Dynamic analysis of complex panel count data

Panel count data occur in many fields including clinical, demographical and industrial studies and an extensive literature has been established for their regression analysis. However, most of the existing methods apply only to the situations where both covariates and their effects are constant or one of them may be time-dependent. In the first part of this dissertation, we consider a situation where both covariates and their effects may be time-dependent and an estimating equation-based approach is developed for estimating those time-varying effects. In the method, B-spline functions are employed to approximate time-dependent coefficients and the asymptotic properties of the proposed estimators are established. To assess the performance of the proposed approach, an extensive simulation study is conducted and suggests that it works well in practical situations. An application to the China Health and Nutrition Survey (CHNS) study is provided. In practice, there could exist more than one type of event of interest, such as two types of tumor recurrence, leading to multivariate panel count data. The second part of this dissertation considers marginal mean model for multivariate panel count data with time-dependent coefficient and covariate effects, which has limited previous research. Based on the conditional estimating equation method developed for time-dependent covariates, we approximate the coefficients by B-splines, hence allow both coefficients and covariates to be time-dependent. Simulation studies show that the proposed estimation procedures work well for practical situations. The methodology is again applied to the China Health and Nutrition Survey (CHNS) study. When we consider time-varying covariates and coefficients effects, most of the previous study focused on the proportional mean model because the likelihood function under the rate model involves intractable integration. However, the rate model is more realistic and efficient. Hence, in the third part of this dissertation, we propose a semi-parametric MLE method under the rate model for panel count data with time-dependent covariates and time-varying effects. B-spline functions are employed again to approximate time-dependent coefficients and an efficient Expectation-Maximization-type algorithm is developed to overcome the computational difficulty. The resulting estimators are shown to be consistent and asymptotically efficient. Monte Carlo simulation studies demonstrate that the proposed method enjoys desirable finite-sample properties. An application to The Young Women's Project (YWP) is provided

Measuring CEO Talent and Its Impact on Firm Performance: A Theoretical Integration and Empirical Analysis

This paper analyzes whether CEOs who are good contrarian investors, good forecasters, or good market timers can run their firms better. Besides using the timing measure which combines returns before and after the trade, we also use the past return measure to estimate the contrarian aspect of CEO trades and the future return measure to assess CEO’s ability to forecast stock returns. Our results suggest that CEOs’ managerial talent and valuation ability are primarily related to CEOs’ past return measure, while high post-trade returns indicate the expropriation motive for CEO trades. Overall, we obtain strong evidence to support the idea that CEOs who are good contrarian investors tend to run their firms better on average than other CEOs

Multiplicity of positive solutions for a fourth-order quasilinear singular differential equation

This paper is concerned with the multiplicity of positive solutions of boundary value problem for the fourth-order quasilinear singular differential equation $$(|u''|^{p-2}u'')''=\lambda g(t)f(u),\quad 0<t<1,$$ where $p>1$, $\lambda>0$. We apply the fixed point index theory and the upper and lower solutions method to investigate the multiplicity of positive solutions. We have found a threshold $\lambda^*<+\infty$, such that if $0<\lambda\leq\lambda^*$, then the problem admits at least one positive solution; while if $\lambda \lambda^*$, then the problem has no positive solution. In particular, there exist at least two positive solutions for $0<\lambda<\lambda^*$

The Impact of a New App Channel on Physicians’ Performance: Evidence From Online Healthcare Natural Experiment

Besides the web browser, the introduction of the mobile app in online healthcare systems has resulted in an additional touchpoint for users. Drawing on the Media Richness theory, we aim to reveal the effect of the mobile app channel on physicians’ performance in the online health communities (OHCs). We provide direct empirical evidence on a large-scale dataset from one of the largest Chinese OHCs, Haodf, and propose a natural experiment to show the casual effect. Our results demonstrate that the introduction of the app channel to OHCs for patients has a positive impact on physicians’ responses and rating performance on the online platforms, especially for male physicians from high-ranking hospitals