Experimental Design

There are three main experimental designs, each with their own strengths and weaknesses.

INDEPENDENT MEASURES

1. Separate groups of people are used in separate conditions.
2. Strengths
   • Order effects (when participants become aware of or bored with an experimental procedure) are avoided.
3. Weaknesses
   • Individual differences between groups, otherwise called “participant variables”, may affect the results (what if one group has people who have a naturally higher IQ than people in the other group?) – this lowers the validity of the experiment.
   • A larger amount of participants are needed in this experimental design.

REPEATED MEASURES

1. Same group of people is used in different conditions.
2. Strengths
   • Participant variable problems are avoided.
   • This experimental design requires less participants because the same group is re-used.
3. Weaknesses
   • Order effects are very likely to occur; participants may become bored, aware or tired.

MATCHED PAIRS

1. Testing separate groups of people but each group’s members have their own common characteristics (e.g. same sex, same eye colour, same ethnicity, etc.).
2. Strengths
   • The issue of participant variables is greatly reduced.
   • Order effects are totally avoided.
3. Weaknesses
   • It is pretty much impossible to match people exactly on every characteristic; unless maybe they are identical twins – and even then, it is usually just matching physical characteristics.
   • It is very time-consuming to find lots of people that match each other so closely.

Solutions to reduce/eliminate order effects:
Counterbalancing means to change and alternate the order in which participants are performing different conditions during an experiment. For example: Group 1 does Activity A then Activity B and then Group 2 does Activity B and then Activity A.

Randomisation means to present material in random orders during an experiment.

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