Lesson 7: Experimental Designs

Question 1

What is the experimental design of a study?

Answer 1

How the participants are assigned to different conditions. There are three main types of experimental designs, independent groups, repeated measures and matched pairs.

Question 2

Independent groups

Answer 2

Different pps are used in each of the conditions, therefore each group of participants is independent from one another. Pps are usually randomly allocated to each condition to balance out any participant variables.

Question 3

Advantages of independent groups

Answer 3

• Order effects will not occur as there are different pps in each condition. Order effects are when the sequence in which pps take part in conditions influences their performance or behaviour. E.g. in a memory test, pps may get better with practice- pps may also get tired or fatigued when being asked to take part in more than one condition.
• The chance of demand characteristics is reduced as pps do only one condition each and so have less chance to guess the purpose of the study.
• The same task/materials can be used in both conditions as pps are always naïve to the task.

Question 4

Disadvantages of independent groups experimental design

Answer 4

• More pps are needed for this experimental design.
• There is a chance that the different results between the two conditions are due to participant variables rather than manipulation of the independent variable

Question 5

Repeated measures

Answer 5

Each participant is tested in all conditions of the experiment

Question 6

Advantages of repeated measures experimental design

Answer 6

As the same people are measured in all conditions there are no participant variables between the conditions.

Half as many participants are needed compared to an independent groups design.

Question 7

Disadvantages of repeated measures experimental design

Answer 7

Order effects may affect the results. Can be avoided using counterbalancing, which is when half the pps do condition A first and condition B second and half the pps do condition B first and A second. Counterbalancing does not eliminate order effects, which will be present because there are two separate tasks to be completed by each person. However, counterbalancing controls the impact of order effects (boredom, practise) and allows effects to be distributed evenly across both conditions.

Demand characteristics are more likely to occur as pps are involved in the entire study.

Experimental design takes more time, especially if a time gap between conditions is required.

Question 8

Matched pairs

Answer 8

The different participants are used in all the conditions, just as with the independent groups design. However, pps in the two groups are matched on characteristics important for that study, such as age, gender, level of education etc. identical twins are often used in matched pair designs.

Question 9

Advantages of matched pairs experimental design

Answer 9

Less risk of order effects

Less risk of demand characteristics

Participant variables are unlikely as the groups have been closely matched

Question 10

Disadvantages of matched pairs experimental design

Answer 10

Twice as many participants are required compared with a repeated measures design

The matching process is incredibly difficult; even two closely matched individuals have different levels of motivation and fatigue at any given time.

The matching process is incredibly time consuming.