Varying-Coefficient Models for Lagged Association and Mediation in Longitudinal Studies
Open AccessLongitudinal studies, in which study subjects are observed over time, allow researcher to study the changes in the outcome of interest and make inference on the association between exposure and outcome. When looking at the relationship between exposure and outcome, especially over time, two important issues need to be considered. First, due to the often delayed onset of outcome manifestation relative to exposure, the effect of exposure on outcome may be lagged in time. That is, the current outcome values may be more associated to exposure values at some previous time points, rather then the current ones. Second, the effect of exposure on outcome may be mediated by some intermediate agents, i.e., mediators. As time progresses, the direct effect and the mediated/indirect effect of exposure may be changing over time, with potential lag in both the pathway from exposure to mediator and that from mediator to outcome.The analysis of longitudinal data has many theoretical challenges, primarily due to the correlation between observations from one subject. Varying-coefficient model (VCM), by incorporating the interpretability of parametric models and flexibility of nonparametric models, is a natural tool to handle longitudinal data. However, the application of VCM in the analysis of lagged association and mediation has been limited. Hence, in this thesis, we propose to extend VCM to 1) incorporate lag in the coefficient functions, for direct assessment on the effect of exposure over time and over lag, and 2) model the mediation process, which will allow for the evaluation of direct and indirect effect of exposure on outcome over time. In our study, the proposed routines for lagged association evaluation have been shown to be effective in recovering the true time lag that characterize the association between exposure and outcome, i.e., finding time where exposure values are most predictive of the current outcome. Moreover, in mediation analysis, the lagged association at each pathway of the mediation relationship can be evaluated, which is useful in examining not only the time-varying but also the delayed relationship in mediation.
- All rights reserved
Notice to Authors
If you are the author of this work and you have any questions about the information on this page, please use the Contact form to get in touch with us.