Research Methods in Education

By Louis Cohen; Lawrence Manion et al. | Go to book overview
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4

Sampling

Introduction
The quality of a piece of research not only stands or falls 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 and 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 and 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 twenty 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’. So 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:
1 the sample size;
2 the representativeness and parameters of the sample;
3 access to the sample;
4 the sampling strategy to be used.

The decisions here will determine the sampling strategy to be used.

-92-

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