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   6696篇
  免费   693篇
  国内免费   1篇
  2022年   49篇
  2021年   86篇
  2020年   57篇
  2019年   70篇
  2018年   85篇
  2017年   81篇
  2016年   115篇
  2015年   214篇
  2014年   250篇
  2013年   309篇
  2012年   339篇
  2011年   382篇
  2010年   254篇
  2009年   231篇
  2008年   302篇
  2007年   372篇
  2006年   325篇
  2005年   291篇
  2004年   327篇
  2003年   297篇
  2002年   245篇
  2001年   178篇
  2000年   150篇
  1999年   147篇
  1998年   121篇
  1997年   82篇
  1996年   84篇
  1995年   77篇
  1994年   80篇
  1993年   82篇
  1992年   113篇
  1991年   84篇
  1990年   121篇
  1989年   84篇
  1988年   75篇
  1987年   89篇
  1986年   75篇
  1985年   69篇
  1984年   50篇
  1983年   56篇
  1982年   56篇
  1981年   52篇
  1980年   50篇
  1979年   54篇
  1977年   40篇
  1976年   49篇
  1975年   36篇
  1974年   42篇
  1972年   35篇
  1971年   39篇
排序方式: 共有7390条查询结果,搜索用时 218 毫秒
11.
Measurement of insulin-like growth factor-1 (IGF-I) has utility for the diagnosis and management of growth disorders, but inter-assay comparison of results has been complicated by a multitude of reference standards, antibodies, detection methods, and pre-analytical preparation strategies. We developed a quantitative LC-MS method for intact IGF-I, which has advantages in throughput and complexity when compared to mass spectrometric approaches that rely on stable isotope dilution analysis of tryptic peptides. Since the method makes use of full-scan data, the assay was easily extended to provide quantitative measurement of IGF-II using the same assay protocol. The validated LC-MS assay for IGF-I and IGF-II provides accurate results across the pediatric and adult reference range and is suitable for clinical use.  相似文献   
12.
13.
OBJECTIVE--To identify the attitudes of general practitioners towards the use of thermometers in general practice. DESIGN--Postal questionnaire survey. SETTING--All general practitioners in the catchment area of Frimley Park Hospital, Surrey. SUBJECTS--145 general practitioners. MAIN OUTCOME MEASURES--Answers to questions covering a variety of aspects concerning the use of thermometers in general practice. RESULTS--116 (80%) doctors replied. Seven doctors did not have any method of taking a patient''s temperature; up to 12 more doctors did not use their thermometers and 56 doctors used them infrequently, less than once a fortnight. Mercury glass thermometers were most commonly used (80 doctors; 69%), but only 8% of doctors used them correctly. Six doctors failed to clean their thermometers between patients. The study failed to identify the roles of axillary and rectal temperature readings. CONCLUSION--There is a wide variation in attitudes towards the use of thermometers in general practice.  相似文献   
14.
15.
16.
Woodlots are forest islands embedded within an urban matrix, and often represent the only natural areas remaining in suburban areas. Woodlots represent critical conservation areas for native plants, and are important habitat for wildlife in urban areas. Invasion by non-indigenous (NIS) plants can alter ecological structure and function, and may be especially severe in remnant forests where NIS propagule pressure is high. Woody shrubs in the Family Berberidaceae have been well documented as invaders of the forest–urban matrix in North America. Mahonia bealei (Berberidaceae) is a clonal shrub native to China, and is a popular ornamental in the Southeastern United States. Mahoni bealei is listed as “present” on some local and state floras, but almost nothing is known regarding its invasion potential in the United States. We sampled 15 woodlots in Clemson, South Carolina, to assess the invasion of M. bealei and other woody non-indigenous species (NIS). M. bealei invaded 87% of the woodlots surveyed and species richness of NIS on these woodlots varied from 5 to 14. Stepwise-multiple regression indicated that less canopy cover and older M. bealei predicted greater abundance of M. bealei , and that not all subdivisions were equally invaded (P < 0.0001; r2 = 0.88). The impact of M. bealei on native flora and fauna may be considerable, and it is likely to continue to spread in the Southeastern United States. M. bealei should be recognized as an aggressive invader in the Southeastern United States, with the potential for negative impacts on native flora and fauna.  相似文献   
17.
18.
19.
20.
Hurricane risk characteristics are examined across the U. S. Gulf of Mexico coastline using a hexagonal tessellation. Using an extreme value model, parameters are collected representing the rate or λ (frequency), the scale or σ (range), and the shape or ξ (intensity) of the extreme wind distribution. These latent parameters and the 30-year return level are visualized across the grid. The greatest 30-year return levels are located toward the center of the Gulf of Mexico, and for inland locations, along the borders of Louisiana, Mississippi, and Alabama. Using a geographically weighted regression model, the relationship of these parameters to sea surface temperature (SST) is found to assess sensitivity to change. It is shown that as SSTs increase near the coast, the frequency of hurricanes in these grids decrease significantly. This reinforces the importance of SST in areas of likely tropical cyclogenesis in determining the number of hurricanes near the coast, along with SSTs along the lifespan of the storm, rather than simply local SST. The range of hurricane wind speeds experienced near Florida is shown to increase with increasing SSTs (insignificant), suggesting that increased temperatures may allow hurricanes to maintain their strength as they pass over the Florida peninsula. The modifiable areal unit problem is assessed using multiple grid sizes. Moran’s I and the local statistic G are calculated to examine spatial autocorrelation in the parameters. This research opens up future questions regarding rapid intensification and decay close to the coast and the relationship to changing SSTs.  相似文献   
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