1. Ch 8: Experimental Design Ch 9: Conducting Experiments

2. Uses of Experimental Research
Test hypotheses derived from theories
Test the effectiveness of a treatment or program
Examine the causes of behavior

3. Conducting Experimental Research
Manipulate independent variable to see effect on dependent variable
Compare groups in terms of their scores on the dependent variable
All other variables kept constant through direct experimental control and/or randomization

4. Independent Variable
This variable is manipulated (controlled) by the experimenter and has at least two different levels (conditions)
1000 mg
0 mg=placebo

5. Manipulating the Independent Variable
Straightforward manipulations
Amount of substance administered
Written instructions
Verbal material
Visual material
Staged manipulations
Often employ confederates
Events are staged or manipulated to:
Create some psychological state
Simulate some situation that occurs in the real world

6. Dependent Variable
This variable is measured by the experimenter and is used to determine the effect of the independent variable
no pain mild moderate severe

7. Measuring the Dependent Variable
Types of measures
Self-report measures
Behavioral measures
Physiological measures

8. Sensitivity of the Dependent Variable
The independent variable can appear to have no effect on the dependent variable when there is a
Ceiling effect—participants quickly reach the maximum performance level
Floor effect—a task is so difficult that hardly anyone can perform well

9. Posttest-only Design
Obtain two equivalent groups of participants (R=random assignment)
Introduce the independent variable
Measure the effect of the independent variable on the dependent variable

10. Posttest-only Design
Involves random assignment of individuals to the different conditions of the independent variable

11. Random assignment
A method for placing subjects in conditions prior to implementing the independent variable
Every individual has the same chance of being placed in a given condition

12. Pretest-posttest design
Same as a posttest-only design but adds a pretest before the experimental manipulation
Allows the researcher to ascertain if the groups are equivalent at the beginning of the experiment
Example: Are kids healthier
on some dimension (weight,
stamina) after going through
an athletics program?

13. Advantages of the pretest-posttest design
Assess equivalency with small sample size
Assess mortality (attrition or
dropout factor)

14. Mortality
Mechanical subject loss: equipment failure or experimenter error leads to loss
Selective subject loss: some characteristic of participant is responsible for loss

15. Disadvantages of the pretest-posttest design
Time consuming
Awkward to administer
Sensitizes participants to what is being studied
Demand characteristics:
Cues and information a
participant uses to guide
their behavior in a
psychological study
“Is research on cultural acceptance?”

16. Demand characteristics
Possible solutions:
disguise pretest
embed the pretest in another measure (filler questions)
concealed observation
Placebo control group —used to assess whether participants’ expectancies contribute to the outcome of an experiment

17. Internal validity
Occurs when we are able to confidently state that the independent variable caused the differences we observe
Causal inferences can be made when internal validity is present

18. Confounding
This occurs when the variable of interest and a different potential independent variable are allowed to covary
Represents an alternative explanation for a study’s findings
Threatens internal validity

19. Other Threats to Internal Validity
Intact groups
Extraneous variables
Experimenter effects

20. Intact groups
This occurs when groups are formed prior to the start of an experiment
Selection differences: systematic ways in which people can differ

21. Selection differences
Characteristics of people that differ or vary:
Physical characteristics: sex, race
Social characteristics: ethnicity, religion, marital status
Personality characteristics: extraversion, emotional stability
Mental health characteristics: depression, anxiety

22. Extraneous variables
Other practical considerations that may confound an experiment

23. Experimenter effects
Biases that occur when experimenters’ expectancies regarding the outcome of the experiment influence their behavior toward participants in different conditions
Control by automating procedures as much as possible

24. Double-blind experiment
A procedure in which both the participants and the experimenters are unaware of which condition is being administered
Controls for both demand characteristics and experimenter effects

25. Experimental control techniques
Methods for ruling out other possible causes for the outcome

26. Assigning Participants to Experimental Conditions
Independent groups design
Participants randomly assigned to conditions
Participants are in only one group
Meaningfulness
Low-meaningful
High-meaningful
15 randomly assigned participants
Another 15
randomly assigned participants

27. Assigning Participants to Experimental Conditions
Repeated measures design
The same participants are in all of the groups
Meaningfulness
Low-meaningful
High-meaningful
15 participants
The SAME 15
participants

28. Repeated Measures Design
Advantages
Fewer participants
Extremely sensitive to statistical differences (more likely to detect an effect of the IV on the DV)
Disadvantages
Order effects
Practice effects
Fatigue effects
Contrast effects

29. Minimizing order effects
Counterbalancing
Complete counterbalancing—all possible orders of presentation are included in the experiment

30. Matched pairs design
Ensures groups are equivalent on the matching variable prior to the IV
Match participants on a particular characteristic
After matching, randomly assign to experimental conditions