Experimental Design

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Experimental design refers to the way in which participants are used in experiments

Independent Groups:

An independent groups design is when two separate groups of participants experience two different conditions of the experiment. The performance/responses of the two groups would then be compared. For example in the energy drink experiment, this would involve;

- one group of participants drinking the energy drink (experimental condition)

- another group of participants drinking water (control condition)

The participants that occupy the different groups are not the same. If a researcher found a mean difference between the groups on the DV this may be more to do with individual differences than the effects of the IV

Less economical than repeated measures as each participant contributes a single result only

Order effects are not a problem

Participants are less likely to guess the aims

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Repeated Measures:

A repeated measures design is where all the participants take part in all conditions of the experiment. For example, in the energy drink experiment;

- All participants would be tested with the energy drink

- All participants would be tested again later with water

The two sets of data would then be compared to see if there was a difference

Each participant has to do at least two tasks and the order of these may be significant

Order effects may arise because repeating two tasks could create boredom or fatigue that might cause deterioration in performance on the second task, so it matters what order the tasks are in. Alternatively participants performance may improve through the effects of practice, especially on a skill based task - in this case participants would perform better on the second task. Order acts as a confounding variable

It is very likely participants will work out the aim of the study so there may be more demand characteristics

Participant variables are controlled and fewer participants are needed

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Matched Pairs:

In a matched pairs design, participants are paired together on a variable/variables relevant to the experiment. For example in the energy drink experiment we might observe participants interacting in a room before the experiment begins and select the two people that appear to be the chattiest. One of the pair would be placed in the experimental condition the other in the control condition. We would then match up the rest of the participants and place them in each condition.

Participants only take part in a single condition so order effects and demand characteristics are less of a problem

Participants can never be matched exactly meaning there are differences that may affect the DV

Matching may be time consuming and expensive, particularly if a pre-test is required, so this is less economical than other designs