1. Experimental Research

2. Experiments Begin with a Hypothesis Modify Something in a Situation
Compare Outcomes
Cases or People are Termed “Subjects”

3. Random Assignment Probability of Equal Selection
Allows Accurate Prediction
An Alternative to Random Assignment is Matching

4. Parts of the Classic Experiment
Treatment or Independent Variable
Dependent Variable
Pretest
Posttest
Experimental Group
Control Group
Random Assignment

5. Variations on Experimental Design
Pre-experimental Design
One-shot Case Study
One-group Pretest-Posttest Design
Static Group Comparison
Quasi-Experimental and Special Designs

6. Types of Validity External Validity Internal Validity
Do the results apply to the broader population?
Internal Validity
Is the independent variable responsible for the observed changes in the dependent variable?

7. Confounding Variables That Threaten Internal Validity
Maturation
Changes due to normal growth or predictable changes
History
Changes due to an event that occurs during the study, which might have affects the results

8. Confounding Variables That Threaten Internal Validity
Instrumentation
Any change in the calibration of the measuring instrument over the course of the study
Regression to the Mean
Tendency for participants selected because of extreme scores to be less extreme on a retest
Selection
Any factor that creates groups that are not equal at the start of the study

9. Confounding Variables That Threaten Internal Validity
Attrition
Loss of participants during a study; are the participants who drop out different from those who continue?
Diffusion of treatment
Changes in participants” behavior in one condition because of information they obtained about the procedures in other conditions

10. Subject Effects Participants are not passive
They try to understand the study to help them to know what they “should do”
This behavior termed “subject effects”
Participants respond to subtle cues about what is expected (termed “demand characteristics”)
Placebo effect: treatment effect that is due to expectations that the treatment will work

11. Experimenter Effects
Any preconceived idea of the researcher about how the experiment should turn out
Compensatory effects

12. Types of Control Procedures
General control procedures (applicable to virtually all research)
Control over subject and experimenter effects
Control through the selection and assignment of participants
Control through specific experimental design

13. Principles of Experimental Design
Control the effects of lurking variables on the response, most simply by comparing two or more treatments
Randomize
Replicate

14. Randomization
The use of chance to divide experimental units into groups is called randomization.
Comparison of effects of several treatments is valid only when all treatments are applied to similar groups of experimental units.

15. How to randomize? Flip a coin or draw numbers out of a hat
Use a random number table
Use a statistical software package or program
Minitab

16. Statistical Significance
An observed effect so large that it would rarely occur by chance is called statistically significant.

17. A few more things…
Double-blind: neither the subjects nor the person administering the treatment knew which treatment any subject had received
Lack of realism is a major weakness of experiments. Is it possible to duplicate the conditions that we want?