Questions of value, questions of magnitude: An exploration and application of methods for comparing indirect effects in multiple mediator models.

Mediation analysis is widely used to test and inform theory and debate about the mechanism(s) by which causal effects operate, quantitatively operationalized as an indirect effect in a mediation model. Most effects operate through multiple mechanisms simultaneously, and a mediation model is likely to be more realistic when it is specified to capture multiple mechanisms at the same time with the inclusion of more than one mediator in the model. This also allows an investigator to compare indirect effects to each other.

Mediation, Moderation, and Conditional Process Analysis: Concepts, Computations, and Some Common Confusions.

This work provides a conceptual introduction to mediation, moderation, and conditional process analysis in psychological research. We discuss the concepts of direct effect, indirect effect, total effect, conditional effect, conditional direct effect, conditional indirect effect, and the index of moderated mediation index, while providing our perspective on certain analysis and interpretation confusions that sometimes arise in practice in this journal and elsewhere, such as reliance on the causal steps approach and the Sobel test in mediation analysis, misinterpreting the regression coefficients in a model that includes a product of variables, and subgroups mediation analysis rather than conditional process analysis when exploring whether an indirect effect depends on a moderator. We also illustrate how to conduct various analyses that are the focus of this paper with the freely-available PROCESS procedure available for SPSS, SAS, and R, using data from an experimental investigation on the effectiveness of personal or testimonial narrative messages in improving intergroup attitudes.

Regression-based statistical mediation and moderation analysis in clinical research: Observations, recommendations, and implementation.

There have been numerous treatments in the clinical research literature about various design, analysis, and interpretation considerations when testing hypotheses about mechanisms and contingencies of effects, popularly known as mediation and moderation analysis. In this paper we address the practice of mediation and moderation analysis using linear regression in the pages of Behaviour Research and Therapy and offer some observations and recommendations, debunk some popular myths, describe some new advances, and provide an example of mediation, moderation, and their integration as conditional process analysis using the PROCESS macro for SPSS and SAS. Our goal is to nudge clinical researchers away from historically significant but increasingly old school approaches toward modifications, revisions, and extensions that characterize more modern thinking about the analysis of the mechanisms and contingencies of effects.

Two-condition within-participant statistical mediation analysis: A path-analytic framework.

Researchers interested in testing mediation often use designs where participants are measured on a dependent variable Y and a mediator M in both of 2 different circumstances. The dominant approach to assessing mediation in such a design, proposed by Judd, Kenny, and McClelland (2001), relies on a series of hypothesis tests about components of the mediation model and is not based on an estimate of or formal inference about the indirect effect. In this article we recast Judd et al.

An Index and Test of Linear Moderated Mediation.

I describe a test of linear moderated mediation in path analysis based on an interval estimate of the parameter of a function linking the indirect effect to values of a moderator-a parameter that I call the index of moderated mediation. This test can be used for models that integrate moderation and mediation in which the relationship between the indirect effect and the moderator is estimated as linear, including many of the models described by Edwards and Lambert ( 2007 ) and Preacher, Rucker, and Hayes ( 2007 ) as well as extensions of these models to processes involving multiple mediators operating in parallel or in serial. Generalization of the method to latent variable models is straightforward.

Injury news coverage, relative concern, and support for alcohol-control policies: an impersonal impact explanation.

Research on the impersonal impact hypothesis suggests that news (especially print) coverage of health and safety risks primarily influences perceptions of risk as a societal issue, and not perceptions of personal risk. The authors propose that the impersonal impact of news-impact primarily on concerns about social-level risks-will mediate effects of news stories on support for public health policies; such effects substantively matter as evidence suggests health policies, in turn, have important effects on protective behaviors and health outcomes. In an experiment using 60 randomly selected violent crime and accident news stories manipulated to contain or not contain reference to alcohol use as a causative factor, the authors find that the effect of stories that mention alcohol as a causative factor on support for alcohol-control policies is mediated by social-level concern and not by personal-level concern.

Statistical mediation analysis with a multicategorical independent variable.

Virtually all discussions and applications of statistical mediation analysis have been based on the condition that the independent variable is dichotomous or continuous, even though investigators frequently are interested in testing mediation hypotheses involving a multicategorical independent variable (such as two or more experimental conditions relative to a control group). We provide a tutorial illustrating an approach to estimation of and inference about direct, indirect, and total effects in statistical mediation analysis with a multicategorical independent variable. The approach is mathematically equivalent to analysis of (co)variance and reproduces the observed and adjusted group means while also generating effects having simple interpretations.

The relative trustworthiness of inferential tests of the indirect effect in statistical mediation analysis: does method really matter?

A content analysis of 2 years of Psychological Science articles reveals inconsistencies in how researchers make inferences about indirect effects when conducting a statistical mediation analysis. In this study, we examined the frequency with which popularly used tests disagree, whether the method an investigator uses makes a difference in the conclusion he or she will reach, and whether there is a most trustworthy test that can be recommended to balance practical and performance considerations. We found that tests agree much more frequently than they disagree, but disagreements are more common when an indirect effect exists than when it does not.

Self-efficacy mediates the impact of craving on smoking abstinence in low to moderately anxious patients: results of a moderated mediation approach.

Little is known about the effect of craving on smoking abstinence among cardiac patients who smoked prior to admission and the mechanisms that might facilitate success in smoking cessation after discharge from hospital. This study examined the mediating effect of self-efficacy on the relationship between craving and smoking abstinence and how this mechanism may be contingent on emotional state at the time of hospital admission. Cardiac patients who smoked prior to admission were recruited from cardiac nursing units in Dutch hospitals.

