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Two experiments

- PPs are divided into two groups. Each PP is timed completing a jigsaw puzzle. In one group, the PPs complete the puzzle on their own. In the other group, the PPs all complete the puzzle in the same room.

- A group of PPs is recruited. Each PP is timed completing two jigsaw puzzles. First, they complete a puzzle on their own. Then, they complete another puzzle whilst all in the same room.

Experimental designs

- An experiment must have two (or more) conditions but those conditions can be arranged in a number of ways.

Experimental designs

- Independent groups
- Repeated measures
- Matched pairs

Compare the results for the two groups

Independent groupsRecruit a group of participants

Divide them into two

This group does the experimental task with the IV set for condition 1

This group does the experimental task with the IV set for condition 2

Measure the DV for each group

Compare the results for the two conditions

Repeated measuresRecruit a group of participants

Condition 1 Condition 2

The group does the experimental task with the IV set for condition 1

The group repeats the experimental task with the IV set for condition 2

Compare the results for the matched pairs

Matched participantsRecruit a group of participants

Find out what sorts of people you have in the group

Recruit another group that matches them one for one

Treat the experiment as independent measures

Condition 1 Condition 2

Independent groups

- What sorts of problem are likely to arise from using two different groups of people?

Problem:

The natural variation between the individuals (participant variables) in each group may affect the DV measurements, making it look as if the IV has had an effect when it actually hasn’t

Control:

After the PPs have been recruited, they should be randomly assigned to their groups. This should ensure the groups are similar, on average.

Repeated measures

- What sorts of problem are likely to arise from using the same group of people twice?

Problem:

Doing both conditions may (1) give PPs practice on the task; (2) make them bored or tired; (3) allow them to work out the aim of the study, all of which might affect the DV measurement. (4) Reuse of stimuli is not possible.

Control:

Divide the PPs into two groups. Half does condition A first, then condition B. The rest do condition B then condition A. DV measurements for the conditions A and B are then compared (counterbalancing).

Matched pairs

- What sorts of problem are likely to arise from using matched pairs of PPs?

Problem:

Several problems: (1) time consuming; (2) an exact match is rarely possible; (3) if one PP drops out you lose 2 PPs’ data.

Control:

Members of each pair should be randomly assigned to conditions. However, this does not solve all these problems.

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