1. Chapter 8
Experimental Design

2. Confounding variables and internal validity
The means by which independent variables are kept constant
Confounding variable
A variable that fluctuates along with the independent variable
Confounding occurs when the effects of 2 IVs are intertwined making it unsure of which IV is responsible for the observed effect
Eliminating possible confounds makes for good experimental design
Internal validity
When the results of an experiment can confidently be attributed to the effect of the IV

3. Find the confounding variables
A researcher is interested in the ability of schizophrenic patients to judge different time durations. It is hypothesized that loud noise will negatively affect judgment.
Subjects are tested
Quietly in a sound proof room or
Noisily in a nurse’s office w/ a stereo playing music at a loud and constant volume
Due to scheduling problems, Risk III patients are available only on Mondays. Risk I patients are available only on Thursdays.
Also, hearing tests are scheduled for Thursdays, so the sound proof room is only available on Monday.

4. Basic experiments Posttest-only design Researcher must
Obtain 2 equivalent groups
Introduce the independent variable
Measure the effect of the independent variable on the dependent variable
R = random assignment
Experimental group R
Measure
Participants
Control group R
Measure

5. Basic experiments Posttest-only design
Avoid selection differences where people selected differ in any way
Accomplish equivalence by randomly assigning participants
If groups were equally selected and there were no confounds, any difference in the DV groups must be attributed to the IV

6. Basic experiments Pretest-posttest design Researcher must
Give pretest before experimental manipulation
The larger the sample, the less likely groups will differ before manipulating the independent variable (IV)
R = random assignment R
Experimental group
Pretest
Treatment
Posttest
Participants
Posttest
Control group
Pretest R

7. Basic experiments
Advantages .
Disadvantages

8. Basic experiments Solomon four-group design
½ participants = only posttest
½ participants = pretest and posttest
The effect of the pretest can be directly assessed
Possible outcomes
Results are the same with or without a pretest
A difference b/w the experimental and control groups w/ a pretest BUT no difference with no pretest

9. Assigning participants
Two methods
Random assignment (independent groups design)
Participants are in all conditions (repeated measures design
Advantages
Needs less participants; Less training of participants
Sensitive to finding statistically significant differences
Individual differences can be seen and explained
Disadvantages
Order effects: the order of presenting the treatments affects the dependent variable.
Performance on the 2nd task may be better than the 1st

10. Assigning participants
Types of order effects
Practice effect
Fatigue effect
Contrast effect

11. Should the IV be between- or a within-subjects design?
A neuroscientist hypothesizes that damage to the primary visual cortex is more permanent in older animals
A sensory psychologist predicts that it is easier to distinguish slightly different shadings of grays under daylight than under fluorescent light
A developmental psychologist predicts cultural differences in moral development

12. Counterbalancing Used in repeated measures designs
Complete counterbalancing
All possible orders of presentation are included in the experiment
Latin square
A limited set of orders where
Each condition appears at a ordinal position
Each condition precedes and follows each condition at one time

13. Counterbalancing Randomized blocks
Each repetition in the basic experiment is called a block of trials
To control for order effects the order of presentation can be randomly determined

14. Matched pairs design Match people on a certain characteristic
Rank participants based on scores on the matched variable
Members of each pair are randomly assigned to the conditions in the experiment
Each treatment is given to one subject in each pair
In the two different groups one of which will act as the control, the other as the experimental.
Ensures that groups are equivalent
Drawbacks
Time consuming
Needs a large sample pool

15. Developmental research designs
Cross sectional method
Persons of different ages are studied at only one point in time
Developmental change is not observed but based on comparisons of different cohorts
More common and yields immediately useful results
Longitudinal method
The same group of people is observed at different time points in time as they grow older
Over the course of the study people may move, die, drop out…
Sequential method
Combines both methods
First phase is cross sectional then they are studied longitudinally
Two or more age cohorts are tested at two or more times.