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   1376篇
  免费   150篇
  2023年   9篇
  2021年   27篇
  2020年   27篇
  2019年   33篇
  2018年   27篇
  2017年   26篇
  2016年   32篇
  2015年   81篇
  2014年   68篇
  2013年   74篇
  2012年   94篇
  2011年   97篇
  2010年   61篇
  2009年   48篇
  2008年   74篇
  2007年   76篇
  2006年   63篇
  2005年   66篇
  2004年   55篇
  2003年   55篇
  2002年   63篇
  2001年   16篇
  2000年   17篇
  1999年   17篇
  1998年   8篇
  1997年   6篇
  1996年   14篇
  1995年   11篇
  1994年   13篇
  1993年   5篇
  1992年   11篇
  1991年   11篇
  1990年   15篇
  1989年   15篇
  1988年   7篇
  1987年   8篇
  1986年   8篇
  1985年   14篇
  1984年   9篇
  1983年   11篇
  1982年   6篇
  1981年   7篇
  1979年   9篇
  1977年   5篇
  1976年   6篇
  1974年   6篇
  1973年   9篇
  1972年   6篇
  1970年   5篇
  1968年   5篇
排序方式: 共有1526条查询结果,搜索用时 312 毫秒
111.

Purpose

Asthma during adolescence can induce social, psychological, and behavioral problems. We examined the impact of asthma and other allergic diseases on psychological symptoms and health risk behaviors among South Korean adolescents.

Methods

In this population-based cross-sectional study, 3192 adolescents (10–18 years of age) participating in the 2008–2011 Korean National Health and Nutrition Examination Survey were enrolled. Psychological problems associated with clinically diagnosed asthma, allergic rhinitis, and atopic dermatitis were assessed using questionnaires and surveys. Data was analyzed using logistic regression to determine the association of depression with allergic disease while controlling for age, sex, body mass index, smoking experience, and alcohol use.

Results

Asthma and atopic dermatitis were associated with a higher prevalence of depression (17.2% and 13%, respectively). After adjusting for the covariates, asthma patients were approximately two times as likely to have depression as non-allergic participants (odds ratio, 1.81; 95% confidence interval, 1.22–2.68). Psychosocial stress significantly increased in the following order: no allergy, any allergy without asthma, asthma only, and asthma with any allergy (p for linear trend = 0.01). The asthma without other allergies group showed the highest prevalence of cigarette smoking (p = 0.007).

