Sampling
Teaching Research Methods: Resources for
HE Social Sciences Practitioners
Session Objectives
By the end of the session you will be able to:
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Explain what sampling means in research
List the different sampling methods available
Have had an introduction to confidence levels
and confidence intervals
Teaching Research Methods: Resources for
HE Social Sciences Practitioners
Sampling
• Sample frame = the target population you wish to research
• Census = all the respondents in the sample frame
participating in the research
• Sample = a smaller group selected from the sample frame
to participate in the research
• Sample method = the technique used to select the sample
Teaching Research Methods: Resources for
HE Social Sciences Practitioners
Representativeness
• The aim of any sample is to represent the
characteristics of the sample frame.
• There are a number of different methods
used to generate a sample.
• As a researcher you will have to select the
most appropriate method meet the
requirements of your research.
Teaching Research Methods: Resources for
HE Social Sciences Practitioners
Sampling Methods
Task – 10 mins – Small groups
• Match up the type of sampling with the
descriptions
Teaching Research Methods: Resources for
HE Social Sciences Practitioners
Sampling
•
Sampling methods can be split into two
distinct groups:
1. Probability samples
2. Non-probability samples
Teaching Research Methods: Resources for
HE Social Sciences Practitioners
Sampling
Probability Samples
Probability samples offer each respondent an
equal probability or chance at being included in
the sample.
They are considered to be:
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Objective
Empirical
Scientific
Quantitative
Representative
Teaching Research Methods: Resources for
HE Social Sciences Practitioners
Sampling
Non Probability Samples
A non probability sample relies on the
researcher selecting the respondents.
They are considered to be:
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•
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Interpretivist
Subjective
Not scientific
Qualitative
Unrepresentative
Teaching Research Methods: Resources for
HE Social Sciences Practitioners
Probability Sampling Methods
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Random Sampling
Systematic Random Sampling
Stratified Random Sampling
Cluster Random Sampling
Quota Random Sampling
Multi-Stage Sampling
Teaching Research Methods: Resources for
HE Social Sciences Practitioners
Random Sampling
• This involves selecting anybody from the sample
frame entirely at random.
• Random means that each person within the
sample frame has an equal chance of being
selected.
• In order to be random, a full list of everyone
within a sample frame is required.
• Random number tables or a computer is then
used to select respondents at random from the
list.
Teaching Research Methods: Resources for
HE Social Sciences Practitioners
Systematic Random Sampling
• This selection is like random sampling but
rather than use random tables or a computer
to select your respondents you select them in
a systematic way.
• E.g. every tenth person on the college list is
selected.
Teaching Research Methods: Resources for
HE Social Sciences Practitioners
Stratified Random Sampling
• An appropriate group is decided upon i.e.
female, male, 16 –18 year olds and the
participants are picked randomly from within
the strata
Teaching Research Methods: Resources for
HE Social Sciences Practitioners
Cluster Random Sampling
• Similar to stratified sampling
but the groups are selected
for their geographical location
• i.e. school children within a
particular school.
• The school is the cluster with
the children being selected
randomly from within the
cluster
Teaching Research Methods: Resources for
HE Social Sciences Practitioners
Quota Random Sampling
• Having decided on the characteristics of the
sample frame, a sample is selected to meet
these characteristics.
• E.g. if the sample frame is car drivers and
the car driving population is 55% male and
45% female then the quota would require
the same proportions.
• Participants would be selected to fill this
quota using the random method
Teaching Research Methods: Resources for
HE Social Sciences Practitioners
Non-probability Sampling
• Convenience Sampling
• Snowball Sampling
• These non-probability methods can be used
in conjuncture with the cluster, quota or
stratified methods, however they will remain
non-probability samples
Teaching Research Methods: Resources for
HE Social Sciences Practitioners
Convenience Sampling
• This involves selecting the nearest and
most convenient people to participate in
the research.
• This method of selection is not
representative and is considered a very
unsatisfactory way to conduct research.
Teaching Research Methods: Resources for
HE Social Sciences Practitioners
Snowball Sampling
• This type of sampling is used when the research is
focused on participants with very specific
characteristics such as being members of a gang.
• Having identified and contacted one gang member
the researcher asks to be put in touch with any
friends or associates who are also gang members.
• This type of sampling is not representative
however is useful, especially where the groups in
the research are not socially organised i.e. they do
not have clubs or membership lists.
Teaching Research Methods: Resources for
HE Social Sciences Practitioners
Sampling Methods
Task – 10 mins – Small groups
• Return to the task and see how many you got
right and put right any you got wrong.
Teaching Research Methods: Resources for
HE Social Sciences Practitioners
Quantitative Research - Sample
Size
• When conducting probability sampling it is important to use a
sample size that is appropriate to the aims and objectives of
the research.
• There are tables recommending sample size (see de Vaus,
1996 pp 71-72) but as a general rule the smaller the total
sample frame the larger the sample ratio needs to be.
• A common error is to assume that the sample should be a
certain percentage of the population, for example 10%. In
reality there is no such relationship and it only the size of the
sample that is important.
• A probability sample size of 100+ is considered a large enough
sample to conduct statistical analysis
Teaching Research Methods: Resources for
HE Social Sciences Practitioners
Statistics and Samples
• When presenting your research you need to be able
to demonstrate, how representative of the whole
population the sample data you have collected is.
• There are two statistical test used to do this:
• Standard Error
• Confidence Levels
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Standard Error
• Using the standard deviation of the population and
the sample size a statistical calculation can measure
the degree of error likely to occur between the
results of a sample and the results of a census, this is
call the standard error.
• The larger the sample the lower the standard error.
• When a probability sample of 100+ is undertaken
the distribution can usually be assumed to be
normal
• When the sample has normal distribution, we can
use the z score approach to obtain confidence limits
for the sample mean.
Teaching Research Methods: Resources for
HE Social Sciences Practitioners
Confidence Levels
• Confidence levels are calculated using the
Central Limit Theorem
• Using this and the sampling error we can then
use the area below the normal distribution
curve to make predictions about our sample.
• As well as making predictions we can use the
properties of the normal distribution curve to
provide us with confidence levels
• There are three confidence levels 68%, 95%
and 99%
Teaching Research Methods: Resources for
HE Social Sciences Practitioners
Confidence Levels
• The concept does not mean that we are 95% sure that
a single sample mean lies within these limits.
• The 95% confidence limits mean that if we drew many
samples, and find the mean for each, then we can
expect 95% of the sample means to lie within the
stated limits.
• 95% confidence is considered acceptable in social
research, medical research often requires 99%
confidence
Teaching Research Methods: Resources for
HE Social Sciences Practitioners
Confidence Levels – Bell Curve
Teaching Research Methods: Resources for
HE Social Sciences Practitioners
Review
• Can you explain what sampling means in
research?
• Can you list the different sampling methods
available?
• Have had an introduction to confidence levels
and sample error?
Teaching Research Methods: Resources for
HE Social Sciences Practitioners
Further Reading
• Drummond, A. (1996) Research methods for
therapists. Cheltenham, Nelson Thornes
• Fielding J and Gilbert N (2000) Understanding social
statistics London: Sage
• Thomas J R and Nelson J K (2001) Research methods
in physical activity 4th Ed, Leeds, Human Kinetics
• Trochim W (2007) available at
www.socialresearchmethods.net/kb/sampstat.php
Teaching Research Methods: Resources for
HE Social Sciences Practitioners