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This session examines the role of moderation and mediation in understanding the relationship between cell phones and gas station fires, highlighting statistical methods and their applications.
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RStats Statistics and Research Camp 2014 Moderation and Mediation Session 2 Todd Daniel PhD RStats Institute
“So far, we have been unable to document any incidents that were sparked by a cellular telephone. In fact, many researchers have tried to ignite fuel vapors with a cell phone and failed.” Petroleum Equipment Institute “The wireless industry has done studies on the potential for wireless phones to create sparks…there is no documented incident where the use of a wireless phone was found to cause a fire or explosion at a gas station.” Federal Communications Commission https://www.youtube.com/watch?v=AIlsJPNMQss
Hmmmm… • More gas station fires occur to women • Women are more likely to re-enter the car • Women are less likely to touch the car when exiting • Conclusion: static electricity not cell phones Isn’t this more useful?
Stats Tell Us What? Stats tell us what. In what way? How? By which pathway? Under what circumstances? Grow from whether and if to how and when
Next Steps • NHST tells us whether • Correlation and Regression tell us if • Mediationanswers how • Moderationanswers when
Moderation • The combined effect of two variables on another • Conceptually known as moderation • In statistical terms: an interaction effect Moderator Predictor Outcome
Example • Do violent video games make teens aggressive? • Participants • 442 youths • IV: Number of hours spent playing video games per week • DV: Aggression • Moderator: Callous (unemotional) traits
Conceptual moderation model If calloustraits are a moderator then the strength or direction of the relationship between game playing and aggression is affected by callous (unemotional) traits. Callous Traits Game Playing Aggression
The Statistical Moderation Model Predictor Outcome Moderator Predictor x Moderator
Centering variables • The interaction term makes the b’s for the main predictors uninterpretable in many situations • For this reason, it is common to transform the predictors using grand mean centering • Centeringrefers to the process of transforming a variable into deviations around a fixed point
Simple slopes equations of the regression of aggression on video games at three levels of callous traits
How do I do that? PROCESS www.afhayes.com Plug in for SPSS
Mediation Statistical Model Mediation: when the relationship between a predictor variable and outcome variable can be explained by their relationship to a third variable (the mediator) Mediator c a b Predictor Predictor Outcome Outcome c' Simple Relationship Mediated Relationship
Baron & Kenny, (1986) • Mediation is tested through three regression models: • Predicting the outcome from the predictor variable • Predicting the mediator from the predictor variable • Predicting the outcome from both the predictor variable and the mediator
Baron & Kenny, (1986) Four conditions of mediation: • The predictor must significantly predict the outcome variable. • The predictor must significantly predict the mediator. • The mediator must significantly predict the outcome variable. • The predictor variable must predict the outcome variable less strongly in model 3 than in model 1. Mediator a b Predictor Outcome c'
Limitations of Baron & Kenny’s (1986) Approach • How much of a reduction in the relationship between the predictor and outcome is necessary to infer mediation? • people tend to look for a change in significance, which can lead to the ‘all or nothing’ thinking that p-values encourage
Sobel Test • An alternative is to estimate the indirect effect and its significance using the Sobel Test (Sobel, 1982) • If the Sobel test is significant, there is significant mediation
Effect Sizes of Mediation Kappa-squared (k2) (Preacher & Kelley, 2011)
Example of a Mediation Model Indirect Effect Relationship Commitment a b Pornography Consumption Infidelity Direct Effect c' Analysis is conducted in PROCESS
Reporting Mediation Analysis There was a significant indirect effect of pornography consumption on infidelity though relationship commitment, b = 0.127, BCa CI [0.023, 0.335]. This represents a relatively small effect, κ2= .041, 95% BCa CI [.008, .104].
Reporting Mediation Analysis Model of pornography consumption as a predictor of infidelity, mediated by relationship commitment. The confidence interval for the indirect effect is a BCa bootstrapped CI based on 1000 samples. Relationship Commitment b = -0.47, p = .028 b = -0.27, p < .001 Direct Effect, b = 0.46, p = .02 Indirect Effect, b = 0.13, 95% CI [0.02, 0.34] Pornography Consumption Infidelity
Anything else? • You can do mediation and moderation together • Conditional Process Analysis
197 male amateur golfers Stress at Home Self-Efficacy Social Support Task Performance Rees & Freeman, 2009
Typicality of OutgroupMbr. Rationality of argument Attribution Positive or Negative Interaction w/ Outgroup Attitude about Outgroup Popan et al. (2010)
172 female freshmen White v. Non-White Race Social anxiety Parental Attachment Satisfaction Satisfaction with Friends Parade et al. (2010)
Slogan did (not) emphasize saving money Awareness of advertising intent Was the advertisement intended to persuade Behavioral Prime Persuasion Focus Persuasive Intent Brand logos v. Brand logos + slogans $0 to $500 on imaginary shopping spree Advertising Tactic Willingness to Spend $ Laran, Dalton, and Andrade (2011)
“Static electricity has caused fires at gas stations…for this reason, you should not re-enter your vehicle while you are refueling, since static electricity caused by friction from your clothing’s contact with the car seat can ignite the gas when you get back out of the car to complete the refueling process.” Ohio State Bar Association website
