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
What is a moderator?- Definition- Example of moderationWhat is a mediator?- Definition- Example of mediation
Moderator A variable that changes the impact of one variable on another. Predictor Outcome Moderator
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. Media Attention to Unemployment Presidential Performance Handling Unemployment Overall Presidential Performance
Mediation of Moderation Media Attention to Unemployment Accessibility of Unemployment Performance Assessment Unemployment Presidential Performance Overall Presidential Performance
Unemployment Attitudes Unemployment Unemployment Story
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
Not Mere Accessibility – Persuasion? Moderation of Mediation of Moderation!
Media Attention to Unemployment Belief that Media Personnel Believe the Issue is Important for the Nation Trust in the Media Political Knowledge Belief that the Issue is Important for the Nation Unemployment Presidential Performance Overall Presidential Performance
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.
.44 .11* .04 .31**
Laboratory Experiment Conditions Unpaced
Candidate A 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.
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
Candidate A Abortion 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. 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* .00 .55** .04 .38*
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).