Theses and Dissertations from UMD

Permanent URI for this communityhttp://hdl.handle.net/1903/2

New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM

More information is available at Theses and Dissertations at University of Maryland Libraries.

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Author: search.filters.author.Li, Ying

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    Decentralized Transportation Model In Vehicle Sharing
    (2023) Li, Ying; Ryzhov, Ilya; Applied Mathematics and Scientific Computation; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    This dissertation introduces the concept of decentralization to address the rebalancing challenges in bike-sharing systems and proposes a model known as the Decentralized Route Assignment Problem (DRAP) under specific assumptions. The primary contributions of this research include the formulation of the DRAP and the derivation of theoretical results that facilitate its transformation into a lower-dimensional global optimization problem. This transformation enables efficient exploration using modern search methods. An extended version of DRAP, called DRAP-EA, is also proposed for further analysis by introducing more agents into the system. Various solution approaches, such as branch-and-cut, hill-climbing, and simulated annealing, are explored and customized to enhance their performance in the context of rebalancing. Two simulated annealing methods, Gurobi with warm-start, and an extension of the local search algorithm are implemented on 24 instances derived from a comprehensive case study for experimental evaluation. The experimental results consistently demonstrate the superior performance of the simulated annealing methods. Furthermore, a comparison between SA and SA-PS is conducted, and the obtained solutions are visualized to help further explore the spatial patterns and traffic flows within the bike-sharing system.
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    Exploring the Full-information Bifactor Model in Vertical Scaling with Construct Shift
    (2011) Li, Ying; Lissitz, Robert W.; Measurement, Statistics and Evaluation; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    To address the lack of attention to construct shift in IRT vertical scaling, a bifactor model is proposed to estimate the common dimension for all grades and the grade-specific dimensions. The bifactor model estimation accuracy is evaluated through a simulation study with manipulated factors of percent of common items, sample size, and degree of construct shift. In addition, the unidimensional IRT (UIRT) estimation model that ignores construct shift is examined to represent the current practice for IRT vertical scaling; comparisons on parameter estimation accuracy of the bifactor and UIRT models are discussed. The major findings of the simulation study are (1) bifactor models are well recovered overall, even though item discrimination parameters are underestimated to a small degree; (2) item discrimination parameter estimates are overestimated in UIRT models due to the effect of construct shift; (3) person parameters of UIRT models are less accurately estimated than that of bifactor models, and the accuracy decreases as the degree of construct shift increases; (4) group mean parameter estimates of UIRT models are less accurate than that of bifactor models, and a large effect due to construct shift is found for the group mean parameter estimates of UIRT models. The real data analysis provides an illustration of how bifactor models can be applied to a problem involving for vertical scaling with construct shift. General procedures for testing practice are also discussed.