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Methods – Experiments
We can divide the main methods of investigation into the experimental
method and non-experimental methods.
The Experimental Method
The experimental method is usually taken to be the most scientific of all
methods, the 'method of choice'. The main problem with all the nonexperimental methods is lack of control over the situation. The experimental
method is a means of trying to overcome this problem. The experiment is
sometimes described as the cornerstone of psychology: This is partly due to
the central role experiments play in many of the physical sciences and also to
psychology's historical view of itself as a science. A considerable amount of
psychological research uses the experimental method.
An experiment is a study of cause and effect. It differs from nonexperimental methods in that it involves the deliberate manipulation of one
variable, while trying to keep all other variables constant.
Experiments in the Laboratory:
In psychological experiments (like experiments in other fields) we try to keep
all aspects of the situation constant except one - the one we are looking at.
For example, suppose we want to investigate which of two methods is more
successful at teaching children to read. The aspect that we vary is called the
independent variable (IV) and we change this in a very precise way. In this
example the teaching method is the independent variable. We call the factor
which we then measure, in our example it would be some measure of the
children’s reading ability, the dependent variable (DV), because, if our ideas
are correct, it depends on the independent variable. In our example, the
children’s reading ability depends on the teaching method used.
The variable which is being manipulated by the researcher is therefore called
the independent variable and the dependent variable is the change in
behaviour measured by the researcher.
All other variables which might affect the results and therefore give us a false
set of results are called confounding variables (also referred to as random
variables). Examples of confounding variables in the example given might
include the following

Differences in the instructions given by an experimenter or in the stimulus
materials being used (which could be overcome by standardising
instructions and materials foe all those taking part)

Differences between participants, e.g. in their age (which could be
eliminated as a variable by using a single age group, or alternatively it
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could be made more constant by ensuring that the age structure of each of
the groups taking part in the experiment is very similar).
By changing one variable (the IV) while measuring another (the DV) while
we control all others, as far as possible, then the experimental method allows
us to draw conclusions with far more certainty than any non-experimental
method. If the IV is the only thing that is changed then it must be responsible
for any change in the dependent variable.
Probably the commonest way to design an experiment in psychology is to
divide the participants into 2 groups, the experimental group and the control
group, and then introduce a change for the experimental group and not the
control group. Suppose we wish to see if people sit at a library table for a
shorter time if someone comes and sits at the same table than if they remain
alone. First we must measure the average amount of time people sit when
they are alone. This is the control condition and it gives us a baseline against
which to judge our results. Then we send a confederate to sit at the same
table and we measure the average amount of time the person sits there. This
is the experimental condition.
A control group, then, is a group for whom the experimenter does not change
the IV. The experimental and control groups must be matched on all
important characteristics, e.g. age, sex, experience etc.
Design of Experiments
There are three basic experimental designs
Independent measures design: If 2 groups in an experiment consist of
different individuals then this is an independent measures design. For
example, if we are trying to discover if girls are less aggressive than boys,
then we obviously need 2 separate groups, namely boys and girls.
An independent measures design has an advantage resulting from the
different participants used in each condition - there is no problem with order
effects (order effects are described below). However, the design also has
disadvantages. The most serious is the potential for error resulting from
individual differences between the groups of participants taking part in the
different conditions. Also if participants are in short supply, then an
independent groups design may represent an uneconomic use of those
available to participate, since twice as many participants are needed to obtain
the same amount of data as would be required in a two-condition repeated
measures design.
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Repeated measures design: Sometimes, however we can use the same
individuals and test them on 2 or more separate occasions. Suppose, for
example, we want to find out if people react more quickly to an auditory
stimulus (like a bell) or to a visual stimulus (like a light). We can use the same
participants and try them out with both types of stimulus. This is called a
repeated measures design and is often more accurate than the independent
measures design. However, it introduces other confounding variables which
we must be careful to control; namely practice effects or fatigue (these are
called order effects). Suppose the participants in the above example were all
asked to react as quickly as possible to a light, it may be because they were
unfamiliar with the procedure and they were quicker with the bell simply
because they had practice. On the other hand, if they were quicker with the
light it may be because they had become tired or bored by the time the
second lot of trials were held.
In order to control the effects of
fatigue/boredom and practice we would give half the participants the light
condition first, then the bell and reverse the order for the other half. This is
known as counterbalancing.
The key advantage of the repeated measures design is that individual
differences between participants are removed as a potential confounding
variable (you may recall that this was a major drawback of the independent
measures design). Also the repeated measures design requires fewer
participants, since data for all conditions derive from the same group of
participants.
The design also has its disadvantages. The range of potential uses is smaller
than for the independent groups design. For example, it simply is not possible
to use two different reading schemes to teach young children to read within
the same group of children. Only an independent groups design could be
employed. There is a potential disadvantage resulting from order effects,
although as has been noted already these order effects can be minimised.
