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As you continue planning your research project you need
to think about how you’re going to choose your participants.
By now you should have decided what type of people
you need to contact. For some research projects, there
will be only a small number of people within your research
population, in which case it might be possible to contact
everyone. This is called a census. However, for most projects,
unless you have a huge budget, limitless timescale
and large team of interviewers, it will be difficult to speak
to every person within your research population.
SAMPLING
Researchers overcome this problem by choosing a smaller,
more manageable number of people to take part in their
research. This is called sampling. In quantitative research,
it is believed that if this sample is chosen carefully using the
correct procedure, it is then possible to generalise the results
to the whole of the research population. For many
qualitative researchers however, the ability to generalise
their work to the whole research population is not the goal.
Instead, they might seek to describe or explain what is happening
within a smaller group of people. This, they believe,
might provide insights into the behaviour of the wider research
population, but they accept that everyone is different
and that if the research were to be conducted with another
group of people the results might not be the same.
Sampling procedures are used everyday. Market researchers
use them to find out what the general population think
about a new product or new advertisement. When they report
that 87% of the population like the smell of a new
brand of washing powder, they haven’t spoken to the
whole population, but instead have contacted only a sample
of people which they believe are able to represent the
whole population. When we hear that 42% of the population
intend to vote Labour at the next General Election,
only a sample of people have been asked about their voting
intentions. If the sample has not been chosen very carefully,
the results of such surveys can be misleading. Imagine
how misleading the results of a ‘national’ survey on voting
habits would be if the interviews were conducted only in
the leafy suburbs of an English southern city.
Probability samples and purposive samples
There are many different ways to choose a sample, and the
method used will depend upon the area of research, research
methodology and preference of the researcher.
Basically there are two main types of sample:
- probability samples
- purposive samples
In probability samples, all people within the research population
have a specifiable chance of being selected. These
types of sample are used if the researcher wishes to explain,
predict or generalise to the whole research population.
On the other hand, purposive samples are used if
description rather than generalisation is the goal. In this
type of sample it is not possible to specify the possibility
of one person being included in the sample. Within the
probability and purposive categories there are several different
sampling methods.
The best way to illustrate these sampling methods is to
take one issue and show how the focus of the research
and the methodology leads to the use of different sampling
methods. The area of research is ‘school detention’
and in Table 3 you can see that the focus and sampling
techniques within this topic can be very different, depending
on the preferences of the researcher, the purpose of
the research and the available resources.
SAMPLE SIZE
The first question new researchers tend to ask is ‘how
many people should I speak to?’ This obviously depends
on the type of research. For large scale, quantitative surveys
you will need to contact many more people than you
would for a small, qualitative piece of research. The sample
size will also depend on what you want to do with your
results. If you intend to produce large amounts of cross
tabulations, the more people you contact the better.
It tends to be a general rule in quantitative research that
the larger the sample the more accurate your results.
However, you have to remember that you are probably restricted
by time and money – you have to make sure that
you construct a sample which will be manageable. Also,
you have to account for non-response and you may need
to choose a higher proportion of your research population
as your sample to overcome this problem. If you’re interested
in large-scale quantitative research, statistical methods
can be used to choose the size of sample required for
a given level of accuracy and the ability to make generalisations.
If your research requires the use of purposive sampling
techniques, it may be difficult to specify at the beginning
of your research how many people you intend to contact.
Instead you continue using your chosen procedure such as
snowballing or theoretical sampling until a ‘saturation
point’ is reached. This was a term used by Glaser and
Strauss (1967) to describe that time of your research when
you really do think that everything is complete and that
you’re not obtaining any new information by continuing.
In your written report you can then describe your sampling
procedure, including a description of how many
people were contacted.
SUMMARY
- If it is not possible to contact everyone in the research
population, researchers select a number of people to
contact. This is called sampling.
There are two main types of sampling category – probability
samples and purposive samples.
- In probability samples, all people within the research
population have a specifiable chance of being selected.
- Only within random samples do participants have an
equal chance of being selected.
- Purposive samples are used if generalisation is not the
goal.
- The size of sample will depend upon the type and purpose
of the research.
- Sample sizes should take into account issues of non-response.
- Remember that with postal surveys it might be difficult
to control and know who has filled in a questionnaire.
Will this affect your sample?
- In some purposive samples it is difficult to specify at
the beginning of the research how many people will
be contacted.
- It is possible to use a mixture of sampling techniques
within one project which may help to overcome some
of the disadvantages found within different procedures.
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