Experimental Designs           

There are three basic experimental designs

Independent measures (or group) design  

If two 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.

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. 

Allocation of participants in three different experimental designs

1.  The independent measures design

Participants (Ps) may be allocated to the conditions randomly.  For example:

2   Repeated measures design

Each participant undertakes all conditions of the experiment.  For example:

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 :

Test yourself on this matching quiz and this one.