A play-the-winner-type urn design with reduced variability

We propose a new adaptive allocation rule, the drop-the-loser, that randomizes subjects in the course of a trial comparing treatments with dichotomous outcomes. The rule tends to assign more patients to better treatments with the same limiting proportion as the randomized play-the-winner rule. The new design has significantly less variable allocation proportion than the randomized play-the-winner rule. Decrease in variability translates into a gain in statistical power. For some values of success probabilities the drop-the-loser rule has a double advantage over conventional equal allocation in that it has better power and assigns more subjects to the better treatment. Acknowledgments. I thank Stephen Durham, the associate editor, and the referees for their helpful suggestions.     

Family and sex-specific U.S. immigration from Europe, 1870–1910: A panel data study of rates and composition

This paper develops and estimates models of family and sex-specific emigration, as well as the sex composition of this emigration, from 12 European source countries to the U.S. for the period 1870–1910. The models are based on the distinction between economic migrants (males, single females, and some married females) and tied or trailing migrants (females) and are estimated with panel data, including data that relate to the occupational/industrial structure of male and female economic activity in source countries. Hausman-Taylor instrumental variable estimates suggest that although both males and females responded to labor-market signals, males were more responsive than females to per capita GDP differences. Moreover, compared to the rest of Europe, Ireland, and Scandinavia were the sources of many young, single male, and female migrants, who responded strongly to gaps in economic opportunities. In fact, much of the European response to such gaps appears to be due to migrants from Ireland and Scandinavia. Females tended to originate in English-speaking countries and countries that were agriculturally oriented. Service and manufacturing jobs in source countries discouraged the migration of females relative to males. Males tended to follow recent migrants more than females, but females responded more to long-term influences as measured by stocks of migrants from their source countries who had previously settled in the U.S. Countries with high birth rates had relatively fewer female emigrants, whereas those with high rates of natural increase 20 years earlier experienced relatively more male emigration. Intact families, other family members (including family-strategy male migrants and trailing female migrants), and single males and females responded strongly to economic incentives, but the singles were most responsive followed by family-strategy males.     

试论生态林业系统的时空观

生态林业系统具有时间、空间两个序列的开发前景。长周期,大跨度的生态林业生产可以分解为复合的短周期方式,使生态意识与经济效益有机结合。空间组合的多层次立体开发有效地利用了光、热、水、土资源,并获得最大生物量;地域组合着眼宏观,注重研究大型骨干及网络性生态工程问题。     

Competition as a track for preventing illegal immigration

Rich countries often face sizeable illegal migration. This paper suggests that these countries would use the financial aid which they give to the source countries as an instrument to prevent illegal immigration. The core of this policy is to allow the source countries to compete for the pre-determined aid, which would be distributed according to the cross-proportion of the apprehended illegal aliens. Moreover, we show that it may be beneficial for the rich country to split the source countries into competing pairs rather than allowing all of them to compete jointly. The rich country has basically two policy means: funds allocated to strengthening its border control; and the foreign aid given to the source countries. The multi-country general equilibrium model presented shows how the rich country, by choosing an appropriate mix of these two policy means, can minimize the number of illegal immigrants subject to its budget constrain.Received: 15 August 2002, Accepted: 21 January 2004, JEL Classification: F22Nava Kahana, Tikva Lecker: The authors are grateful to two anonymous referees for their valuable comments.     

对我国生态环境保护的初步探讨

中国是世界上最大的发展中国家,由于现代工农业生产而造成了严重的环境污染,尤其是大量使用农用化学品所导致的生态失调与环境破坏,已严重威胁到人类生存环境及生活健康。今后社会发展必须大力提倡与推广无公害、无污染的清洁生产,全面实现生产的"零污染"与废物的"零排放"、保护环境与追求生态和谐的发展目标。     

黑河流域生态环境保护和建设

黑河是我国西北地区一个较大的内陆河，该流域由于人口压力，林牧矛盾，水资源粗放利用，出现了严重的土地沙化，盐化，沙尘暴等生态问题，本文提出应执行抢救性水源林生态环境建设，合理配置和科学利用水资源，实施跨流域调水等补水工程，构建生态农业体系的发展方略，实现黑河流域生态良性循环，促进区域经济，社会的可持续发展。     

循环经济的本质与我国发展循环经济的路径选择

本文从循环经济的基本理论入手,探讨了循环经济概念的渊源,指出循环经济是一种全新的发展模式,并且论述了发展循环经济的必要性,分析了中国发展循环经济面临的问题,最后提供了推进循环经济的路径选择:加大宣传发展循环经济的重要意义;不断调整和优化产业结构;建立和完善循环经济政策和体制;为循环经济发展提供有力的技术支撑;探索建立绿色国民经济核算体系;加强国际合作,追踪先进理论和科技等八项措施.     

岷江上游生态重建的模式

本文在分析岷江上游干旱河谷区生态环境问题的基础上，阐述了该区生态退化是脆弱的自然生态环境和人类的不合理经济营活动综合作用的结果，指出生态恢复与重建必须与经济建设紧密结合，走生态经济发展之路，将岷江上游干温河谷区划分为干旱河谷粮果林生态我，中山，亚高山林药菌生态林业区，高山灌丛草甸，草甸生态特业区，并提出了不同的生态经济类型区的生态恢复与重建模式，最后，论述了生态恢复与重建的工程措施，生物措施及管理措施。     

Definitions and progress of ecosystem health and ecological security

1. Introduction Development of human industrial civilization and predatory exploitation of natural resources have in- duced tremendous changes of ecological environment. In recent years, territorial and global environmental problems have increased, such as greenhouse effect, depletion of ozone, global warming, water scarcity, water pollution, acid rain, decrease of biodiversity, soil erosion desertification etc. These changes seriously intimidate the health and security of ecosystems, and dist…     

四川生态建设中存在的问题

本文在介绍四川省生态建设概况的基础上，分析了四川生态建设中存在的问题，指出四川应增强生态意识，把生态建设纳入农业可持续发展战略，确保生态农业建设顺利进行。