A compound Rayleigh survival model and its application to randomly censored data

Received: May 5, 1999; revised version: June 15, 2000     

Woody vegetation and canopy fragmentation along a forest-to-urban gradient

To identify patterns that can be used to predict vegetation and landscape characteristics in urban environments, we surveyed the species composition and size of woody plants, as well as the landscape structure of forest canopies, along a forest-to-urban gradient near Oxford, Ohio, USA. The gradient included six sites of increasingly urban land-use: a preserve, a recreational area, a golf course, a residential subdivision, apartment complexes, and a business district. We recorded species identity and stem diameter for all woody plants greater than 3 cm diameter at breast height (DBH) to examine the distribution of individual species as well as overall community composition. We used digitized aerial photographs to compare the spatial characteristics of the forest canopy at each site. We found predictable patterns in species diversity (Shannon index), spatial heterogeneity in species composition (mean percent dissimilarity), and all measures of patch fragmentation (canopy cover and patch number and size). There were clear differences in tree density and total basal area between forested sites and developed sites, but there was little resolution among developed sites. Species richness and average DBH showed no clear pattern, suggesting that landscaping preference largely determined these values. We present a modified version of an intermediate heterogeneity model that can be used to predict diversity patterns in urban areas. We discuss probable mechanisms that led to these patterns and the potential implications for animal communities in urban environments.     

The percentile residual life up to time t0: Ordering and aging properties

Motivated by practical issues, a new stochastic order for random variables is introduced by comparing all their percentile residual life functions until a certain instant. Some interpretations of these stochastic orders are given, and various properties of them are derived. The relationships to other stochastic orders are studied and also an application in reliability theory is described. Finally, we present some characterization results of the decreasing percentile residual life up to time t₀ aging notion.     

Transitions in welfare participation and female headship

This study uses data from the 1990, 1992, 1993 and 1996 panels of the Survey of Income and Program Participation to examine how welfare policies and local economic conditions contribute to women's transitions into and out of female headship and into and out of welfare participation. It also examines whether welfare participation is directly associated with longer spells of headship. The study employs a simultaneous hazards approach that accounts for unobserved heterogeneity in all of its transition models and for the endogeneity of welfare participation in its headship model. The estimation results indicate that welfare participation significantly reduces the chances of leaving female headship. The estimates also reveal that more generous welfare benefits do not directly contribute to headship but rather contribute indirectly to headship by increasing the chances that a mother will enter welfare and consequently remain a single mother for longer. More generous Earned Income Tax Credit benefits are associated with more stable arrangements for both headship and welfare participation. Other measures of welfare policies, including indicators for the adoption of welfare waivers and the implementation of Temporary Assistance for Needy Families programs, are generally not significantly associated with headship or welfare receipt. Better economic opportunities are estimated to increase headship but reduce welfare participation among unmarried mothers.     

线性规划的梯度投影算法

Ｋａｒｍａｒｋａｒ算法是解线性规划的多项式算法，但其具有数值不稳定的缺点，同时，由于它属于内点法，在算法终止时所得的点始终是一个近似最优解。文中给出的梯度投影法，可以穿过区域内部，或穿过区域的边界的相对内部，证明了该方法将在有限步终止。     

连续铸造灰铁管内沟形成机理的探讨

本文通过测量内结晶器的变形,研究了内沟与石墨化膨胀及内结晶器变形的关系,指出了产生内沟的原因及其位置,为减少内沟提供了一定根据。     

Evolution of recurrent asthma event rate over time in frailty models

Summary. To model the time evolution of the event rate in recurrent event data a crucial role is played by the timescale that is used. Depending on the timescale selected the interpretation of the time evolution will be entirely different, both in parametric and semiparametric frailty models. The gap timescale is more appropriate when studying the recurrent event rate as a function of time since the last event, whereas the calendar timescale keeps track of actual time. We show both timescales in action on data from an asthma prevention trial in young children. The frailty model is further extended to include both timescales simultaneously as this might be most relevant in practice.     

初始点任意的广义梯度投影算法

利用广义梯度投影与罚函数技巧，将等式与不等式约束问题化成一个无约束问题。给出了一个初始点任意的广义梯度投影算法，削弱了文献［８］的条件，罚参数在计算中自动调整，在迭代次数适当大时成为常数，并证明了算法具有全局收敛性，在实际应用中也容易实现。     

Do Forbearance Plans Help Mitigate Credit Card Losses?

In this paper, we examine how reinstated (i.e., re-aged) credit card accounts are likely to default again. Our sample data reveal that about 22% of the re-aged accounts default again, mostly in the first 24 months after reinstatement. We also find that a FICO score (public information) is a better predictor of a second default, while a payment behavioral score (private information) is a better predictor of a first default. Furthermore, the average FICO score of the 78% of the re-aged borrowers who did not default again rises about 20 points, an improvement in their relative risk profile overall. These findings suggest that the re-aging program provides a second chance for liquidity-constrained borrowers who would have otherwise defaulted on their debt.