RESEARCH METHOD COHEN ok

TRUE EXPERIMENTAL DESIGNS 275
motorcycle accidents, and see which group seem to
have sustained the greatest traumas. It may be that
the lorry accident victims had the greatest trauma,
followed by the motorcycle victims, followed by
the bus victims. Now, although it is not possible
to say with 100 per cent certainty what caused
the trauma, one could make an intelligent guess
that those involved in lorry accidents suffer the
worst injuries. Here we look at the outcomes and
work backwards to examine possible causes. We
cannot isolate, control or manipulate variables, but
nevertheless we can come to some likely defensible
conclusions.
In the outline of research designs that follows
we use symbols and conventions from Campbell
and Stanley (1963):
X represents the exposure of a group to an
experimental variable or event, the effects of
which are to be measured.
O refers to the process of observation or
measurement.
Xs andOs inagivenrowareappliedtothe
same persons.
Left to right order indicates temporal sequence.
Xs and Os vertical to one another are
simultaneous.
R indicates random assignment to separate
treatment groups.
Parallel rows unseparated by dashes represent
comparison groups equated by randomization,
while those separated by a dashed line
represent groups not equated by random
assignment.
True experimental designs
There are several variants of the ‘true’ experimental
design, and we consider many of these
below (see http://www.routledge.com/textbooks/
9780415368780 – Chapter 13, file 13.2. ppt):
the pretest-post-test control and experimental
group design
the two control groups and one experimental
group pretest-post-test design
the post-test control and experimental group
design
the post-test two experimental groups design
the pretest-post-test two treatment design
the matched pairs design
the factorial design
the parametric design
repeated measures designs.
The laboratory experiment typically has to identify
and control a large number of variables, and
this may not be possible. Further, the laboratory
environment itself can have an effect on the
experiment, or it may take some time for a
particular intervention to manifest its effects (e.g.
aparticularreadinginterventionmayhavelittle
immediate effect but may have a delayed effect in
promoting a liking for reading in adult life, or may
have a cumulative effect over time).
A ‘true’ experiment includes several key
features:
one or more control groups
one or more experimental groups
random allocation to control and experimental
groups
pretest of the groups to ensure parity
post-test of the groups to see the effects on the
dependent variable
one or more interventions to the experimental
group(s)
isolation, control and manipulation of independent
variables
non-contamination between the control and
experimental groups.
If an experiment does not possess all of these
features then it is a quasi-experiment: it may look
as if it is an experiment (‘quasi’ means ‘as if’) but
it is not a true experiment, only a variant on it.
An alternative to the laboratory experiment
is the quasi-experiment or field experiment,
including:
the one-group pretest-post-test
the non-equivalent control group design
the time series design.
We consider these below. Field experiments have
less control over experimental conditions or
extraneous variables than a laboratory experiment
Chapter 13