There are three main experimental designs, each with their own strengths and weaknesses.
Independent Measures
- Separate groups of people are used in separate conditions.
Strengths
- Order effects (when participants become aware of or bored with an experimental procedure) are avoided.
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
- Same group of people is used in different conditions.
Strengths
- Participant variable problems are avoided.
- This experimental design requires less participants because the same group is re-used.
Weaknesses
- Order effects are very likely to occur; participants may become bored, aware or tired.
Matched Pairs
- 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.).
Strengths
- The issue of participant variables is greatly reduced.
- Order effects are totally avoided.
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.