RESEARCH METHOD COHEN ok

274 EXPERIMENTS AND META-ANALYSIS
Frequently in learning experiments in classroom
settings, the independent variable is a stimulus
of some kind, a new method in arithmetical
computation for example, and the dependent
variable is a response, the time taken to do
twenty sums using the new method. Most empirical
studies in educational settings, however, are quasiexperimental
rather than experimental. The single
most important difference between the quasiexperiment
and the true experiment is that
in the former case, the researcher undertakes
his study with groups that are intact, that is
to say, the groups have been constituted by
means other than random selection. In this
chapter we identify the essential features of true
experimental and quasi-experimental designs, our
intention being to introduce the reader to the
meaning and purpose of control in educational
experimentation.
In experiments, researchers can remain relatively
aloof from the participants, bringing a degree
of objectivity to the research (Robson 2002: 98).
Observer effects can distort the experiment, for
example researchers may record inconsistently, or
inaccurately, or selectively, or, less consciously,
they may be having an effect on the experiment.
Further, participant effects might distort the experiment
(see the discussion of the Hawthorne
effect in Chapter 6); the fact of simply being
in an experiment, rather than what the experiment
is doing, might be sufficient to alter
participants’ behaviour.
In medical experiments these twin concerns
are addressed by giving placebos to certain
participants, to monitor any changes, and
experiments are blind or double blind. In blind
experiments, participants are not told whether
they are in a control group or an experimental
group, though which they are is known to the
researcher. In a double blind experiment not
even the researcher knows whether a participant
is in the control of experimental group – that
knowledge resides with a third party. These
are intended to reduce the subtle effects of
participants knowing whether they are in a control
or experimental group. In educational research it
is easier to conduct a blind experiment rather
than a double blind experiment, and it is even
possible not to tell participants that they are
in an experiment at all, or to tell them that
the experiment is about X when, in fact, it
is about Y, i.e. to ‘put them off the scent’.
This form of deception needs to be justified;
a common justification is that it enables the
experiment to be conducted under more natural
conditions, without participants altering their
everyday behaviour.
Designs in educational experimentation
There are several different kinds of experimental
design, for example:
the controlled experiment in laboratory conditions
(the ‘true’ experiment): two or more
groups
the field or quasi-experiment in the natural
setting rather than the laboratory, but
where variables are isolated, controlled and
manipulated.
the natural experiment in which it is not possible
to isolate and control variables.
We consider these in this chapter (see http://
www.routledge.com/textbooks/9780415368780 –
Chapter 13, file 13.1. ppt). The laboratory experiment
(the classic true experiment) is conducted
in a specially contrived, artificial environment, so
that variables can be isolated, controlled and manipulated
(as in the example of the wheat seeds
above). The field experiment is similar to the laboratory
experiment in that variables are isolated,
controlled and manipulated, but the setting is the
real world rather than the artificially constructed
world of the laboratory.
Sometimes it is not possible, desirable or
ethical to set up a laboratory or field experiment.
For example, let us imagine that we wanted
to investigate the trauma effects on people in
road traffic accidents. We could not require a
participant to run under a bus, or another to stand
in the way of a moving lorry, or another to be
hit by a motorcycle, and so on. Instead we might
examine hospital records to see the trauma effects
of victims of bus accidents, lorry accidents and