• 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.
• Selecting and recruiting subjects
– Random sampling
• Better external validity
• rarely happens
• Introductory psychology students
• 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?
• One Independent Variable: Two-groups
designs
– two versions
• two independent groups design
• two matched groups design
• 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
Assignment to groups
• Random assignment
– every subject has an equal chance of being
placed in either of the treatment conditions.
• Random selection
– external validity
• Random assignment
– internal validity
• An example of a two independent
groups experiment
– Loftus and Burns (1982)
• Another independent groups
experiment
– Brady (1958)
• 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)
• 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
• 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
• 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
•
•
•
•
•
•
•
•
Group 1
1 = 61
2 = 65
3 = 71
4 = 60
5 = 83
6 = 58
mean – 66.33
Group 2
1 = 72
2 = 92
3 = 76
4 = 83
5 = 95
6 = 75
mean – 82.17
• 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
• Now randomly assign one member from each
pair to groups (flip a coin)
Gp1
58
65
72
76
83
95
mean =73.17
Gp2
60
61
71
75
83
92
mean = 73.67
• 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
• Methods of matching
– Precision matching
• Same score
– Range matching
• Specified range
– Rank ordered matching
• All subjects included
• 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
• 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