216 research outputs found
Predicting the Number of Future Events
No full textThis paper describes prediction methods for the number of future events from a population of units associated with an on-going time-to-event process. Examples include the prediction of warranty returns and the prediction of the number of future product failures that could cause serious threats to property or life. Important decisions such as whether a product recall should be mandated are often based on such predictions. Data, generally right-censored (and sometimes left truncated and right-censored), are used to estimate the parameters of a time-to-event distribution. This distribution can then be used to predict the number of events over future periods of time. Such predictions are sometimes called within-sample predictions and differ from other prediction problems considered in most of the prediction literature. This paper shows that the plug-in (also known as estimative or naive) prediction method is not asymptotically correct (i.e., for large amounts of data, the coverage probability always fails to converge to the nominal confidence level). However, a commonly used prediction calibration method is shown to be asymptotically correct for within-sample predictions, and two alternative predictive-distributionbased methods that perform better than the calibration method are presented and justified.This is a manuscript of an article published as Tian, Qinglong, Fanqi Meng, Daniel J. Nordman, and William Q. Meeker. "Predicting the number of future events." Journal of the American Statistical Association (2021). doi:10.1080/01621459.2020.1850461. Posted with permission.</p
Information Asymmetry and Venture Capital Investment Decisions
No full text© 2024 Fanqi MengThis investigation embarks on a nuanced analysis of information asymmetry’s role in the decision-making processes of venture capital (VC) investment, emphasising the significant yet underexplored influence of geographically structured social connections on mitigating information asymmetry between venture capitalists and entrepreneurs. Venture capitalists are pivotal in financing and overseeing the development of early-stage companies and wield significant asymmetric information. Despite the critical role of venture capitalists, the ways in which this information is leveraged by social connections throughout various stages of the VC investment process have not been extensively examined. This thesis aims to address two fundamental questions concerning the effect of asymmetric information on VC decision-making, using novel data sources. First, it investigates whether the possession of superior asymmetric information facilitates VC deal formation, particularly during the matching and screening processes. Second, this thesis explores whether the presence of asymmetric information influences post-investment management behaviours, specifically the staging process. Each of these questions will be examined in separate chapters, providing comprehensive insights into the role of information asymmetry in VC decision-making.
Chapter 3 investigates the effect of social connectedness on VC investments and entrepreneurial performance using innovative data. Our findings reveal that VC investors exhibit a higher propensity to invest in start-ups located in regions where they have strong social connections, even when these regions are geographically distant. The results hold in both the US private market and the global private market. Additionally, we observe that VC–entrepreneur matches characterised by stronger social ties positively influence subsequent performance, benefiting both VC investors and entrepreneurial firms. Furthermore, we attribute this positive correlation to the reduction in agency costs and the ability to select higher-quality entrepreneurial firms during the pitching and screening process. Our research underscores the pivotal role of social networks in shaping VC–entrepreneur interactions and post-investment outcomes, providing valuable insights into VC decision-making and performance.
