Two experiments

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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.

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Todays session
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Experimental designs

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

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Experimental designs

• Independent groups
• Repeated measures
• Matched pairs

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Independent groups
Recruit 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
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Repeated measures
Recruit 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
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Matched participants
Recruit 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
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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 hasnt
Control After the PPs have been recruited, they
should be randomly assigned to their groups.
This should ensure the groups are similar, on
average.
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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).
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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.