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Title
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A comparison of two mediation analysis methods with sequential mediators
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Creator
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Bennett, Kyle
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Date
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2019
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Collection
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Electronic Theses & Dissertations
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Description
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"Two methods for mediation analysis with sequential mediators were compared using multiple simulation scenarios. The performances of each method were assessed using three key metrics: relative bias, root mean square error, and coverage. The methods shared both similarities and key differences and some modification and adjustment were necessary to perform comparable simulations across the scenarios. Overall performance was assessed primarily using relative bias, where each simulated effect...
Show more"Two methods for mediation analysis with sequential mediators were compared using multiple simulation scenarios. The performances of each method were assessed using three key metrics: relative bias, root mean square error, and coverage. The methods shared both similarities and key differences and some modification and adjustment were necessary to perform comparable simulations across the scenarios. Overall performance was assessed primarily using relative bias, where each simulated effect estimate was compared to a "true" effect generated by simulating from a theoretical super population. Simulation scenarios included correctly specified models using both methods and various mis-specified estimation models by incorrectly specifying a critical parameter in the model to assess the performance and robustness of each mediation analysis method. The results of the simulations suggest that one method was particularly more resilient to mis-specification of the model over the other, and that proper specification of the marginal structural model is also critical to minimizing bias and maximizing coverage."--Page ii.
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Title
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Improvements in fine-scale estimation and evaluation of geographic variables using climate data in East Africa
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Creator
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Hession, Sarah L.
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Date
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2011
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Collection
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Electronic Theses & Dissertations
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Description
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Global environmental change has surfaced as a critical issue to both the scientific community and the general public. One aspect of particular concern involves climate change, which will exert impacts on ecosystems and economies, presenting considerable challenge to human adaptation. In Africa, a continent that is vulnerable due to multiple stressors and low adaptive capacity, climate change is expected to significantly affect both people and ecosystems. Adaptation strategies are being...
Show moreGlobal environmental change has surfaced as a critical issue to both the scientific community and the general public. One aspect of particular concern involves climate change, which will exert impacts on ecosystems and economies, presenting considerable challenge to human adaptation. In Africa, a continent that is vulnerable due to multiple stressors and low adaptive capacity, climate change is expected to significantly affect both people and ecosystems. Adaptation strategies are being developed using information from studies that evaluate the impacts of climate variability and climate change in Africa. Recommendations are made for local development of adaptation strategies due to the heterogeneity of climate change and its effects on East Africa's climate. However, global climate change models are coarse in scale and mask much of the local variation in regional climate, indicating the need for higher resolution climate data. This dissertation addresses this need by comparing spatially explicit statistical methods of interpolation and prediction, both theoretically and empirically; expanding upon the method of universal kriging by incorporating complex feedback relationships that may produce simultaneity between precipitation and its covariates; and evaluating precipitation patterns over space in East Africa through a case study. Mechanisms of precipitation have been considered in detail, expanding upon many other spatially explicit applications of prediction methods to date. Further, spatially explicit inferential regression models have been developed to better understand spatial patterns and variability in East African precipitation. Predicted maps of precipitation, generated at a resolution of 1 kilometer, accurately reflect the mesoscale influences of topography and the presence of large water bodies (i.e., Lake Victoria) as well as the seasonal influences of the passing of the intertropical convergence zone (ITCZ). In terms of prediction, the spatially explicit methods considered herein clearly outperformed a global data set (i.e., the CRU TS 3.1) in terms of error and ability to reflect local variability. The method of local ordinary kriging generally outperformed the multivariate kriging techniques, indicating that precipitation patterns in areas of high topographic variability, such as East Africa, may be modeled as well or better using local search neighborhoods in the kriging process rather than using complex multivariate regression models. However, additional work to improve the multivariate regression models and overall levels of correlation are expected to yield improved prediction results. Furthermore, the case study successfully demonstrated that the newly developed method of universal kriging with instrumental variables performs similarly to other standard methods of estimation, and perhaps better in the presence of significant measurable simultaneity.
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Title
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From synthesis to behavioral activity in streams : investigations of putative sea lamprey pheromone components
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Creator
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Brant, Cory Olaf
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Date
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2011
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Collection
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Electronic Theses & Dissertations
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Description
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The sea lamprey (Petromyzon marinus L.) has become a model species in the study of bile acid production and release into the environment where these compounds function as intraspecific chemical signals. Throughout the later stages of their life history, sea lampreys have been shown to rely upon pheromone communication to mediate reproduction. Laboratory and stream behavioral bioassays have implicated 3-keto petromyzonol sulfate (3kPZS) as a lamprey mating pheromone, but the full function of...
Show moreThe sea lamprey (Petromyzon marinus L.) has become a model species in the study of bile acid production and release into the environment where these compounds function as intraspecific chemical signals. Throughout the later stages of their life history, sea lampreys have been shown to rely upon pheromone communication to mediate reproduction. Laboratory and stream behavioral bioassays have implicated 3-keto petromyzonol sulfate (3kPZS) as a lamprey mating pheromone, but the full function of this bile alcohol derivative remains to be elucidated. Further, the biosynthesis, regulation, and release of 3kPZS and other putative components of the pheromone remain only partially characterized. In Chapter 1 of this thesis, I observed the behaviors of migratory females to the presence of 3kPZS in streams across a typical migratory season. In Chapter 2, the synthesis, transport, and release of several steroid-derived compounds in adult male sea lampreys were further examined using analytical chemistry and molecular biology-based approaches in adult males. The data presented here further characterize the male mating pheromone in sea lamprey, contribute to the understanding of pheromone communication in vertebrates, and provide implications for controlling the invasive species in the Laurentian Great Lakes.
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