1. Experimental Design

2. Learning Objectives: To develop an understanding of experimental design. Success Criteria: 1) Take part in a discussion about IVs and DVs. 2) Answer a past paper question on IVs and DVs. 3) Complete page 10 of your booklet on experimental design.

3. Research Methods To conduct an experiment you need… IV & DV
Hypotheses (directional & non-directional)
Controls
Operationalisation

4. IV and DV Independent variables – the thing you change
This variable is manipulated by the researcher and is assumed to effect the DV
Dependent variables – the thing you measure
This variable is measured by the researcher and is affected by the independent variable

5. Discussion activity
Which is the independent variable and dependent variable for each of the following research questions...
Does the weather affect mood?
Do boys and girls like different music?
Do we lose our memory as we get older?
Can a person be conditioned to fear spiders?
Is there less attention in classes on a Friday afternoon?
Is there such a thing as a born killer?

6. Task
Identify the IV and DV from each of the examples on page 9 of your booklet.

7. Past Exam Question AO3 = 2 marks
IV    The interview, type of interview, method of interview, ‘standard interview or / and cognitive interview’, whether or not cognitive interview.
DV   Number of items recalled, recall, what they remembered.
A psychologist used an independent groups design to investigate whether or not a cognitive interview was more effective than a standard interview, in recalling information. For this experiment, participants were recruited from an advertisement placed in a local paper. The advertisement informed the participants that they would be watching a film of a violent crime and that they would be interviewed about the content by a male police officer. The psychologist compared the mean number of items recalled in the cognitive interview with the mean number recalled in the standard interview. Identify the independent variable and the dependent variable in this experiment. (2 marks)

8. Experimental Design In a lab experiment we manipulate an IV
There are usually two values of the IV e.g.
Noise or no noise
Chocolate or no chocolate
These determine the conditions (groups) of the experiment
These groups can be arranged in several different ways

9. Experimental Design
There are three techniques psychologists can use to design their experiment:
Independent groups design
Repeated measures design
Matched pairs design
See page 10 of your booklet

10. Independent Groups Design
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
Compare the results for the two groups

11. Participant Variables
Variation between participants can affect DV
Could mask an effect (false negative)
Could imply an effect where none exists (false positive)
This is a problem with independent measures
You can control this by random assignment to groups, or
Use repeated measures or matched participants instead

12. Repeated Measures Design
Recruit a group of participants
Condition 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 two conditions

13. Counterbalancing
This is a really important control when using repeated measures
Reduces ‘carry over’ effects (when you learn from first attempt and apply it to the second)…so…
Half participants do condition A then B
Other half do condition B then A
Fully counterbalanced: ABBA

14. Fatigue, Boredom & Practice
Carrying out a task repeatedly leads to changes in performance
Deterioration as participants become tired or bored, or
Improvement due to practice
This is a problem with repeated measures, so
Leave a long gap between conditions, or
Counterbalance the design, or
Use independent measures or matched participants

15. Matched Pairs Design Compare the results for the matched pairs
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
Compare the results for the matched pairs

16. Attrition
When participants drop out of a study (attrition) data is lost
Fewer data = less powerful study
This can be a problem with matched participants (and repeated measures if there is a gap between conditions)
Loss of a participant in these designs means losing their data from both conditions

17. In a nut shell... Experimental Design What it means
Independent measures design
One group does one thing, and one does another
Repeated measures design
We all do one thing, then we all do the other thing
Matched pairs design
It’s independent measures, but the groups are deliberately balanced

18. Independent Groups Design
Which experimental design has been used in this famous study? Was it a good choice?
Loftus & Palmer (1974) took 45 students from a Californian university. They all watched a film clip of two cars colliding. Then they were split into 5 groups of 9. Each group was asked a question – “About how fast were the cars going when they hit/smashed/bumped/collided/contacted each other?” Estimates of speed were found to vary according to which verb had been used in the question.
Independent Groups Design

19. Repeated measures Design
Which experimental design has been used in this famous study? Was it a good choice?
Kiecolt-Glaser (1984) took blood samples from 75 first year medical students a) one month before their final examinations (relatively low stress), and (b) during the examinations (high stress). She found that they had reduced immune systems during times of stress.
Repeated measures Design

20. Independent Groups Design
Which experimental design has been used in this famous study? Was it a good choice?
Nisbett & Wilson divided students into 2 groups. They watched 2 different videos of the same lecturer. In one he gave advice to students in a warm and friendly manner; in the other video he said exactly the same words but in a cold and aloof way. Students were asked to score him on likeability. The ones who had seen the warm and friendly video scored the lecturer higher but they didn’t know why!
Independent Groups Design

21. Repeated measures Design
Which experimental design has been used in this famous study? Was it a good choice?
 Stroop (1935) gave 70 college undergraduates a sheet of paper to read where the words matched the colour of the ink in which they were printed (e.g. ‘pink’ was printed in pink ink). The participants were asked to name the ink colour. They were also asked to read a sheet where the words did not match the ink colour (e.g. ‘pink’ would be printed in blue ink). Participants were asked to name the ink colour. He found that responses were slower when ink and word did not match. Counterbalancing was used to eliminate order effects.
Repeated measures Design

22. Independent Groups Design
Which experimental design has been used in this famous study? Was it a good choice?
Bandura, Ross and Ross (1961) tested 74 preschoolers. They were put into groups:
a control group;
single sex groups who saw a violent model of either same sex or a different sex;
and single-sex groups who saw a non-violent model of either the same sex or a different sex.
Children who saw a model who was violent were more likely to imitate violent acts, especially if the model was the same sex as them.
Independent Groups Design

23. Tasks On page 10 of your booklet is a grid. You need to complete this.
Give a brief description of each type of experimental design.
Give details of the advantages and disadvantages of each method.
The textbooks on the shelf will help.
Extension: Complete the questions at the bottom of page 10 onto your own paper.