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   6151篇
  免费   12篇
电工技术   8篇
化学工业   333篇
机械仪表   11篇
建筑科学   11篇
矿业工程   1篇
能源动力   5篇
轻工业   514篇
石油天然气   1篇
无线电   19篇
一般工业技术   57篇
冶金工业   5169篇
原子能技术   4篇
自动化技术   30篇
  2013年   85篇
  2011年   8篇
  2010年   7篇
  2009年   7篇
  2005年   9篇
  2004年   7篇
  2003年   10篇
  2000年   7篇
  1999年   143篇
  1998年   1535篇
  1997年   860篇
  1996年   585篇
  1995年   341篇
  1994年   315篇
  1993年   336篇
  1992年   59篇
  1991年   105篇
  1990年   78篇
  1989年   101篇
  1988年   106篇
  1987年   101篇
  1986年   92篇
  1985年   58篇
  1984年   25篇
  1983年   34篇
  1982年   46篇
  1981年   52篇
  1980年   68篇
  1979年   25篇
  1978年   50篇
  1977年   165篇
  1976年   365篇
  1975年   43篇
  1974年   26篇
  1973年   24篇
  1972年   31篇
  1971年   29篇
  1970年   23篇
  1969年   12篇
  1968年   13篇
  1967年   12篇
  1966年   12篇
  1965年   15篇
  1964年   12篇
  1963年   9篇
  1962年   8篇
  1959年   7篇
  1958年   8篇
  1956年   10篇
  1955年   8篇
排序方式: 共有6163条查询结果,搜索用时 93 毫秒
91.
OBJECTIVES: The authors describe the relation of provider characteristics to processes, costs, and outcomes of medical care for elderly patients hospitalized for community-acquired pneumonia. METHODS: Using Medicare claims data, Medicare beneficiaries discharged from Pennsylvania hospitals during 1990 with community-acquired pneumonia were identified. Claims data were used to ascertain mortality, readmissions, use of procedures and physician consultations, and the costs of care. The relationship of these measures to provider characteristics was analyzed using regression techniques to adjust for patient characteristics, including comorbidity and microbial etiology. RESULTS: Among 22,294 pneumonia episodes studied, 30-day mortality was 17.0%. After adjusting for patient characteristics, 30-day mortality and readmission rates were unrelated to hospital teaching status or urban location or to physician specialty. Use of procedures and physician consultations was more common and costs were 11% higher among patients discharged from teaching hospitals compared with nonteaching hospitals. Similarly, costs were 15% higher at urban hospitals compared with rural hospitals. General internists and medical subspecialists used more procedures and had higher costs than family practitioners. CONCLUSIONS: Processes and costs of care for community-acquired pneumonia varied by provider characteristics, but neither mortality nor readmission rates did. These differences cannot be explained by clinical variables in the database. Further studies should determine whether less costly patterns of care for pneumonia, and perhaps other conditions, could replace more costly ones without compromising patient outcomes.  相似文献   
92.
BACKGROUND AND PURPOSE: Norway has the highest reported incidence of hip fractures in western Europe. Little is known about the epidemiology of falls in Norway where the winter season is long and dark. The objective of this work was to study reported falls and their consequences among elderly Norwegians living at home. METHODS: A cross-sectional design was used for the study. Interviews were performed in the homes of 431 subjects, aged 67-97 years, living at home. Information on falling was gathered through six questions: whether the subject had fallen during the last six months, and if so, how many falls they had, where the last fall occurred, its perceived reason, the activity the subject had been engaged in when the fall occurred, and the resulting injury. RESULTS: In all, 24.1% of subjects reported falling during the last six months, and 9.5% had suffered more than one fall. Falls were most frequently linked to external events (63.1%). Outdoor falls were more frequent (59.0%; 95% CI = 51.2-82.0) than indoor falls. Older subjects were associated with more frequent indoor falls (p < 0.05), but gender was not significant. Fifty-one per cent of subjects had fallen while walking and 53% had suffered an injury from the last fall. In 13.4% of the women and 16.2% of the men, the last fall had resulted in a fracture. CONCLUSIONS: Compared to the results of other studies from industrialized Western countries, a similar crude fall rate, similar frequency and similar type of injury were found. However, in contrast to other studies, no gender difference was observed with regard to falling, place of falling and fracture rate.  相似文献   
93.
