Experimental research involves manipulating an independent variable to test its effect on a dependent variable. Key aspects of experimental design include assigning participants randomly to conditions, using control groups, counterbalancing order effects, and blinding participants and researchers to reduce bias. Well-designed experiments allow researchers to draw causal conclusions about the impact of an intervention.

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

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

16. Demand characteristics
o 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
Low-
meaningful
High-
meaningful
15 randomly
assigned
participants
Another 15
randomly
assigned
participants
Meaningfulness

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

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
1. 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