The Importance of Testing Mediation and Moderation

Jon A. Krosnick

Departments of Communication,

Political Science,

and Psychology

Stanford University

Outline for Today

• Introduction: Defining and Testing Mediation and Moderation

• Example: News Media Priming

• Example: Public Learning About Policy Issues

Introduction

The Audience Participation Portion of Today’s Show

What is a moderator?

- Definition- Example of moderation

What is a mediator?

- Definition- Example of mediation

Moderator

A variable that changes the impact of one variable on

another.

Predictor Outcome

Moderator

Testing a Moderator Hypothesis

Don’t Presume Linearity

Mediator

The mechanism by which one variable affects another

variable

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) test

On the exact variance of products. Journal of the American Statistical Association, 55, 708-713.

First Example:

News Media Priming

News Media Priming

• News media attention to an issue increases the impact of performance assessments in the domain on overall presidential evaluations.

Presidential Performance Handling Unemployment

Overall Presidential Performance

Media Attention to Unemployment

Mediation of Moderation

Unemployment Presidential Performance

Overall Presidential Performance

Media Attention to Unemployment

Accessibility of UnemploymentPerformance Assessment

Unemployment

Story

Unemployment

Attitudes

Unemployment

Study Design

- Pre-exposure questionnaire

- 5 non-political TV news stories

2 stories on either:

Crime Pollution Unemployment

- Post-exposure questionnaire

Measures

• Overall presidential performance• Presidential performance handling:

– Crime– Pollution– Unemployment

• Accessibility (response latency)• Political knowledge• Trust in the news media

Moderation

of Mediation

of Moderation!

Not Mere Accessibility – Persuasion?

Unemployment Presidential Performance

Overall Presidential Performance

Media Attention to Unemployment

Belief that Media Personnel Believe the Issue is Important for the NationTrust

in the Media

Political Knowledge

Belief that the Issue is Important for the Nation

Conclusion

• Understanding moderation and mediation increase confidence in the causality claim

• Understanding mediation changed the political character of the effect (not victims of the architecture of the mind)

• Understanding moderation changed the normative spin of the effect (not naïve people who lack political expertise – people choose to be influenced)

Example Two:

How Do People Learn About Politics?

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

Analysis: Hierarchical Linear Modeling

Participant

Taxes Capital Defense

Punishment Spending

CR RM CR RM CR RM

Effect of Importance on Knowledge

b=.46

SE=.23

p<.05

N=366

The Usual Approach

Stop There.

Smile.

Fold Up Tent.

Go Home.

.11*

.04

.31**

.44

Laboratory Experiment Conditions

Unpaced

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 alternative source of energy.

The U.S. should send more troops to Saudi Arabia to oppose Iraq’s invasion of Kuwait. In order to protect American jobs, we need more restrictions on the number of goods imported.

Candidate A

Laboratory Experiment Conditions

Unpaced

Paced

“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.”

Laboratory Experiment Conditions

Unpaced

Paced

Elaboration Time

“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 45 seconds to think about what you read.”

Laboratory Experiment Conditions

Unpaced

Paced

Elaboration Time

Topic Labels

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 alternative source of energy.

The U.S. should send more troops to Saudi Arabia to oppose Iraq’s 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

Measures

• Free Recall

• Recognition Memory

• Attitude Importance

Alternative Hypothesis

Actual

Knowledge

Volume

Perceived

Knowledge

Volume

Attitude Importance

.37*

.27*

.38*

.55** .04 .00

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

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 political ads prime racial attitudes during campaigns. American Political Science Review,96, 75-90.

Tali Mendelberg The Race Card: Campaign Strategy, Implicit Messages, and the Norm of Equality (Princeton Univ Press, 2001).