2. PowerPoint presentation to accompany Research Design Explained 4th edition ; ©2000 Mark Mitchell & Janina Jolley Chapter 7 The Multiple Group Experiment

3. PowerPoint presentation to accompany Research Design Explained 4th edition ; ©2000 Mark Mitchell & Janina Jolley Overview l The advantages of using more than two values of an independent variable l Analyzing multiple group experiments

4. PowerPoint presentation to accompany Research Design Explained 4th edition ; ©2000 Mark Mitchell & Janina Jolley The Advantages of Using More Than Two Values of an Independent Variable l Comparing more than two kinds of treatment l Comparing two or more treatments against each other and against no treatment l Better ability to estimate the effects of different amounts (levels) of a treatment* l Better ability to rule out confounding variables*

5. PowerPoint presentation to accompany Research Design Explained 4th edition ; ©2000 Mark Mitchell & Janina Jolley Comparing More than Two Levels (Amounts) of an Independent Variable l May help discover significant relationships l May help more accurately map the functional relationship** l Summary: Multi-level experiments have more external validity than simple experiments

6. PowerPoint presentation to accompany Research Design Explained 4th edition ; ©2000 Mark Mitchell & Janina Jolley A straight line is not the only line that you can draw between two points.

7. PowerPoint presentation to accompany Research Design Explained 4th edition ; ©2000 Mark Mitchell & Janina Jolley Using Multiple Groups to Improve Construct Validity of Experiments l Multi-level experiments can call attention to a possible problems with your control group** l Using multiple imperfect control groups to make up for not having the perfect control group

8. PowerPoint presentation to accompany Research Design Explained 4th edition ; ©2000 Mark Mitchell & Janina Jolley No Med High Amount of Treatment

9. PowerPoint presentation to accompany Research Design Explained 4th edition ; ©2000 Mark Mitchell & Janina Jolley Analysis of Multiple Group Experiment l Within groups variability: A pure measure of error l Between groups variability: Error + (possibly) treatment l Comparing between groups variance to within groups variance: The differences between groups due to more than random error l Why an F of 1 does not show that the treatment had an effect

10. PowerPoint presentation to accompany Research Design Explained 4th edition ; ©2000 Mark Mitchell & Janina Jolley To See If an F above 1 Is Significant, Use an F-Table l Choose significance level l Look under correct degrees of freedom –First (Treatment) df = Number of treatment groups -1 (G-1) –Second (Error) df= Number of participants - # of groups (N-G)

11. PowerPoint presentation to accompany Research Design Explained 4th edition ; ©2000 Mark Mitchell & Janina Jolley The Meaning of Statistical Significance in ANOVA l Beyond ANOVA: Pinpointing a significant effect –Post hoc t-tests among group means: Which groups differ? –Post hoc trend analysis: What is the shape of the relationship?

12. PowerPoint presentation to accompany Research Design Explained 4th edition ; ©2000 Mark Mitchell & Janina Jolley Concluding Remarks l Why use multiple group experiments? –To compare several treatments –To increase external validity –To increase construct validity l Why know about how to analyze such experiments? –Read results of such studies –Design such studies –Analyze results of such studies