Assignment 6: Mediation example
Please identify a quantitative research article evaluating mediation in your field and provide the citation.
Roman Shrestha, Pramila Karki, Tania B. Huedo-Medina, Michael Copenhaver, 2017 “Intent to Use Pre-exposure Prophylaxis (PrEP), HIV Risk Behaviors, and Self-Report Neurocognitive Symptoms by High-Risk Drug Users: A Mediation Analysis” Journal of the Association of Nurses in AIDS Care Available from https://ucsf.idm.oclc.org/login?url=https://doi.org/10.1016/j.jana.2017.04.005
What is the primary discipline of the authors?
Roman Shrestha, MPH, is a doctoral student, Department of Community Medicine & Health Care, University of Connecticut Health Center, Farmington, Connecticut
Pramila Karki, RN, is a graduate student, Department of Allied Health Sciences, University of Connecticut, Storrs, Connecticut
Tania B. Huedo-Medina, Ph.D., is an Assistant Professor, Department of Allied Health Sciences, University of Connecticut, Storrs, Connecticut
Michael Copenhaver, Ph.D., is an Associate Professor, Department of Allied Health Sciences, University of Connecticut, Storrs, Connecticut
Draw a DAG representing the implicit or explicit causal model explored in this paper (you do not need to post your DAG, but we will try to discuss in class).
What is the exposure of interest? Neurocognitive impairment
What is the outcome of interest? Intent to use pre-exposure prophylaxis
What is the hypothesized mediator of interest and how is it measured?
Mediator: HIV risk behavior.
Measurement: HIV risk behavior was assessed using a validated HIV Risk-taking Behavior Scale (HRBS). The HRBS is a brief, 11-item questionnaire developed to measure both drug- and sex-related behaviors that put individuals at risk of either contracting or transmitting HIV. The overall HRBS score is obtained by adding responses to all items on a scale (range = 0-55), with higher scores representing higher levels of HIV risk behavior
Participants are asked to self-report drug- and sex-related HIV risk behaviors during the past 30 days.
Describe the modeling approach and briefly report the estimated total, direct, and indirect effects (if these are reported).
The authors use the 4-step approach of mediation analysis proposed by Baron and Kenny. These four steps include
1. Show that the exposure variable is correlated with the outcome – this step established that there is an effect that may be mediated (total effect B = 0.974)
2. Show that the exposure variable is correlated with the mediator – this step treats the mediator as an outcome variable (exposure-mediator B =0.706
3. Show that the mediator affects the outcome variable (mediator-outcome B = 0.194
4. Show the effect of exposure variable on outcome when controlling for the mediator (direct effect B = 0.538)
They also use the Sobel test and bootstrap method to estimate the indirect effect. B = 0.075
Lastly, the control for covariates, which include age, gender, sexual orientation, ethnicity, marital status, education, income and employment
If the direct effect is reported, would you describe this as a natural direct effect, a controlled direct effect, or something else?
The direct effect denoted as c1 is the controlled direct effect
Do you think there is potential measurement error in the mediator and how would that affect the results?
The mediator is measured via self-report on a standardized scale. I think measurement error in the mediator could occur if there is differential reporting among those with neuro-cognitive impairment. It is possible that those with neuro-cognitive impairment misreport their risky sexual behavior due to memory impairment as opposed to those who are more functional.
This error is likely to result in the effect of the mediator on the outcome being underestimated, and the effect of the exposure on the outcome (direct effect) is likely over-estimated (if the product of the coefficients of exposure-mediator and mediator-outcome is positive -which is typical). The over-estimation of the direct effect is exacerbated to the extent by which the exposure-mediator Standard error is large.
Do you think there are unmeasured confounders of the mediator-outcome association and how would that affect the results of the mediation analysis? Per my DAG, the covariate marital status confounds the mediator-outcome association and not the exposure variable and outcome association.
Do you have any critiques of the paper?
My main issue with this paper is that given the numerous similar covariates confounding exposure-outcome, mediator-outcome, and exposure-mediator pathways, this paper would have benefited from a sensitivity analysis which would have allowed them to assess how robust their direct and indirect effect estimates are to violations in the four assumptions of mediation (there is also no mention of whether the data meets the assumptions of the OLS)
I also think that the authors should have controlled for the exposure variable in establishing the effect of the mediator on the outcome. This is because it is not enough just to correlate the mediator with the outcome because the mediator and the outcome may be correlated because they are both caused by the exposure variable.
One of the key assumptions for mediation analysis that there should be no mediator-outcome confounder that is itself affected by the exposure. Although not depicted in the DAG. – It is easy to see/imagine how neurocognitive impairment (exposure) itself affects confounders of the mediation-outcome path such as education, employment status and income.