RESEARCH METHOD COHEN ok

282 EXPERIMENTS AND META-ANALYSIS
design, and offers considerable control potential,
as it is exactly the same person receiving different
interventions. (see http://www.routledge.com/
textbooks/9780415368780 – Chapter 13, file
13.11. ppt). Order effects raise their heads here:
the order in which the interventions are sequenced
may have an effect on the outcome; the first intervention
may have an influence – a carry-over
effect – on the second, and the second intervention
may have an influence on the third and so on.
Further, early interventions may have a greater effect
than later interventions. To overcome this it
is possible to randomize the order of the interventions
and assign participants randomly to different
sequences, though this may not ensure a balanced
sequence. Rather, a deliberate ordering may have
to be planned, for example, in a three-intervention
experiment:
Group 1 receives intervention 1 followed by
intervention 2, followed by intervention 3.
Group 2 receives intervention 2 followed by
intervention 3, followed by intervention 1.
Group 3 receives intervention 3 followed by
intervention 1, followed by intervention 2.
Group 4 receives intervention 1 followed by
intervention 3, followed by intervention 2.
Group 5 receives intervention 2 followed by
intervention 1, followed by intervention 3.
Group 6 receives intervention 3 followed by
intervention 2, followed by intervention 1.
Repeated measures designs are useful if it
is considered that order effects are either
unimportant or unlikely, or if the researcher
cannot be certain that individual differences will
not obscure treatment effects, as it enables these
individual differences to be controlled.
Aquasi-experimentaldesign:the
non-equivalent control group design
Often in educational research, it is simply
not possible for investigators to undertake true
experiments, e.g. in random assignation of
participants to control or experimental groups.
Quasi-experiments are the stuff of field experimentation,
i.e. outside the laboratory (see http://
www.routledge.com/textbooks/9780415368780 –
Chapter 13, file 13.12. ppt). At best, they may
be able to employ something approaching a true
experimental design in which they have control
over what Campbell and Stanley (1963) refer to
as ‘the who and to whom of measurement’ but
lack control over ‘the when and to whom of exposure’,
or the randomization of exposures – essential
if true experimentation is to take place. These
situations are quasi-experimental and the methodologies
employed by researchers are termed quasiexperimental
designs. (Kerlinger (1970) refers to
quasi-experimental situations as ‘compromise designs’,
an apt description when applied to much
educational research where the random selection
or random assignment of schools and classrooms is
quite impracticable.)
Quasi-experiments come in several forms, for
example:
Pre-experimental designs: the one group
pretest-post-test design; the one group posttests
only design; the post-tests only nonequivalent
design.
Pretest-post-test non-equivalent group design.
One-group time series.
We consider these below.
Apre-experimentaldesign:theonegroup
pretest-post-test
Very often, reports about the value of a
new teaching method or interest aroused by
some curriculum innovation or other reveal
that a researcher has measured a group on a
dependent variable (O 1 ), for example, attitudes
towards minority groups, and then introduced an
experimental manipulation (X), perhaps a tenweek
curriculum project designed to increase
tolerance of ethnic minorities. Following the
experimental treatment, the researcher has again
measured group attitudes (O 2 ) and proceeded to
account for differences between pretest and posttest
scores by reference to the effects of X.
The one group pretest-post-test design can be
represented as:
Experimental O 1 X O 2