EXPERIMENTAL DESIGN

1. EXPERIMENTAL DESIGN Chapter 7

2. Outline for Tonight�s Class Why we need experimental research. Random groups design. Challenges to internal validity. How to analyze and interpret results. Establishing external validity. Two alternative experimental designs.

3. Why do we need experimental research? The four goals of psychological research: description, prediction, understanding and creating change. Psychologists want to understand the �why� of behavior. Experimental research allows us to establish cause-and-effect relationships.

4. Examples of Experiments Moreland and Beach (1992) � asked a female research assistant to attend a college class 15 times during one semester, another to attend 10 times, another 5 times, and one to not attend at all. At the end of the semester all 4 women entered the classroom and students were asked to fill out surveys rating how much they liked each woman.

5. 100 heart patients were prescribed 100mg of aspirin every day, 100 other heart patients were prescribed 75gm, 100 were prescribed 50mg, and 100 were prescribed a placebo (sugar pill). After two months, the blood pressure of all 500 patients was measured and compared to their initial blood pressure.

6. Experimental Control Have to have control in order to state with confidence that the IV caused the changes in the DV. Three conditions needed to make a causal inference: covariation, time-order relationship, and the elimination of other plausible alternative causes.

7. How is balancing accomplished? You want to have two comparable groups. Random groups design allows you to form comparable groups prior to beginning the experiment. Random selection Random assignment

8. Threats to Internal Validity Testing intact groups (those formed prior to the start of the experiment). Extraneous variables. Subject loss. Mechanical subject loss. Selective subject loss. Expectations held by participants and experimenters.