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Chapter 8

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Michele Oliver

on 2 July 2014

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Transcript of Chapter 8

Chapter 8 Presentation
Confounding Variables & Internal Validity
A confounding variable is another variable that occurs along with the independent variable.

The problem is that you can’t determine whether it is the independent variable or the confounding variable that is having an effect on the dependent variable.

What’s the relationship between confounding variables and internal validity?

Confounding variables compromise internal validity because you can’t draw conclusions about the IV affecting the DV.

Basic Experiments
The Posttest-Only Design

Here's where the IV has 2 levels, then the IV is introduced, followed by measuring the effect of the IV on the DV.

I hope that you enjoyed learning about experimental designs!
Chapter 8
Experimental Design
The 3 Types of Experimental Designs
The 3 Types of Experimental Designs
Basic Experiments (cont'd)
Pretest-Posttest Design

A pretest is given to each group prior to introduction of the experimental manipulation

Assures that groups are equivalent at the beginning of the experiment

Learning Objectives
Describe the relationship between a confounding variable and the internal validity of an experiment
Describe the posttest-only and the pretest-posttest design
Contrast and compare independent groups design with a repeated measures design
Summarize the advantages and disadvantages of using a repeated measures design
Describe a matched pairs design, including reasons to use this design

One More Design - The Matched Pairs Design
Participants are matched to either the dependent measure or a variable that is strongly related to the dependent variable. The matched pairs design results in a greater ability to detect a statistically significant effect of the IV.

1. Match on a participant characteristic
2. Obtain a measure of the matching variable from each participant
3. Rank order participants from highest to lowest based on their score on the matching variable
4. Create matched pairs that are approximately equal on the characteristic
5. Members of each pair are randomly assigned to the conditions in the experiment

Assigning Participants
Independent Groups Design
Participants are only in one group.

For example, either they've been assigned to the Control group or the Experimental group.
Repeated Measures Design
Participants are in ALL conditions
Here are some advantages...
You don’t need as many participants because participants act as their own control group.

You are more likely to find a significant finding with a repeated measures design

Here are some disadvantages...
Order Effects – the order of Task 1 followed by Task 2 may have a different effect than the order of Task 2 followed by Task 1

Practice Effects – Performance improves over time, with practice

Fatigue Effects – Performance deteriorates over time

Contrast (or Carryover) Effects – The effects from Task 1 carryover and have an effect on Task 2

What can we do???
Counterbalancing (either complete or use Latin Squares) can help with order effects

Offering rest periods between treatments can help with fatigue and contrast effects
Full transcript