28,989 research outputs found
Modeling long-term longitudinal HIV dynamics with application to an AIDS clinical study
A virologic marker, the number of HIV RNA copies or viral load, is currently
used to evaluate antiretroviral (ARV) therapies in AIDS clinical trials. This
marker can be used to assess the ARV potency of therapies, but is easily
affected by drug exposures, drug resistance and other factors during the
long-term treatment evaluation process. HIV dynamic studies have significantly
contributed to the understanding of HIV pathogenesis and ARV treatment
strategies. However, the models of these studies are used to quantify
short-term HIV dynamics ( 1 month), and are not applicable to describe
long-term virological response to ARV treatment due to the difficulty of
establishing a relationship of antiviral response with multiple treatment
factors such as drug exposure and drug susceptibility during long-term
treatment. Long-term therapy with ARV agents in HIV-infected patients often
results in failure to suppress the viral load. Pharmacokinetics (PK), drug
resistance and imperfect adherence to prescribed antiviral drugs are important
factors explaining the resurgence of virus. To better understand the factors
responsible for the virological failure, this paper develops the
mechanism-based nonlinear differential equation models for characterizing
long-term viral dynamics with ARV therapy. The models directly incorporate drug
concentration, adherence and drug susceptibility into a function of treatment
efficacy and, hence, fully integrate virologic, PK, drug adherence and
resistance from an AIDS clinical trial into the analysis. A Bayesian nonlinear
mixed-effects modeling approach in conjunction with the rescaled version of
dynamic differential equations is investigated to estimate dynamic parameters
and make inference. In addition, the correlations of baseline factors with
estimated dynamic parameters are explored and some biologically meaningful
correlation results are presented. Further, the estimated dynamic parameters in
patients with virologic success were compared to those in patients with
virologic failure and significantly important findings were summarized. These
results suggest that viral dynamic parameters may play an important role in
understanding HIV pathogenesis, designing new treatment strategies for
long-term care of AIDS patients.Comment: Published in at http://dx.doi.org/10.1214/08-AOAS192 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Pure Exporter: Theory and Evidence
This paper provides the first evidence about pure exporters (i.e., firms exporting all of their output to the foreign market) -- a phenomenon overlooked and cannot be explained in the existing literature. It then offers a generalized model of Melitz (2003) for examining the existence and behavior of pure exporters. In particular, pure exporters arise when the export market is sufficiently large -- a situation more likely to hold in developing countries as opposed to large developed countries; and their productivity levels are above those of non-exporters, but below those of firms having both domestic sales and export. These theoretical predictions are borne out in a data of Chinese manufacturing firms for the period of 1998-2005.Pure Exporter, Firm Heterogeneity, Exporting Behavior
Contract Enforcement and Family Control of Business: Evidence from China
Family control of business is prevalent in developing economies, and one of the leading theories suggests that it is a response to weak contract enforcement in such economies. In this paper, we investigate the impacts of contract enforcement on the degree of family control of business using a sample of China's private enterprises. It is found that weaker contract enforcement is associated with the higher degree of family control of business. Our results are robust to the control for omitted variables and reserve causality issues, to the adjustment for the sample attrition bias, to the use of a sub-sample, and to the inclusion of other explanations for the family control of business.Family Control of Business, Contract Enforcement, China's Private Enterprises
Determinants of Entrepreneurial Activities in China
The institutional environment – including protection of private properties and contract enforcement – has been rather unfavorable for the emergence and development of China’s private enterprises. This is in sharp contrast to the case of the developed economies where the institutional environment is conductive to the entrepreneurial activities and only the personal attributes of would-be entrepreneurs determine their entrepreneurship decision. We thus propose a theoretical framework for the entrepreneurship decision in China with a focus on the role of institutional environment. Using a life-histories survey data of 2,854 respondents from twenty cities in China, we find strong support for the impacts of the institutional environment and its interactions with other determinants of entrepreneurship decision.
Anti-dark and Mexican-hat solitons in the Sasa-Satsuma equation on the continuous wave background
In this letter, via the Darboux transformation method we construct new
analytic soliton solutions for the Sasa-Satsuma equation which describes the
femtosecond pulses propagation in a monomode fiber. We reveal that two
different types of femtosecond solitons, i.e., the anti-dark (AD) and
Mexican-hat (MH) solitons, can form on a continuous wave (CW) background, and
numerically study their stability under small initial perturbations. Different
from the common bright and dark solitons, the AD and MH solitons can exhibit
both the resonant and elastic interactions, as well as various
partially/completely inelastic interactions which are composed of such two
fundamental interactions. In addition, we find that the energy exchange between
some interacting soliton and the CW background may lead to one AD soliton
changing into an MH one, or one MH soliton into an AD one.Comment: 12 pages, 6 figure
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