Mediation Moderation1

8/13/2019 Mediation Moderation1

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The Importance of Testing

Mediation and Moderation

Jon A. Krosnick

Departments of Communication,

Political Science,

and Psychology

Stanford University


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Outline for Today

Introduction: Defining and Testing Mediation

and Moderation

Example: News Media Priming

Example: Public Learning About Policy Issues


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Introduction


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The Audience Participation

Portion of Todays Show


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What is a moderator?

- Definition

- Example of moderation

What is a mediator?

- Definition

- Example of mediation


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Moderator

A variable that changes theimpact of one variable on

another.

Predictor Outcome

Moderator


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Testing a Moderator Hypothesis


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Dont Presume Linearity


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Mediator

The mechanism by which onevariable affects another

variable


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Testing Mediation

Step 1: IV DV

Step 2: IV Mediator

Step 3: Mediator DV

Step 4: Effect of IV on DV is significantly reduced by

controlling for the mediator: Sobel (1982) test

(http://www.unc.edu/~preacher/sobel/sobel.htm)

Goodman (1960) testOn the exact variance of products.Journal of the American Statistical

Association, 55, 708-713.


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First Example:

News Media Priming


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News Media Priming

News media attention to an issue increases

the impact of performance assessments in the

domain on overall presidential evaluations.

PresidentialPerformanceHandling

Unemployment

OverallPresidentialPerformance

Media Attentionto Unemployment


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Mediation of Moderation

UnemploymentPresidentialPerformance



Accessibility ofUnemployment

Performance Assessment


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UnemploymentStory

Unemploymentttitudes

Unemployment


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Study Design

- Pre-exposure questionnaire

- 5 non-political TV news stories

2 stories on either:

Crime Pollution Unemployment

- Post-exposure questionnaire


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Measures

Overall presidential performance

Presidential performance handling:

CrimePollution

Unemployment

Accessibility (response latency) Political knowledge

Trust in the news media


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Moderation

of Mediation

of Moderation!

Not Mere AccessibilityPersuasion?


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UnemploymentPresidentialPerformance



Belief that MediaPersonnel Believe theIssue is Importantfor the NationTrust

in theMedia

PoliticalKnowledge

Belief that the Issueis Important for the

Nation


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Conclusion

Understanding moderation and mediationincrease confidence in the causality claim

Understanding mediation changed thepolitical character of the effect (not victimsof the architecture of the mind)

Understanding moderation changed thenormative spin of the effect (not navepeople who lack political expertisepeoplechoose to be influenced)


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Example Two:

How Do People Learn About Politics?


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October 13, 1988 Presidential Debate

George H. W. Bush vs. Michael Dukakis

Day Before: 134 people interviewed

Day After: 63 reinterviewed

- Cued Recall

- Recognition Memory

- Attitude Importance


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Analysis: Hierarchical Linear Modeling

Participant

Taxes Capital DefensePunishment Spending

CR RM CR RM CR RM


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Effect of Importance on

Knowledgeb=.46

SE=.23

p


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The Usual Approach

Stop There.

Smile.

Fold Up Tent.

Go Home.


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.11*

.04

.31**

.44


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Laboratory ExperimentConditions

Unpaced


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A woman should have the right to obtain an abortion if she so chooses.

The death penalty is an acceptable punishment for convicted murderers.

We should build more nuclear power plants to provide an alternativesource of energy.

The U.S. should send more troops to Saudi Arabia to oppose Iraqs

invasion of Kuwait.

In order to protect American jobs, we need more restrictions on the

number of goods imported.

Candidate A


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Unpaced

Paced


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You will have 20 seconds to read five statements

made by each candidate.

After 20 seconds, the statements will disappear, so

read as quickly as you can.


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Unpaced

Paced

Elaboration Time


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You will have 20 seconds to read five statements

made by each candidate.

After 20 seconds, the statements will disappear, so

read as quickly as you can.

Once the statements disappear, you will have 45seconds to think about what you read.


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Unpaced

Paced

Elaboration Time

Topic Labels


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A woman should have the right to obtain an abortion if she so chooses.

The death penalty is an acceptable punishment for convicted murderers.

We should build more nuclear power plants to provide an alternativesource of energy.

The U.S. should send more troops to Saudi Arabia to oppose Iraqs

invasion of Kuwait.In order to protect American jobs, we need more restrictions on the

number of goods imported.

Candidate A

Abortion

Death Penalty

Nuclear Power

Iraq

Imports


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Measures

Free Recall

Recognition Memory

Attitude Importance


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Alternative Hypothesis

Actual

Knowledge

Volume

Perceived

Knowledge

Volume

AttitudeImportance


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.37*

.27*

.38*

.55** .04 .00


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Conclusions

You can test mediation with panel data

You can test mediation via moderation

manipulations in the lab

You can test mediation with cross-sectional

data with instrumental variables and 2SLS


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Broader Conclusion

Test mediation (and moderation)!

Two examples where this was not done:

Valentino, N. A., Hutchings, V. L., & White, I.K. (2002). Cues that matter: How politicalads prime racial attitudes duringcampaigns.American Political Science

Review,96, 75-90.Tali Mendelberg The Race Card: Campaign

Strategy, Implicit Messages, and the Normof Equality (Princeton Univ Press, 2001).

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