Chapter 8: Experimental Design

Question 1

Extraneous and Confounding Variables —Key Points—

Answer 1

Main point: Most outcomes are influenced by multiple variables

• Purpose of experiments is to isolate the causal factor
• Other factors that influence the DV are problematic

Question 2

Extraneous Variables

Answer 2

Variables other than the IV that could affect the outcome/DV

Question 3

Confounding Variables

Answer 3

An extraneous variable that that varies systematically with the IV

A confounding variable makes it impossible to claim causation from IV

Question 4

Extraneous Vs. Confounding Variables

Answer 4

Extraneous Variables generally just add variability, while Confounding Variables can create misleading conclusions about causality.

Question 5

Test Prep — Identify the Confounding Variable:

Research Question: Does explaining to someone else enhance comprehension?
* Explanation condition: Read an article about echolocation in bats (10 min). After reading the article once, attempt to explain it to someone else, with access to the article the whole time (10 min).
* No explanation condition: Read an article about echolocation in bats (10 min).
* DV: Take a test that measures comprehension (both groups get this test, obvi).

1. Identify the confounding variable.
2. Describe why/how that confounding variable disrupts internal validity
3. Fix the experiment

Answer 5

Answers:

1. Confounding variable: Time with the article
   (The explanation group has TWICE the amount of time with the article, compared to the control group…)
2. Disruption of Internal Validity: If participants in the explanation condition score better than participants in the no explanation condition, we don’t know if (a) explanation improves comprehension, (b) extra study time improves comprehension, or (c) both.
3. Fix the experiment:
   Ensure both groups spend the same amount of time with the material. For example:
   a) Explanation condition: Read for 10 minutes, then explain for 10 minutes.
   b) No explanation condition: Read for 10 minutes, then engage in another active task related to the material for 10 minutes (e.g., summarizing in writing or answering questions about the article).

This adjustment controls for time and engagement differences, isolating the effect of explaining on comprehension.

Question 6

Confounding Variables Relationship to Internal Validity

Answer 6

Confounding Variables = a THREAT to Internal Validity!!!

• Internal Validity only exists when the results of an experiment can be confidently attributed the effect of the IV.
• When a confounding variable is present, we cannot determine which variable is responsible for the effect

Question 7

Basic Experiment Types:

Answer 7

Between-Subjects Design: Each group/participant experiences ONE condition
* Between-Subjects Design = Independent Groups Design

Within-Subjects Designs: Each participant experiences ALL conditions (only one group).
* Within-Subjects Designs = Repeated Measures Desig

Question 8

Assigning participants to conditions: Between-subjects/Independent Groups Design

Answer 8

• Subjects are randomly assigned.
• Random assignment prevents systematic biases

Question 9

Assigning participants to conditions: Within-subjects/Repeated Measures Design

Answer 9

All subjects experience every conditions

Question 10

Assigning participants to conditions: Matched pairs Design

Answer 10

Participants are paired/matched based on similar participant variables (ie: age, sex, hight, etc.) before being assigned a condition.

Ensures that the groups are equivalent (on the matching variable) before introducing the IV manipulation.

Question 11

Advantages & Disadvantages of Within-Groups Designs

Answer 11

Advantages:
* More confidence when making statistical inferences
* Fewer subjects required

Disadvantages:
* Potential Risk of Order Effects

Question 12

Order Effects

Answer 12

• Practice/learning effect
• Fatigue effect
• Carryover effect:

Question 13

Practice/ Learning Effect

Answer 13

Definition: Performance increases due to practice in the experiment (warm-ups)

Solution: reduced by adding pre-experimental practice trials

Question 14

Fatigue Effect

Answer 14

Definition: Performance decreases due to fatigue

Solution: Reduced by allowing breaks during the experiment

Question 15

Carry-Over Effect

Answer 15

Definition: Persistent influence of one level of the IV on the next level of the IV

Solution: Reduced by allowing breaks during the experiment

Question 16

Controlling for Order Effects

Answer 16

Counterbalancing and Randomization

Question 17

Counterbalancing

Answer 17

A method of controlling for order effects in a repeated measures design by switching (balancing) the match between condition and stimuli

Question 18

Randomization

Answer 18

randomly determining the order for each subject

Question 19

Matched Pairs Design: When are they Useful?

Answer 19

When a specific participant variable is very likely to affect the DV

When the sample size is NOT particularly large

Question 20

Pretest Design

Answer 20

Testing the level of the DV before introducing the IV

• Reveals weather or not participant groups are fair/the same

Question 21

Posttest Design

Answer 21

A typical experiment uses a post test design. Testing the level of the DV after introducing IV

Question 22

Pretest-Posttest Design

Answer 22

Measure the DV before introducing the IV and then measure the DV again after IV is introduced