首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   8455篇
  免费   325篇
  2020年   109篇
  2019年   156篇
  2018年   188篇
  2017年   211篇
  2016年   267篇
  2015年   191篇
  2014年   173篇
  2013年   2548篇
  2012年   239篇
  2011年   258篇
  2010年   286篇
  2009年   250篇
  2008年   223篇
  2007年   241篇
  2006年   240篇
  2005年   119篇
  2004年   118篇
  2003年   108篇
  2002年   111篇
  2001年   124篇
  2000年   87篇
  1999年   108篇
  1998年   76篇
  1997年   61篇
  1996年   75篇
  1995年   85篇
  1994年   81篇
  1993年   86篇
  1992年   78篇
  1991年   64篇
  1990年   60篇
  1989年   76篇
  1988年   66篇
  1987年   78篇
  1986年   74篇
  1985年   88篇
  1984年   75篇
  1983年   99篇
  1982年   79篇
  1981年   83篇
  1980年   77篇
  1979年   73篇
  1978年   78篇
  1977年   67篇
  1976年   76篇
  1975年   72篇
  1974年   56篇
  1973年   59篇
  1972年   64篇
  1970年   49篇
排序方式: 共有8780条查询结果,搜索用时 15 毫秒
191.
Participants in a statewide family preservation program were classified on the basis of income level and enrollment in public assistance. These variables, along with other economic, parent, and child risk factors, were used to predict the probability of out-of-home placements at the close of this intervention program. Results revealed that neither low income nor receiving public assistance was predictive of having a child removed from the home. Higher levels of child-centered risk and parent-centered risk were the only consistent predictors of placement outcomes. Additional analyses revealed that family characteristics (e.g., history of psychiatric care, prior child placements, and involvement with the legal system) also were typical of families experiencing out-of-home placements. This study challenges the stereotype of low-income or welfare families being at greater risk of having a child removed from their home. Findings are discussed in the context of recent welfare reform initiatives and implications for future family policy research.  相似文献   
192.
Macrophysical climatic modeling (MCM) is based on the relationship between large-scale atmospheric dynamics and synoptic climatology to develop simulations of late Pleistocene and Holocene climate in specific localities. Climatic events are calculated at 200-year intervals for the last 14,000 and 500 years for the period from 14,000 to 40,000 B.P. The model has been applied to more than 200 localities in Africa. We present examples from different parts of the continent at locations of archaeological significance. The results indicate that the transition from glacial maximum to postglacial conditions shows both temporal and inter- and intraregional variability. In addition to long-term differences among early, middle, and late Holocene climatic regimes, the model also shows incidences of sharp, abrupt events at some intervals. The applications of MCM invite comparison with inferences based on proxy data and assist in formulating and cross-examining socioecodynamic models based on climatic change, e.g., the continuity of cultural interactions along the Mediterranean littoral, the emergence and spread of cattle pastoralism, and the depopulation of the Sahara during the glacial maximum.  相似文献   
193.
194.
195.
This paper describes the use of electron spin resonance spectroscopy to estimate the degree of heating of quartzite cobbles from hearths on the floor of the archaeological remains of an eighteenth-century Dutch colonial slave lodge. A novel technique based on the comparison of line intensities for the E’and O-2 centres in quartz distinguished successfully between cobbles which had been heated to estimated temperatures ranging from 300 °C to 450° C and controls from an adjacent stream bed. This inexpensive and simple technique could be applied to a wide range of archaeological problems involving the thermal history of objects consisting of or containing quartz.  相似文献   
196.
197.
ABSTRACT. Average monthly price data from twelve hinterland markets and the Houston port price for wheat are studied in a cointegration framework using the Engle-Granger "two-step" procedure and Johansen's maximum likelihood procedure. Out-of-sample forecasts from an error correction model are compared to those from a vector autoregression fit to levels and a univariate autoregression fit to first differences. This comparison suggests that modeling these (cointegrated) data as a levels vector autoregression, rather than as an error-correction process, results in significantly higher error bias, but lower error variance, at long horizons.  相似文献   
198.
199.
ABSTRACT. Total cleared area in a von Thünen land-use model drives an atmosphere externality which depresses agricultural productivity uniformly throughout the region. Exogenous events that encourage clearance and use of a larger cultivated area (output price or population increase) exacerbate the externality. Imposition of a simple, corrective tax on land rents does not reverse these patterns but does mitigate the increase in the externality and leaves cultivators with higher incomes than they would obtain without the tax. We examine an optimal tax on land rents, designed to maximize the social value of land rents in the region, and an output tax.  相似文献   
200.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号