Experimental Design

What is the Matched Pairs Design?

Involves using different but similar participants in each condition of an experiment. An effort is made to match the participants in each condition on any important characteristics that might affect performance e.g. on driving ability, alcohol tolerance etc.

What is the Repeated Measures Design?

Involves using the same participants in each condition of an experiment e.g. giving a group of participants a driving test with no alcohol, followed by the same test at a later time after a pint of beer.

What is the Independent Groups Design?

Involves using different participants for each condition of the experiment e.g. giving one group of participants a driving test with no alcohol and a different group of participants the same test after a pint of beer.

What are the advantages of the Matched Pairs Design?

• Participant variables are kept constant between conditions

• Demand characteristics are less of a problem as the participants only participate in one condition, so is naive to the test, and is less likely to guess the aim of the study

What are the disadvantages of the Matched Pairs Design?

• Participant variables can never be perfectly matched in every aspect, also matching participants is very time consuming and difficult

• More participants are required (because each is used only once) and therefore is less economical 

What are the advantages of the Repeated Measures Design?

• Fewer participants are required (because each is used more than once) therefore more economical

• Participant variables which could become extraneous variables are kept constant between condition since the same participants are used in both conditions 

What are the disadvantages of the Repeated Measures Design?

• Order effects such as learning, fatigue and boredom may become constant errors when one condition is done after another e.g. a participant given the same test may do better due to practice

• Demand characteristics may become a problem. As the participant does both conditions of the experiment, they have more clues to pick up on and may ‘guess’ the aim’

What are the advantages of the Independent Groups Design?

• Order effects such as learning, fatigue or boredom don’t occur since the participant only participates in one condition

• Participants cannot guess the aim by comparing conditions (repeated measures design), which may reduce demand characteristics slightly

• Suitable for experiments where repeating the task would change behaviour

What are the disadvantages of the Independent Groups Design?

• More participants are required (because each is used only once) and therefore it is less economical

• Participant variables differ, which could act as extraneous variables unless controlled - participant variables may affect the results 

What is a method of dealing with the limitations of the Independent Groups Design?

Randomly allocate participants to conditions which (theoretically) distribute participant variables evenly.

What is a method of dealing with the limitations of the Repeated Measures Design?

Counterbalancing, alternate the order in which participants do the conditions of the experiment to average out order effects.

What is a method of dealing with the limitations of the Matched Pairs Design?

Restrict the number of variables to match on to make it easier. Conduct a pilot study to consider key variables that might be important when matching.