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Internal Validity Tutorial

Preface

This tutorial is a component of the courseware of the Psychology Centre of Athabasca University. The tutorial is used in Athabasca University's Psychology 404 (Experimental Psychology). It was authored by Dr. David Polson, Adjunct Assistant Professor at the University of Victoria. Dr. Cheuk Ng, Dr. Lyle Grant and Dean Mah of Athabasca University provided various content-related, technical and instructional-design assistance. Some technical notes provide some information about the instructional-design principles on which the tutorial is based.

Introduction

In Part 1 of this tutorial, you will be introduced to nine sources of threat to internal validity. (Campbell, 1969; Campbell & Stanley, 1963) First, some relevant terms are defined. Then, some background explanation for a hypothetical experiment is presented. Finally, each of the nine threats is described, followed by an example and a contrasting nonexample as applied to the hypothetical experiment. An explanation is included of why the example represents a threat to internal validity and why the nonexample is not a threat.

In Part 2 of this tutorial, you will be asked to classify 36 hypothetical experiments as internally valid or not. If not, you must select the threat to internal validity from one of the nine sources introduced in Part 1.

Following some of the vignettes in this tutorial, a reference is made to a "Related Source." This reference is included for students interested in pursuing the subject matter of the vignette. Sometimes the details of the hypothetical experiment are very similar to the related source, but in many cases they are quite different. Thus, the hypothetical experiment is not to be regarded as an accurate description of the related source -- the commonality between the two may simply be similar independent and dependent variables.

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