Subject Assignment

Question 1

Within-subjects design

Answer 1

-Every subject receives each level of IV
-Potential problems:
• Sequence or carry-over effects (minimize by counterbalancing)

Question 2

Between-subjects design

Answer 2

-Each subject receives only one level of IV
-Test diff. groups of subjects
-Potential problems:
• Group diff. could be due to subject diff.
• Increase “bad” variance

Question 3

Counterbalancing

Answer 3

Can eliminate treatment order confounds

1. Complete counterbalancing
2. Partial counterbalancing

Cannot eliminate confounds if:

• treatment produces irreversible change to subject
• asymmetrical transfer

Question 4

Complete counterbalancing

Answer 4

All possible sequences, each used equally often

Question 5

Partial counterbalancing

Answer 5

• Without every possible order

- Ex. Balanced Latin Square

Question 6

Use between-subjects design if:

Answer 6

• Being in one condition makes it impossible to be in other (irreversible change)
• Asymmetrical transfer between conditions
• IV is a subject variable

Advantages:
• Subjects equally naïve
• Less time to test each subject

To minimize bad variance:
• Control irrelevant variations
• Control some subject differences

Question 7

How to assign subjects to treatment groups:

Answer 7

1. Randomized groups

2. Matched groups

Question 8

Randomized groups

Answer 8

• Random assignment of subjects to groups

- Problem: if groups are small

Question 9

Matched groups

Answer 9

• Select sample from population
• Measure subjects on traits to control for
• Use trait scores to pair similar subjects
• One of each pair to each group

Best if:

• Small # subjects
• Traits are expected in influence DV
• Trait is easily and reliably measured

BUT pretest must be used (might elude subjects to nature of test)

Question 10

Ideal conditions for each design

Answer 10

Within-subjects
# subjects: small
*Carryover effects: absent OR easily counterbalanced*
Subject differences: Likely to effect DV
*Real life: Experience all conditions*

Matched groups
# subjects: small
Carryover effects: present AND NOT easily counterbalanced
Subject differences: Likely to effect DV
Real life: DON’T experience all conditions

Random groups
# subjects: large
Carryover effects: present AND NOT easily counterbalanced
Subject differences: unlikely to effect DV
Real life: DON’T experience all conditions