Experiments

1. Experiments • Why would a double-blind experiment be used? • Discuss how the following factors may threaten internal validity: history, maturation, testing, instrumentation, statistical regression, selection biases, experimental mortality, causal time-order, diffusion or imitation of treatments, compensation, compensatory rivalry, and demoralization. • Does a true experimental design address the problem of external validity? • Identify strengths and weaknesses of the experimental method.

2. Experimental Research

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

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

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

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

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

8. 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

9. 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

10. 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

11. 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

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

13. 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

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

15. 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.

16. 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 • www.whfreeman.com/ips

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

18. 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?