Chapter 4 empirically investigates the determinants of staged financing in VC investments and assesses the influence of staged financing influence on post-investment performance. By leveraging innovative proxies to measure outside opportunities and employing state-level non-compete agreements as a quasi-exogenous shock on outside opportunities, this research establishes entrepreneurs’ outside opportunities as a key determinant of VC staged financing, providing the first empirical study in support of the hold-up hypothesis in VC investment. The findings reveal that higher outside opportunities are linked to increased staging, characterised by smaller investments per round and shorter round durations in the US VC market. Furthermore, this research reconciles conflicting empirical findings in the literature by highlighting a positive correlation between the number of financing rounds and entrepreneurial success when entrepreneurs face greater external opportunities. Last, this study enriches the VC and social finance literature by shedding light on the role of social connections in private market investments
Saturable absorption of femtosecond optical pulses in multilayer turbostratic graphene
No full textWe investigate the nonlinear transmission of a ∼280-layer turbostratic graphene sheet for near-infrared amplifier laser pulses (775 nm, Ti:sapphire laser) with a duration of 150-fs and 20-fs. Saturable absorption is observed in both cases, however it is not very strong, amounting to ∼13% transmittance change for the 20-fs (150-fs) pulses at a peak intensity of 30 GW/cm2 (4 GW/cm2). The dependence on incident peak intensity is reproduced well using a theoretical model for the time-dependent saturable absorption, where the excited carriers vacate the photo-excited energy range within 3-5 fs, which we attribute to energy redistribution due to carrier-carrier scattering. This is also supported by spectrally resolved measurements for the 20-fs pulses, which show a marked dependence of the degree of saturation on the photon energy. A key result is that the shorter pulses do not yield a lower saturation fluence, due to the combined effects of the broader excitation bandwidth, and the rapid and broad energy redistribution. We also predict the potential performance of multilayer graphene samples for removing pedestal and pre-pulse structure from ultrafast high-energy pulses
Oscillations de Bloch et échelle de Wannier Stark dans des superréseaux semiconducteurs
No full textTerahertz (THz) electromagnetic field, which lies in the frequency gap between the infrared and microwave, roughly between 1 THz to 10 THz, is highly desirable for both fundamental research and application. Yet tuneable compact THz sources are still not available. On the other hand, ever since first proposed in 1970, semiconductor superlattice provides new playground for various new technique and devices of tremendous research and application interest. In this thesis, an innovative theme, relying on Bloch oscillations in a dc biased semiconductor superlattice, is explored to realize tunable compact THz source THz Bloch oscillator. For doped superlattice Bloch oscillator, we designed quantum cascade super-superlattice structure to realize Bloch oscillations whilst prohibit electrical domain formation. The designed structures were processed into various waveguide and grating devices for electroluminescence detection using Fourier transform infrared spectroscopy (FTIR). The Bloch gain of semi-insulating surface plasmon waveguide device was also measured using THz time domain spectroscopy. Even though the electroluminescence and gain at THz regime were observed, no direct evidence of Bloch emission was confirmed. For undoped superlattice, the THz emission from Bloch oscillations was observed by time domain spectroscopy. At last, the photocurrent corresponding to heavy hole and Wannier Stark Ladder (WSL) states transitions in undoped superlattice was studied. Under CW laser pumping, the photocurrent as function of the applied voltage showed multiple WSL peaks, which indicated laser induced and controllable negative differential conductance (NDC). With increasing pumping power, the nonlinear NDC regime and bistable states were investigated as well.Le champ électromagnétique térahertz (THz) se situe dans l'intervalle de fréquence entre l'infrarouge et les micro-ondes, à peu près entre 1 THz à 10 THz. Ce domaine est hautement souhaitable tant pour la recherche fondamentale que pour les applications. Pourtant des sources THz compacts et accordables ne sont pas encore disponibles. Depuis la première proposition en 1970, les superréseaux semiconducteurs, dans lequel deux couches semi-conductrices atomiques avec bande interdite différente sont disposés périodiquement, fournissent de nouvelles possibilités. De nouvelles techniques et de nouveaux dispositifs deviennent réalisables. Dans cette thèse, les oscillations de Bloch dans des mini-bandes électroniques d’un superréseau polarise et la dispersion du gain associée sont utilisées pour réaliser une source THz compacte et accordable : l’oscillateur de Bloch THz. Un premier ensemble de dispositifs utilisent des réseaux dopes spécifiquement conçus pour éviter la formation de domaine d’accumulation de charges. Ces dispositifs utilisent une surface semi-isolante ou deux surfaces métalliques permettant un guidage par plasmon de surface. Cependant, malgré la réalisation de couplage par les bords ou par un réseau diffractant en surface et des mesures directes ou avec un interféromètre a transformation de Fourrier (FTIR), l’électroluminescence a été observée dans le domaine térahertz, avec un gain qui n’a pas pu etre relie aux oscillations de Bloch. Avec des superréseaux non dope, l'émission THz des oscillations de Bloch a été détectée par spectroscopie dans le domaine temporel. La dépendance de la fréquence d’émission avec le champ électrique appliqué constitue une preuve directe des oscillations de Bloch. L’échelle de Wannier Stark des trous sous pompage optique continu a aussi été observe dans les superréseaux non dopes. Avec l’augmentation de la puissance de pompage optique, les pics du photocourant se décalent et leurs formes deviennent asymétriques. L’évolution est attribue a l’accumulation des porteurs photogénérés dans les deux couches encadrant le superréseau. En outre, pour une puissance de pompage élevée, la bistabilité du photocourant a été également observée
