Markov multi-state models for survival analysis with recurrent events

Markov models are a major class within the system of multi-state models for the analysis of lifetime or event-time data. Applications abound, including the estimation of lifetime of ultra-cold neutrons, the bias correction of the apparent magnitude distribution of the stars in a certain area of the sky, and the survival analysis of clinical trials. This thesis addresses some of the problems arising in the analysis of right-censored lifetime data. Clinical trials are used as examples to investigate these problems. A Markov model that takes a patient's disease development into account for the analysis of right-censored data was first constructed by Fix and Neyman (1951). The Fix-Neyman (F-N) model is a homogeneous Markov process with two transient and two absorbing states that describes a patient's status over a period of time during a cancer clinical trial. This thesis extends the F-N model by assuming the transition rates (hazard rates) to be both state and time dependent. Recurrent transitions between relapse and recovery are allowed in the extended model. By relaxing the condition of time-independent hazard rates, the extension increases the applicability of the Markov models. The extended models are used to compute the model survival functions, cumulative hazard functions that take into consideration of right censored observations as it has been done in the celebrated Kaplan-Meier estimator. Using the Fix-Neyman procedure and the Kolmogorov forward equations, closed-form solutions are obtained for certain irreversible 4-state extended models while numerical solutions are obtained for the model with recurrent events. The 4-state model is motivated by an Aplastic Anemia data set. The computational method works for general irreversible and reversible models with no restriction on the number of states. Simulations of right-censored Markov processes are performed by using a sequence of competing risks models. Simulated data are used for checking the performance of nonparametric estimators for various sample sizes. In addition, applying Aalen's (1978) results, estimators are shown have asymptotic normal distributions. A brief review of some of the literature relevant to this thesis is provided. References are readily available from a vast literature on the survival analysis including many text books. General Markov process models for survival analysis are described, e.g., in Andersen, Borgan, Gill and Keiding (1993)

Mechanism of colloidal sphere self-assembly

Ph.DDOCTOR OF PHILOSOPH

A Hybrid Secure Scheme for Wireless Sensor Networks against Timing Attacks Using Continuous-Time Markov Chain and Queueing Model

Wireless sensor networks (WSNs) have recently gained popularity for a wide spectrum of applications. Monitoring tasks can be performed in various environments. This may be beneficial in many scenarios, but it certainly exhibits new challenges in terms of security due to increased data transmission over the wireless channel with potentially unknown threats. Among possible security issues are timing attacks, which are not prevented by traditional cryptographic security. Moreover, the limited energy and memory resources prohibit the use of complex security mechanisms in such systems. Therefore, balancing between security and the associated energy consumption becomes a crucial challenge. This paper proposes a secure scheme for WSNs while maintaining the requirement of the security-performance tradeoff. In order to proceed to a quantitative treatment of this problem, a hybrid continuous-time Markov chain (CTMC) and queueing model are put forward, and the tradeoff analysis of the security and performance attributes is carried out. By extending and transforming this model, the mean time to security attributes failure is evaluated. Through tradeoff analysis, we show that our scheme can enhance the security of WSNs, and the optimal rekeying rate of the performance and security tradeoff can be obtained. View Full-Tex

Winning over Grassroots Consumers: An Empowerment Perspective of Yu’E Bao

Recent years have witnessed the great potential of technological innovations in helping developing economies tackle financial exclusion, a key obstacle in reducing poverty and accelerating economic growth. Yet, the way that technology innovations should be designed and utilized to deliver inclusive finance has remained obscure. In this study, we undertook an in-depth qualitative case study of Yu’E Bao, an online market fund, which has revolutionized the online finance sector in China. The incredible feat of Yu’E Bao has made financial institutions in China start focusing on potential cumulative wealth of the large population of financially underserved “grassroots consumers”. Drawing insights from empowerment theory, we explicate the mechanisms through which an IT-enabled innovation can successfully engage and leverage the financially underprivileged population to progress towards financial inclusion. Our findings underline important theoretical, economic and societal contributions, viz. exploitation of empowerment mechanisms, acknowledgment of grassroots’ economic potential, and IT-enabled financial inclusion

Empowerment of Grassroots Consumers: A Revelatory Case of a Chinese Fintech Innovation

The recent emergence of financial technology (fintech) innovations offers a promising resolution to financial exclusion via mechanisms that empower financially underprivileged individuals to gain access rights in the traditional financial industry. However, academic research has provided little guidance on how to strategize the IT-enabled empowerment mechanisms for fintech innovations to realize both business success and financial inclusion. In this study, we conduct an in-depth revelatory case study on a novel Chinese fintech innovation, Yu’E Bao, a bellwether for the dramatic transformation of China’s financial industry through the successful empowerment of a large population of financially underprivileged individuals (i.e., “grassroots consumers,” translated from “cao gen” in Chinese). Through our systematic qualitative analysis of news articles collected since the product launch, we derive a two-stage theoretical model, examined through the lens of empowerment, and unravel the mechanisms underlying fintech innovation’s empowerment process on grassroots investment consumers. Most importantly, we elucidate a duality of empowerment through which the catalyst for Yu’E Bao’s success becomes an impediment to Yu’E Bao’s further development. Our study contributes to the literature on information technology-enabled empowerment, empowerment, and fintech. We also elucidate critical implications for various stakeholders, such as governments, policy makers, fintech practitioners, and product designers

Effects of narrow gap wave resonance on a dual-floater WEC-breakwater hybrid system

The effects of gap wave resonance on the performance of a dual-floater hybrid system consisting of an oscillating-buoy type wave energy converter (WEC) and a floating breakwater are important for the design of such a hybrid system. This paper investigates the gap wave resonance by employing a two-dimensional numerical wave flume developed using the Star-CCM + software. The maximum wave elevation in the WEC-breakwater gap and the effects of the gap wave resonance on the performance of the dual-floater hybrid system were studied. The influence of the WEC motion and the geometrical parameters of the hybrid system on the maximum wave elevation were analyzed. The maximum gap wave elevation is essentially controlled by the vertical velocity of the free surface in the WEC-breakwater gap. The gap wave resonance was found to significantly improve the wave energy extraction performance of the hybrid system. This allowed the maximum conversion efficiency to exceed the well-known limit of 0.50 for a symmetric body in single degree-of-freedom motion. The wave resonance frequencies in the WEC-breakwater gap decreased with the increase of the gap width and the WEC draft. Due to the energy extraction of the WEC, the horizontal and vertical forces on the breakwater were reduced by up to 0.79 and 0.59, respectively

Development of a penetration friction apparatus (PFA) to measure the frictional performance of surgical suture

Nowadays there is a wide variety of surgical sutures available in the market. Surgical sutures have different sizes, structures, materials and coatings, whereas they are being used for various surgeries. The frictional performances of surgical sutures have been found to play a vital role in their functionality. The high friction force of surgical sutures in the suturing process may cause inflammation and pain to the person, leading to a longer recovery time, and the second trauma of soft or fragile tissue. Thus, the investigation into the frictional performance of surgical suture is essential. Despite the unquestionable fact, little is actually known on the friction performances of surgical suture-tissue due to the lack of appropriate test equipment. This study presents a new penetration friction apparatus (PFA) that allowed for the evaluation of the friction performances of various surgical needles and sutures during the suturing process, under different contact conditions. It considered the deformation of tissue and can realize the puncture force measurements of surgical needles as well as the friction force of surgical sutures. The developed PFA could accurately evaluate and understand the frictional behaviour of surgical suture-tissue in the simulating clinical conditions. The forces measured by the PFA showed the same trend as that reported in literatures