1. Introduction section of article
Purpose of study and research question(s)
Problems related to study topic
Literature review
Rationale (justification) for the study
Hypotheses

2. Experimental Research

3. Example
Objective: To find out the impact of sunshine on plants’ growth.
Treatment
I.V.
I.V.
Experimental
Control

4. Experimental Research
Researchers manipulate independent variable - 2 levels
And measure the other (dependent variable)
Give treatment to participants and observe if it causes changes in behavior
Compare experimental group (w/ treatment) with a control group (no treatment)
Can say IV caused change in the DV

5. Experimental research design
Measure dependent variable
Control extraneous variables
Hold constant
The only difference between experimental group and control group is the manipulated variable.
Treat groups equally except for treatment.
Randomize effects across treatments
Design to eliminate alternative explanations

6. Random Assignment
A way to assign participants in your sample to the various treatment conditions (groups that will receive different level of the IV)
Any member of your sample has equal chance of being assigned in any treatment group

7. Control Variable
The variable that you would like to keep constant in order to clarify the relationship between Dependent Variable and Independent Variable
‘Confounding” variable (if not controlled)

8. Internal Validity
Ability of your research design to adequately test your hypothesis
Showing that variation in I.V. CAUSED the variation in the D.V. in experiment
In correlational study,
Showing that changes in value of criterion variable relate solely to changes in value to predictor variable

9. Example -- Design:
Question: Does new teaching method work better than traditional method in intro psych course?
Method: Teaches class in morning using new method.
Teaches afternoon class using traditional method.
Both classes will use same book, tests, etc.

10. Findings Students exposed to new method have higher grades.
Concludes: New method better.
Justified?
Why or why not?

11. Confounding
Whenever 2 or more variables combine in a way that their effects cannot be separated = confounding.
Thus, the teaching method study as designed lacks internal validity.

12. Threats to Internal Validity -- 7 Sources of Confounding
Campbell & Stanley (1963)
History
Other events occur that affect results
Maturation
Effect of age or fatigue
Testing

13. Threats to validity continued
Instrumentation
Changes in criteria used by observers or
instrument - (is scale set to 0?)
Statistical regression
If participants selected cause of extreme scores, will tend to be closer to average of pop. upon re-measure.

14. Threats to internal validity cont.
Biased selection of subjects
If participants differ in ways that affect their scores on DV
Experimental mortality
People drop out of study due to frustration, etc. those that remain differ than drop outs.

15. Completely randomized
Independent groups
Completely randomized 2-group experimental design
Advantages
no pretesting or categorizing of participants
adequate to test hypothesis

16. 2-Treatment Within-Subjects Design
Repeated measures
Randomly assign participants to different treatment groups,
Each participant’s performance is measured under Treatment A and again under Treatment B
Advantages reduce error variance
Disadvantages - attrition, carryover effects

17. 2-treatment within-subjects matched design
Repeated measures
When participant characteristic correlates w Dependent Variable
Assess the participants for the characteristics
Group participants w/ matching characteristics.
Matched sets of participants distributed at random, one per group.
Advantages - Effect of error , effect of characteristic distributed evenly across treatment groups.

18. Carryover effects
First treatment alters the behavior observed in next treatment
Sources
learning
fatigue
habituation - repeated exposure -- reduced responsiveness
To effect - (breaks between treatments, change treatment order)