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Bayesian Inference on the Stiefel Manifold: Models, Applications and Algorithms
No full textIn finance, it is crucial to use recent data to model the relationship between the companies since the market environment is evolving constantly. In particular, estimating time-varying covariance matrices has been an important topic for both portfolio optimization and risk management. Market measures such as betas for companies, beta dispersion, and market volatility are also closely related to the eigenvectors and eigenvalues of the covariance matrices. The current approaches for dynamic covariance estimation are focused on vector autoregressive processes and have shared parameters for the eigenvalues and eigenvectors. This inevitably introduces dependencies and fails to reveal the relationships between the model parameters. We contribute to the field of time- varying covariance estimation by proposing a Bayesian autoregressive model on the Stiefel manifold for high dimensional data. Our model considers the eigenvalues and eigenvectors separately, and provides a reliable solution to the relationships between the eigenvalues and eigenvectors. To our knowledge, this is the first attempt for an autoregressive time series model on the Stiefel manifold, and it can be extended to a class of models that are widely applicable to datasets in finance, biology, climate changes, etc.Our Bayesian model involves sampling and inference on the Stiefel manifold, which has been a challenging task. We contribute to Bayesian modeling on the Stiefel manifold by writing a new package using the Stan program. In our StanStiefel package, we extend the sampling method in [1], and propose novel parameter inference methods for popular distributions. Our package takes much less time to generate comparable amount of effective samples than the rstiefel package, especially in high dimensions
“It Belongs to Us”: Chinese Youth’s Imagined Futures
No full textThe imagined future of young people has become a significant area of research in sociology and youth studies, providing insights into youth identities, agency, and autonomy. In western academia, researchers have adopted a dual approach to exploring imagined futures, distinguishing between short-term, personal futures and long-term, societal futures (Cook, 2018; Leahy et al., 2010). This study strives to evaluate the applicability of the western frameworks in a distinctive social context. By examining the future imaginaries of 86 middle-class young people (aged 14-15) in three schools situated in Guangzhou, China, this study posits that there are four categories of future that these adolescents envision: (1) the temporary future; (2) the personal future; (3) the national future; and (4) the societal future. Based on a Bourdieusian analysis, these categories of future imaginaries can be attributed to the hybridity and complexity of Chinese youth’s habitus, field, and capital. Dominated by both western ideologies such as neoliberalism and individualism as well as indigenous philosophies like Confucianism and collectivism, these young people’s imagined futures exhibit diverse constructs, influences, and representations in contrast to those of their western counterparts, contingent on their unique living experiences and socio-cultural circumstances. This research deconstructs the entangled everyday experiences of Chinese youth, providing valuable data to decipher the factors driving the distinctiveness of their imagined futures. Furthermore, this research examines the applicability of western theories in a distinctive social setting, providing perspectives and tools for researchers to study Chinese youth. The practical implications which arise from the methodology of this thesis, centre around a revised adaptation of Pierre Bourdieu’s thinking tools, transforming young people’s imagined futures into processes of social practice rather than simple actions, thus manifesting Bourdieu’s advancement in social research
Research on the development strategy of Higher Education under the digital economy
No full textWith the rapid development of information technology and the rise of the digital economy, higher education is facing unprecedented challenges and opportunities. This article aims to explore the development strategies of higher education in the context of the digital economy, analyze the impact of digital technology on higher education, and discuss how to utilize digital technology to reform the higher education system and cultivate talents that are adaptable to the development of the digital economy. This research holds significant theoretical and practical importance, providing valuable insights for reforming higher education systems and talent cultivation. Keywords: higher education, digital economy, development strategy DOI: 10.7176/JEP/16-3-20 Publication date: March 30th 202
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