Consumer Preference Test Level 1- “h” potato chip vs Level 2 - “g” potato chip 1. How would you rate chip “h” from 1 - 7? Don’t Delicious like it 2. Do you think it could be low fat or regular? 1. How would you rate chip “g” from 1 - 7? 1 Don’t like it 7 Delicious 2. Is it low fat or regular?

Consumer Preference Test ______________ Design ________________ Counterbalance means... How to prevent Carryover effects?

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

Experimental research design 1.Control extraneous variables 1.Hold constant 2.The only difference between experimental group and control group is the manipulated variable. 1.Treat groups equally except for treatment. 3.Randomize effects across treatments 4.Design to eliminate alternative explanations

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

Control Variable The variable you want to keep constant in order to clarify the relationship between Dependent Variable and Independent Variable “Confounding” variable (if not controlled)

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 of predictor variable

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. Example

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

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.

Dr. “Lee’s” study Geography dept. had 10 Mac computers for their students to use Anthro. dept. had 10 IBM-type pc computers. Research question: How does the type of computer affect the quality of students’ papers? Collected papers from each department’s student computer lab

Method: Two graduate students in the English dept. rated the quality of the papers. Results: the quality of the papers was higher in one department than in the other. What can you conclude?

What are the independent variables in this study? What are the dependent variables? What variables are confounded with the independent variable? Does this study have internal validity? Explain. Does study have much external validity?

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

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.

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, … those that remain differ than drop outs.

Within-Subjects/ Groups Design Repeated measures Each participant’s performance is measured under Treatment A and again under Treatment B Advantages reduce error from variation in participants Disadvantages - attrition, carryover effects

Matched random assignment When participant characteristics correlate 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 treatments.

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