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Chapter TenExperiments: Researching Cause and Effect

Key Concepts•The basic experimental concept –

“let’s do something and see what happens”.

•Experimental designs can determine whether variables have a causal relationship

•More sophisticated designs use experimental groups and control groups, and random assignment of subjects into groups.

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Experiment Advantages & Disadvantages

Advantages DisadvantagesDetermine causal relationships. Artificiality of experimental

conditions.

Determine direction of causality.

Questionable validity.

Determine relative influence of variables.

May involve ethical issues such as deception.

Demonstrate influence of one variable on another.

May require large numbers of people.

Control over research conditions.

May be time-consuming.

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Why Experiments?

•Correlation is not causality•Correlation is not causality•Correlation is not causality•Correlation is not causality•Surveys show correlation only.

•Experiments can assess causality.

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The Causality Question:Did A cause B?

To show that A caused B, we must show-

•Time order - ▫ A must precede B.

•Meaningful co-variance - ▫ B must change as A changes.

•Non-spuriousness - ▫ Nothing except A must be influencing

B.

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The Basic Experimental Design

Group A Group B

Treatment X

Group A results Group B results

•Do group results differ?

•If so treatment X explains it (maybe).

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Experiments: Low Level of Control

•Low Control - Field or natural experiments

•Single Group, post-test -

x O1

•One Group, pre-test, post-test -

O1 x O2

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Experiments: Medium Level of Control

Two groups, pre-test, post-test -

(Treatment group) O1 x O2

(Control group) O1 O2

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Experiments: High Level of Control

Two groups, randomized, pre-test, post-test -

(Randomized, treatment) O1 x O2

(Randomized, control) O1 O2

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Experiments: High Level of Control.Solomon Four Group Design.

Group 1 O1 x O2

Group 2 O1 O2

Group 3 x O2

Group 4 O2

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Solomon Four-Group Design: Example.

Before

Program

After

Program

Random

Group 1

First survey Program Second survey

Random

Group 2

First survey Second survey

Random

Group 3

No survey Program First survey

Random

Group 4

No survey First survey

Introducing Communication Research 2e © 2014 SAGE Publications

Chapter Summary

Experiments – • Range from simple observation to sophisticated

factorial designs.• Expose participants to controlled conditions.• Can isolate the effect of specific variables.• Can determine causal relationships among

variables.• Good design requires random assignment to

groups.• Experimental conditions rarely resemble real-life

conditions. • To assess causality requires sophisticated designs

such as the Solomon Four-Group.

Introducing Communication Research 2e © 2014 SAGE Publications

Vocabulary Review

Introducing Communication Research 2e © 2014 SAGE Publications

Web Resource

Web Center for Social Research Methods - http://www.socialresearchmethods.net

Provides an overview of social research methods, including experimental design.

Introducing Communication Research 2e © 2014 SAGE Publications