she turns up on the days of the interviews only
to find that those students who do not enjoy science
have decided to absent themselves from the
science lesson. How can she reach those students
Decisions and problems such as these face
researchers in deciding the sampling strategy to
be used. Judgements have to be made about four
key factors in sampling:
the sample size
representativeness and parameters of the
sample
access to the sample
the sampling strategy to be used.
The decisions here will determine the sampling
strategy to be used (see http://www.routledge.
com/textbooks/9780415368780 – Chapter 4, file
4.1.ppt). This assumes that a sample is actually
required; there may be occasions on which
the researcher can access the whole population
rather than a sample.