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TRUE EXPERIMENTAL DESIGNS 279
textbooks/9780415368780 – Chapter 13, file 13.8.
ppt). So, for example, the designs might be:
Experimental 1 RO 1 X 1 O 2
Experimental 2 RO 3 X 2 O 4
Control RO 5 O 6
This can be extended to the post-test control and
experimental group design and the post-test two
experimental groups design, and the pretest-posttest
two treatment design.
The matched pairs design
As the name suggests, here participants are
allocated to control and experimental groups
randomly, but the basis of the allocation is that
one member of the control group is matched to a
member of the experimental group on the several
independent variables considered important for
the study (e.g. those independent variables that are
considered to have an influence on the dependent
variable, such as sex, age, ability). So, first, pairs of
participants are selected who are matched in terms
of the independent variable under consideration
(e.g. whose scores on a particular measure are the
same or similar), and then each of the pair is
randomly assigned to the control or experimental
group. Randomization takes place at the pair
rather than the group level. Although, as its name
suggests, this ensures effective matching of control
and experimental groups, in practice it may not be
easy to find sufficiently close matching, particularly
in a field experiment, although finding such a
close match in a field experiment may increase the
control of the experiment considerably. Matched
pairs designs are useful if the researcher cannot be
certain that individual differences will not obscure
treatment effects, as it enables these individual
differences to be controlled.
Borg and Gall (1979: 547) set out a useful
series of steps in the planning and conduct of an
experiment:
1 Carryoutameasureofthedependentvariable.
2 Assign participants to matched pairs, based
on the scores and measures established from
Step 1.
3 Randomly assign one person from each pair
to the control group and the other to the
experimental group.
4 Administer the experimental treatment/
intervention to the experimental group and,
if appropriate, a placebo to the control group.
Ensure that the control group is not subject
to the intervention.
5 Carryoutameasureofthedependentvariable
with both groups and compare/measure them
in order to determine the effect and its size on
the dependent variable.
Borg and Gall indicate that difficulties arise
in the close matching of the sample of the
control and experimental groups. This involves
careful identification of the variables on which
the matching must take place. Borg and Gall
(1979: 547) suggest that matching on a number
of variables that correlate with the dependent
variable is more likely to reduce errors than
matching on a single variable. The problem, of
course, is that the greater the number of variables
that have to be matched, the harder it is actually
to find the sample of people who are matched.
Hence the balance must be struck between having
too few variables such that error can occur, and
having so many variables that it is impossible
to draw a sample. Instead of matched pairs,
random allocation is possible, and this is discussed
below.
Mitchell and Jolley (1988: 103) pose three
important questions that researchers need to
consider when comparing two groups:
Are the two groups equal at the commencement
of the experiment
Would the two groups have grown apart
naturally, regardless of the intervention
To what extent has initial measurement error
of the two groups been a contributory factor in
differences between scores
Borg and Gall (1979) draw attention to the need
to specify the degree of exactitude (or variance)
of the match. For example, if the subjects were to
be matched on, say, linguistic ability as measured
in a standardized test, it is important to define the
Chapter 13