Matched pairs design: If we cannot use a repeated measures design it is
sometimes possible to match every subject in one group with a very similar
person in the other group. In order to get the pairing precise enough, it is
common to get one group of participants together and then look round for
partners for everyone. Participants can be matched on variables which are
considered to be relevant to the experiment in question. For example, pairs of
participants might be matched for age, gender and their scores from
intelligence or personality tests.
Although this design combines the key benefits of both an independent and
repeated measures design, achieving matched pairs of participants is a
difficult and time consuming task which may be too costly to undertake.
Successful use of a matched pairs design is heavily dependent on the use of
reliable and valid procedures for pre-testing participants to obtain matched the
pairs.
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Allocation of participants in three different experimental designs
1. The independent group design
Participants (Ps) may be allocated to the conditions randomly. For example:
Condition A
Condition B
P1
P2
P4
P3
P5
P6.....and so on.
2 Repeated measures design
Each participant undertakes all conditions of the experiment. For example:
Condition A
Condition B
P1
P2
P3
P1
P2
P3.....and so on.
3. Matched pairs design
Pairs of participants are matched on appropriate variables relevant to the
experiment; the members of each pair are then allocated to each condition
(sometimes randomly). For example :
Condition A
Condition B
P1a
P2a
P3b
P1b
P2b
P3a.....and so on.
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Advantages of laboratory experiments:
1. Experiments are the only means by which cause and effect can be
established. It has already been noted that an experiment differs from
non-experimental methods in that it enables us to study cause and effect
because it involves the deliberate manipulation of one variable, while
trying to keep all other variables constant. Sometimes the independent
variable (IV) is thought of as the cause and the dependent variable (DV)
as the effect.
2. It allows for precise control of variables.
The purpose of control is to
enable the experimenter to isolate the one key variable which has been
selected (the IV), in order to observe its effect on some other variable (the
DV); control is intended to allow us to conclude that it is the IV, and
nothing else, which is influencing the DV.
3. Experiments can be replicated. We cannot generalise from the results of
a single experiment. The more often an experiment is repeated, with the
same results obtained, the more confident we can be that the theory being
tested is valid. The experimental method consists of standardised
procedures and measures which allow it to be easily repeated.
4. It is also worth noting that an experiment yields quantitative data
(numerical amounts of something) which can be analysed using inferential
statistical tests. These tests permit statements to be made about how
likely the results are to have occurred through chance.
Limitations of laboratory experiments:
1. Artificiality: The experiment is not typical of real life situations. Most
experiments are conducted in laboratories - strange and contrived
environments in which people are asked to perform unusual or even
bizarre tasks. The artificiality of the lab, together with the 'unnatural' things
that the subjects may be asked to do, jointly produces a distortion of
behaviour. Therefore it should be difficult to generalise findings from
experiments because they are not ecologically valid (true to real life).
2. Behaviour in the laboratory is very narrow in its range. By controlling the
situation so precisely, behaviour may be very limited.
3. A major difficulty with the experimental method is demand
characteristics.
Some of the many confounding variables in a
psychology experiment stem from the fact that a psychology experiment is
a social situation in which neither the Subjects or the Experimenters are
passive, inanimate objects but are active, thinking human beings. Imagine
you’ve been asked to take part in a psychology experiment. Even if you
didn’t study psychology, you would be trying to work out what the
experimenter expected to find out. Experimenters too have expectations
about what their results are likely to be. Demand characteristics are all
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the cues which convey to the participant the purpose of the
experiment.
4. The experimental method as used in psychology has a history of using
biased or unrepresentative sampling. George Miller (1962) estimated
that 90% of U.S. experiments have used college students (who are
accessible and 'cheap') and yet the results still tend to be generalised to
the U.S. population as a whole, and often beyond that to Britain, Western
Europe, etc. But there is no reason to believe that U.S. college students
are typical of any other group in terms of gender, age, personality, social
class background or any other subject variable which can influence how
subjects will perform in any experimental situation. What's more, these
students are often psychology students who are required to participate in
research as a course requirement!
5. It has already been noted that a strength of the experimental method is the
amount of control which experimenters have over variables. However it
must also be noted that it is not possible to completely control all variables.
There may be other variables at work which the experimenter is unaware
of. In particular, it is impossible to completely control the mental world of
people taking part in a study.
6. A very major problem with the experimental method concerns ethics. For
example, experiments nearly always involve deceiving participants to
some extent and the very term 'subject' implies that the participant is being
treated as something less than a person. Recently the use of the
experimental method has come under considerable criticism for the way
that researchers often break ethical guidelines. It is also important to
recognise that there are very many areas of human life which cannot be
studied using the experimental method because it would be simply too
unethical to do so.
7. Another issue is to do with normative data. Some researchers consider
that an important advantage which experiments have over, say,
observational techniques is the random assignment of research
participants to experimental conditions. This helps to reduce the problems
of analysis caused by systematic differences between people. Other
psychologists, however, argue that grouping people together in this way,
and trying to cancel out individual differences so that we only look at a
group norm, is limited in how much it can tell us because it ignores what is
special about people.
