Research Design

A controlled experiment needs to be carefully designed so that extraneous variables are minimised. This ensures that the independent variable only (and not these extraneous variables) affects the dependent variable (the behaviour of the participants).

This ensures that it is a true test of the cause- and-effect relationship that the experimenter is trying to establish. For example, if a researcher wants to test hours spent studying on exam recall many extraneous variable may get in the way of the results such as: - Anxiety - Past exam experience - Intelligence

The skill in designing an experiment is to allow every chance for the independent variable (hours spent studying) to cause a change in the dependent variable (exam performance) and to minimise the effects of any extraneous variables (e.g. anxiety) To do this in Psychology experiments, researchers attempt to ensure that experimental and control groups are as similar in all characteristics as possible and for the groups to be treated exactly the same for all variables except the independent variable.

This is achieved in the construction of the groups and how participants are allocated to the groups. This is an essential part of the experimental design. Three different types of experimental design include:

Repeated-measures design This is about repeating the experiment for each participant on more than one occasion. It could mean participants being in the experimented group on one occasion and in the control group on another. This repetition tends to control any extraneous variables due to the differing characteristics of the participants.

One disadvantage with this procedure is the order effect - participants can perform better on the second occasion because of practice or more familiarity with the task. This order effect can be offset to some extent by having half the participants in the experimental group and half in the control group on the first occasion and reversing them on the second. This procedure is called counterbalancing. The assumption then is that the practice effect would equally affect both sets of results.

Matched-subjects design This is about achieving control over extraneous variables by matching the groups of participants on any of those variables not specifically under investigation. It means trying to establish a group with similar characteristics as the other group. The type of characteristics needing to be matched would depend on the nature of the experiment and this would be an essential part of the experimenter’s task when designing the experiment.

For example, in the test of alcohol use on sleep patterns it may be important to take account of the differing effects of alcohol on females and older people. This would mean matching groups so that there is equal numbers of these in each group. It may mean some form of surveying or pre-testing to determine the characteristics of participants so that groups can be matched. Obviously, the experimental designer would need to be aware of specific extraneous variables that may affect the outcome of the experiment and include action to minimise them in the design.

Independent-groups design This is about establishing the groups by random methods, not making any attempt to match characteristics. It could be the toss of a coin or drawing them out of a hat. It means the characteristics of the experimental and control groups are decided by chance and that they are independent of each other. The assumption, of course, is that the inherent characteristics of each group will be the same because they have been decided by chance - with small samples this may not necessarily be so.