Conclusions

In this study, asthma with or without other allergies was significantly related to increases in depression, psychosocial stress, and smoking experience. Thus, care should be taken to adjust treatment to account for the psychological symptoms and health risk behaviors common among asthmatic adolescents.  相似文献   
112.
113.
114.
Aphids cause significant damage to crop plants. Studies regarding predator-prey relationships in fennel (Foeniculum vulgare Mill.) and cotton (Gossypium hirsutum L.) crops are important for understanding essential ecological interactions in the context of intercropping and for establishing pest management programs for aphids. This study evaluated the association among Hyadaphis foeniculi (Passerini) (Hemiptera: Aphididae), Aphis gossypii Glover (Hemiptera: Aphididae) and Cycloneda sanguinea (L.) (Coleoptera: Coccinellidae) in cotton with coloured fibres, fennel and cotton intercropped with fennel. Association analysis was used to investigate whether the presence or absence of prey and predator species can indicate possible interactions between aphids and ladybugs. Significant associations among both apterous and alate H. foeniculi and C. sanguinea were observed in both the fennel and fennel-cotton intercropping systems. The similarity analysis showed that the presence of aphids and ladybugs in the same system is significantly dependent on the type of crop. A substantial amount of evidence indicates that the presence of the ladybug C. sanguinea, is associated with apterous or alate A. gossypii and H. foeniculi in fennel-cotton intercropping system. We recommend that future research vising integrated aphid management taking into account these associations for take decisions.  相似文献   
115.
Volunteers are increasingly being recruited into citizen science projects to collect observations for scientific studies. An additional goal of these projects is to engage and educate these volunteers. Thus, there are few barriers to participation resulting in volunteer observers with varying ability to complete the project’s tasks. To improve the quality of a citizen science project’s outcomes it would be useful to account for inter-observer variation, and to assess the rarely tested presumption that participating in a citizen science projects results in volunteers becoming better observers. Here we present a method for indexing observer variability based on the data routinely submitted by observers participating in the citizen science project eBird, a broad-scale monitoring project in which observers collect and submit lists of the bird species observed while birding. Our method for indexing observer variability uses species accumulation curves, lines that describe how the total number of species reported increase with increasing time spent in collecting observations. We find that differences in species accumulation curves among observers equates to higher rates of species accumulation, particularly for harder-to-identify species, and reveals increased species accumulation rates with continued participation. We suggest that these properties of our analysis provide a measure of observer skill, and that the potential to derive post-hoc data-derived measurements of participant ability should be more widely explored by analysts of data from citizen science projects. We see the potential for inferential results from analyses of citizen science data to be improved by accounting for observer skill.  相似文献   
116.
This study investigates morphological differences between flakes produced via “core and flake” technologies and those resulting from bifacial shaping strategies. We investigate systematic variation between two technological groups of flakes using experimentally produced assemblages, and then apply the experimental model to the Cutting 10 Mid -Pleistocene archaeological collection from Elandsfontein, South Africa. We argue that a specific set of independent variables—and their interactions—including external platform angle, platform depth, measures of thickness variance and flake curvature should distinguish between these two technological groups. The role of these variables in technological group separation was further investigated using the Generalized Linear Model as well as Linear Discriminant Analysis. The Discriminant model was used to classify archaeological flakes from the Cutting 10 locality in terms of their probability of association, within either experimentally developed technological group. The results indicate that the selected independent variables play a central role in separating core and flake from bifacial technologies. Thickness evenness and curvature had the greatest effect sizes in both the Generalized Linear and Discriminant models. Interestingly the interaction between thickness evenness and platform depth was significant and played an important role in influencing technological group membership. The identified interaction emphasizes the complexity in attempting to distinguish flake production strategies based on flake morphological attributes. The results of the discriminant function analysis demonstrate that the majority of flakes at the Cutting 10 locality were not associated with the production of the numerous Large Cutting Tools found at the site, which corresponds with previous suggestions regarding technological behaviors reflected in this assemblage.  相似文献   
117.
118.
In closed‐canopy tropical forest understory, light availability is a significant determinant of habitat diversity because canopy structure is highly variable in most tropical forests. Consequently, variation in canopy cover affects the composition and distribution of plant species via creating variable light environments. Nevertheless, little is known about how variation in canopy openness structures patterns of plant–animal interactions. Because of the great diversity and dominance of ants in tropical environments, we used ant–plant interactions as a focal network to evaluate how variation in canopy cover influences patterns of plant–insect interactions in the Brazilian Amazon rain forest. We observed that small increases in canopy openness are associated with increased diversity of ant–plant interactions in our study area, and this change is independent of plant or ant species richness. Additionally, we found smaller niche overlap for both ants and plants associated with greater canopy openness. We hypothesize that enhanced light availability increases the breadth of ant foraging sources because variation in light availability gives rise to plant resources of different quality and amounts. Moreover, greater light availability promotes vegetative growth in plants, creating ant foraging ‘bridges’ between plants. In sum, our results highlight the importance of environmental heterogeneity as a determinant of ant–plant interaction diversity in tropical environments.  相似文献   
119.
As outlined in NCRP Report No. 160 of the US National Council on Radiation Protection and Measurements (NCRP), the average value of the effective dose to exposed individuals in the United States has increased by a factor of 1.7 over the time period 1982–2006, with the contribution of medical exposures correspondingly increasing by a factor of 5.7. At present, medical contributors to effective dose include computed tomography (50% of total medical exposure), nuclear medicine (25%), interventional fluoroscopy (15%), and conventional radiography and diagnostic fluoroscopy (10%). An increased awareness of medical exposures has led to a gradual shift in the focus of radiation epidemiological studies from traditional occupational and environmental exposures to those focusing on cohorts of medical patients exposed to both diagnostic and therapeutic sources. The assignment of organ doses to patients in either a retrospective or a prospective study has increasingly relied on the use of computational anatomic phantoms. In this paper, we review the various methods and approaches used to construct patient models to include anthropometric databases, cadaver imaging, prospective volunteer imaging studies, and retrospective image reviews. Phantom format types—stylized, voxel, and hybrid—as well as phantom morphometric categories—reference, patient-dependent, and patient-specific—are next defined and discussed. Specific emphasis is given to hybrid phantoms—those defined through the use of combinations of polygon mesh and non-uniform rational B-spline (NURBS) surfaces. The concept of a patient-dependent phantom is reviewed, in which phantoms of non-50th percentile heights and weights are designed from population-based morphometric databases and provided as a larger library of phantoms for patient matching and lookup of refined values of organ dose coefficients and/or radionuclide S values. We close with two brief examples of the use of hybrid phantoms in medical dose reconstruction—diagnostic nuclear medicine for pediatric subjects and interventional fluoroscopy for adult patients.  相似文献   
120.
In vitro scanning mutagenesis strategies are valuable tools to identify critical residues in proteins and to generate proteins with modified properties. We describe the fast and simple All-Codon Scanning (ACS) strategy that creates a defined gene library wherein each individual codon within a specific target region is changed into all possible codons with only a single codon change per mutagenesis product. ACS is based on a multiplexed overlapping mutagenesis primer design that saturates only the targeted gene region with single codon changes. We have used ACS to produce single amino-acid changes in small and large regions of the human tumor suppressor protein p53 to identify single amino-acid substitutions that can restore activity to inactive p53 found in human cancers. Single-tube reactions were used to saturate defined 30-nt regions with all possible codon changes. The same technique was used in 20 parallel reactions to scan the 600-bp fragment encoding the entire p53 core domain. Identification of several novel p53 cancer rescue mutations demonstrated the utility of the ACS approach. ACS is a fast, simple and versatile method, which is useful for protein structure–function analyses and protein design or evolution problems.  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