Mainly because of the above limitations psychologists are increasingly more
likely to use other non-experimental methods - and in particular more
qualitative methods.
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The Field Experiment:
Sometimes it is possible to carry out experiments in a more natural setting,
i.e. in ‘the field ’. A famous example of this is the series of studies carried out
by Piliavin et al (1969) in which they arranged for a person to collapse on an
underground train and waited to see how long it was before the person was
helped. One of the independent variables they used was the appearance of
the ‘victim’: whether he was carrying a walking stick or whether he appeared
to be drunk.
As with the laboratory experiment, the independent variable is still deliberately
manipulated by the researcher.
However it is not possible to have such tight control over variables in the field,
but it does have the advantage of being far less artificial than the laboratory.
Natural Experiments:
In some circumstances, psychologists can take advantage of a natural
situation in order to carry out an investigation in circumstances which they
cannot themselves manipulate. For example, a primary school may decide to
try out a completely new reading scheme and the effects of this could be
compared with a similar school using a different reading scheme. A local
hospital may decide to have mixed wards rather than separate wards for men
and women. The effects on the patients of being in these wards could be
compared with those in single-sex wards.
This is not a true experiment because the psychologist is unable to
manipulate or control variables. For this reason it is sometimes referred to as
a quasi-experiment. It is possible, though to compare two groups, the
equivalent of an experimental and a control group. It has the advantage that
the participants are unaware that they are taking part in an investigation and it
is certainly not as artificial as a laboratory setting.
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Exercise
Independent and Dependent Variables
Remember the independent variable is the variable which the researcher
manipulates. The dependent variable is the one which the researcher
measures.
For the following hypotheses or studies, give the independent variable and the
dependent variable and state whether the hypothesis is one or two tailed:
1.
There is a difference in the speed with which people react to visual and
auditory stimuli.
I.V.
______________________________________________________________
D.V.
______________________________________________________________
2.
Cannabis affects memory.
I.V.
______________________________________________________________
D.V.
______________________________________________________________
3.
Bulls will charge more often when presented with a red rag than when
presented with a blue rag.
I.V.
______________________________________________________________
D.V.
______________________________________________________________
4.
First children learn to speak earlier than second and subsequent
children.
I.V.
______________________________________________________________
D.V.
______________________________________________________________
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5.
Men drive faster than women.
I.V.
______________________________________________________________
D.V.
______________________________________________________________
6.
Imprinting only occurs in the first 36 hours of life.
I.V.
______________________________________________________________
D.V.
______________________________________________________________
7.
Lack of sleep affects learning in 10 year old boys.
I.V.
______________________________________________________________
D.V.
______________________________________________________________
8.
A baby under 9 months of age will not search for a hidden object.
I.V.
______________________________________________________________
D.V.
______________________________________________________________
9.
Social class affects I.Q. scores.
I.V.
______________________________________________________________
D.V.
______________________________________________________________
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10.
Aggressive children are emotionally insecure.
I.V.
______________________________________________________________
D.V.
______________________________________________________________
11.
Absence makes the heart grow fonder.
I.V.
______________________________________________________________
D.V.
______________________________________________________________
12.
It is easier to remember items which are chunked together than it is to
remember individual, unconnected items.
I.V.
______________________________________________________________
D.V.
______________________________________________________________
13.
Stressful experiences increase the likelihood of headaches.
I.V.
______________________________________________________________
D.V.
______________________________________________________________
14.
Time of day affects alertness.
I.V.
______________________________________________________________
D.V.
______________________________________________________________
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Exercise
Hypotheses, variables and designs
For each of the following hypotheses or studies state:
(a) what the I.V. and D.V. are
(b) whether it would be preferable to use a repeated measures or an
independent measures design.
1.
Severe punishment causes anxiety.
I.V.?
______________________________________________________________
___
D.V.?
______________________________________________________________
Repeated or independent measures design?
____________________________________
2.
There is a difference in the ability of grey and white rats in learning to
run a maze.
I.V.?
______________________________________________________________
D.V.?
______________________________________________________________
__
Repeated or independent measures design?
____________________________________
3.
People are more likely to make a risky decision when they are in a
group than when they are alone.
I.V.?
______________________________________________________________
___
D.V.?
______________________________________________________________
__
Repeated or independent measures design?
____________________________________
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4.
Adrenaline increases heart-rate.
I.V.?
______________________________________________________________
___
D.V.?
______________________________________________________________
__
Repeated or independent measures design?
____________________________________
5.
Watching TV programmes is likely to give children nightmares.
I.V.?
______________________________________________________________
___
D.V.?
______________________________________________________________
__
Repeated or independent measures design?
____________________________________
6.
There are cultural differences in susceptibility to visual illusions.
I.V.?
______________________________________________________________
___
D.V.?
______________________________________________________________
__
Repeated or independent measures design?
____________________________________
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7.
Phobias can be reduced using operant conditioning
I.V.?
______________________________________________________________
___
D.V.?
______________________________________________________________
__
Repeated or independent measures design?
____________________________________