Chapter 10 Basic between-subjects designs

1. Chapter 10 • Basic between-subjects designs • The experimental design - is the general structure of the experiment • between-subjects design. • Comparing groups that contain different subjects. • Later we will discuss within subjects designs.

2. Selecting and recruiting subjects • Random sampling • Better external validity • rarely happens • Introductory psychology students

3. How many subjects should be in an experiment? • Too few can be a problem • 1) May not represent the population • 2) May decrease power • Human studies usually require participants than do animal studies • Why?

4. One Independent Variable: Two-groups designs • two versions • two independent groups design • two matched groups design

5. Two independent groups design • Selecting the sample • Typically a convenience sample is used • hypothesis testing • We have predicted that a manipulation will cause a difference between our otherwise equal 2 groups. • This can be accomplished without using a truly random sample

6. Assignment to groups • Random assignment • every subject has an equal chance of being placed in either of the treatment conditions.

7. Random selection • external validity • Random assignment • internal validity

8. An example of a two independent groups experiment • Loftus and Burns (1982)

9. Another independent groups experiment • Brady (1958)

10. Two types of two independent groups designs • 1) Experimental group-Control group design • 2) Two experimental groups design • Experimental group – Control group design • experimental condition = treatment • control condition = no treatment • Loftus & Burns (1982)

11. Two Experimental Groups design • participants are exposed to different levels of the Independent Variable. • Holloway & Hornstein (1976) • Good news vs. Bad News in a waiting room

12. Matched groups designs • Page (1958) • This was performed with an independent groups design, but is a great candidate for a matched groups design • The effect of student comments on papers

13. This design is fine if you have a decent sized N. • If small N - groups might be somewhat unequal at the start • use a matched groups design

14. Group 1 Group 2 • 1 = 61 1 = 72 • 2 = 65 2 = 92 • 3 = 71 3 = 76 • 4 = 60 4 = 83 • 5 = 83 5 = 95 • 6 = 58 6 = 75 • mean – 66.33 mean – 82.17

15. A two-matched groups design will eliminate this problem • How to match • First rank order the scores according to their first paper • 58 • 60 • 61 • 65 • 71 • 72 • 75 • 76 • 83 • 83 • 92 • 95

16. Now randomly assign one member from each pair to groups (flip a coin) Gp1 Gp2 58 60 65 61 72 71 76 75 83 83 95 92 mean =73.17 mean = 73.67

17. Rules about matching • The closer the matching variable is to the dependent variable the better • 1) match using actual dependent variable • Grades and grades • 2) match on closely related variable • Zajonc (1969) • 3) match on related variable • IQ and grades

18. Methods of matching • Precision matching • Same score • Range matching • Specified range • Rank ordered matching • All subjects included

19. When to use a matched groups designs • best when you have small numbers of subjects • Disadvantage • If you match on an unimportant variable you will actually lose power • fewer degrees of freedom

20. One independent variable with multiple levels • Multiple Groups Design • Sometimes more than one group is required to test the hypothesis • Drug studies use dose response c`rves • usually not a simple linear relationship