Increasing support for alcohol-control enforcement through news coverage of alcohol's role in injuries and crime.

Objective: Prior research has shown that the proportion of news stories about violent crimes, car crashes, and other unintended injuries that mention the possible contributing role of alcohol is far lower than the actual proportion of alcohol-related crimes and unintended injuries. An experiment was conducted to test the hypothesis that inclusion of such mention can increase concern about alcohol risks and support for alcohol-control measures, which have elsewhere been shown to decrease alcohol-related problems in community settings. Methodologically, we provide a model for experiments permitting generalization across randomly selected message stimuli.

The Influence of Youth Music Television Viewership on Changes in Cigarette Use and Association with Smoking Peers: A Social Identity, Reinforcing Spirals Perspective.

Prior research has found strong evidence of a prospective association between R movie exposure and teen smoking. Using parallel process latent-growth modeling, the present study examines prospective associations between viewing of music video channels on television (e.g.

Quantifying and Testing Indirect Effects in Simple Mediation Models When the Constituent Paths Are Nonlinear.

Most treatments of indirect effects and mediation in the statistical methods literature and the corresponding methods used by behavioral scientists have assumed linear relationships between variables in the causal system. Here we describe and extend a method first introduced by Stolzenberg (1980) for estimating indirect effects in models of mediators and outcomes that are nonlinear functions but linear in their parameters. We introduce the concept of the instantaneous indirect effect of X on Y through M and illustrate its computation and describe a bootstrapping procedure for inference.

Newspaper Coverage of Cancer Prevention: Multilevel Evidence for Knowledge Gap Effects.

Prior research on knowledge gap effects, in health as well as in other domains, has focused largely on assessing individual-level differences in exposure to news based on self-report of media use. Inherent inferential limitations of this approach are addressed by testing the hypothesis that the relationship between education and cancer prevention knowledge will be moderated by regional differences in U.S.

Computational procedures for probing interactions in OLS and logistic regression: SPSS and SAS implementations.

Researchers often hypothesize moderated effects, in which the effect of an independent variable on an outcome variable depends on the value of a moderator variable. Such an effect reveals itself statistically as an interaction between the independent and moderator variables in a model of the outcome variable. When an interaction is found, it is important to probe the interaction, for theories and hypotheses often predict not just interaction but a specific pattern of effects of the focal independent variable as a function of the moderator.

Asymptotic and resampling strategies for assessing and comparing indirect effects in multiple mediator models.

Hypotheses involving mediation are common in the behavioral sciences. Mediation exists when a predictor affects a dependent variable indirectly through at least one intervening variable, or mediator. Methods to assess mediation involving multiple simultaneous mediators have received little attention in the methodological literature despite a clear need.

Using heteroskedasticity-consistent standard error estimators in OLS regression: an introduction and software implementation.

Homoskedasticity is an important assumption in ordinary least squares (OLS) regression. Although the estimator of the regression parameters in OLS regression is unbiased when the homoskedasticity assumption is violated, the estimator of the covariance matrix of the parameter estimates can be biased and inconsistent under heteroskedasticity, which can produce significance tests and confidence intervals that can be liberal or conservative. After a brief description of heteroskedasticity and its effects on inference in OLS regression, we discuss a family of heteroskedasticity-consistent standard error estimators for OLS regression and argue investigators should routinely use one of these estimators when conducting hypothesis tests using OLS regression.

Further evaluating the conditional decision rule for comparing two independent means.

Many books on statistical methods advocate a 'conditional decision rule' when comparing two independent group means. This rule states that the decision as to whether to use a 'pooled variance' test that assumes equality of variance or a 'separate variance' Welch t test that does not should be based on the outcome of a variance equality test. In this paper, we empirically examine the Type I error rate of the conditional decision rule using four variance equality tests and compare this error rate to the unconditional use of either of the t tests (i.

Addressing Moderated Mediation Hypotheses: Theory, Methods, and Prescriptions.

This article provides researchers with a guide to properly construe and conduct analyses of conditional indirect effects, commonly known as moderated mediation effects. We disentangle conflicting definitions of moderated mediation and describe approaches for estimating and testing a variety of hypotheses involving conditional indirect effects. We introduce standard errors for hypothesis testing and construction of confidence intervals in large samples but advocate that researchers use bootstrapping whenever possible.

Longitudinal analysis of the relationship between symptoms and quality of life in veterans treated for posttraumatic stress disorder.

This study examined how change in posttraumatic stress disorder (PTSD) symptoms relates to change in quality of life. The sample consisted of 325 male Vietnam veterans with chronic PTSD who participated in a randomized trial of group psychotherapy. Latent growth modeling was used to test for synchronous effects of PTSD symptom change on psychosocial and physical health-related quality of life within the same time period and lagged effects of initial PTSD symptom change on later change in quality of life.

SPSS and SAS procedures for estimating indirect effects in simple mediation models.

Researchers often conduct mediation analysis in order to indirectly assess the effect of a proposed cause on some outcome through a proposed mediator. The utility of mediation analysis stems from its ability to go beyond the merely descriptive to a more functional understanding of the relationships among variables. A necessary component of mediation is a statistically and practically significant indirect effect.

Believing is seeing? Investigating the perceived accuracy of criminal psychological profiles.

This study investigated whether perceptions of criminal psychological profiles are influenced by the identity of the profile's author. Police officers were given a profile they were told was written by either a professional profiler or by an unspecified author. When judged in relation to the actual perpetrator of the crime, police officers tended to perceive greater accuracy in a profile when it was labeled as authored by a professional profiler independent of the actual content of the profile.