OBJECTIVE: Prednisolone reduced the progression of joint destruction over 2 yr in early, active rheumatoid arthritis. The response to discontinuation of prednisolone under double-blind conditions is now reported. METHODS: A randomized, double-blind, placebo-controlled trial of prednisolone 7.5 mg daily in addition to routine medication over 2 yr in 128 patients with early rheumatoid arthritis, using radiological progression (changes in the Larsen score) and the development of erosions as primary outcome measures. Study medication was blindly discontinued and follow-up maintained for a further year. Other assessments included disability, joint inflammation, pain and the acute-phase response. RESULTS: Similar results were obtained when all available radiographs were included for each year of assessment (maximum 114) and when only patients with radiographs at all time points were included (75 patients). In these 75, the mean progression in the prednisolone group was 0.21 Larsen units in year 1, 0.04 units in year 2 and 1.01 units in year 3 (P = 0.587, 0.913 and 0.039 for change within each year, respectively). The equivalent placebo group means were 2.34, 1.00 and 1.63 Larsen units (P = 0.001, 0.111 and 0.012; difference between groups: 2.13, 0.96 and 0.67 units, P = 0.082, 0.02 and 0.622). The percentage of hands which had erosions at each time point was: prednisolone group: 27.8, 29.2, 34.7 and 39.2; placebo group: 28.2, 48.7, 59.0 and 66.5. There was little evidence for a flare in clinical symptoms after discontinuation of prednisolone. CONCLUSION: Joint destruction resumed after discontinuation of prednisolone. This corroborates the previously reported therapeutic effect and challenges current concepts of disease pathogenesis.  相似文献   
94.
We surveyed hypnosis researchers and Institutional Review Boards (IRBs) with regard to the ethical evaluation of research protocols. Researchers and IRB administrators were independently surveyed within the same institutions. Both objective and free response items were used to address substantive issues such as deception and at-risk populations, as well as practical matters such as paperwork. Parallel questions allowed a point-counterpoint between researchers and IRBs. Overall, the results suggest that IRBs do not treat hypnosis research differently than other types of research. We end with recommendations for facilitating interactions between hypnosis researchers and their IRBs.  相似文献   
95.
We demonstrate that physiological concentrations of HDL3 inhibit the thrombin-induced platelet fibrinogen binding and aggregation in a time- and concentration-dependent fashion. The underlying mechanism includes HDL3-mediated inhibition of phosphatidylinositol 4,5-bis-phosphate turnover, 1,2-diacylglycerol and inositol 1,4,5-tris-phosphate formation, and intracellular calcium mobilization. The inhibitory effects of HDL3 on inositol 1,4,5-tris-phosphate formation and intracellular calcium mobilization were abolished after covalent modification of HDL3 with dimethylsuberimidate. Furthermore, they could be blocked by calphostin C and bis-indolylmaleimide, 2 highly selective and structurally unrelated protein kinase C inhibitors. However, the inhibitory effects of HDL3 were not blocked by H89, a protein kinase A inhibitor. In addition, HDL3 failed to induce cAMP formation but stimulated the phosphorylation of the protein kinase C 40- to 47-kD major protein substrate. We observed a close temporal relationship between the HDL3-mediated inhibition of thrombin-induced inositol 1,4,5-tris-phosphate formation, intracellular calcium mobilization, and fibrinogen binding and the phosphorylation of the protein kinase C 40- to 47-kD major protein substrate. Taken together, these findings indicate that the HDL3-mediated inhibition of thrombin-induced fibrinogen binding and aggregation occurs via inhibition of phosphatidylinositol 4,5-bis-phosphate turnover and formation of 1,2-diacylglycerol and inositol 1,4,5-tris-phosphate. Protein kinase C may be involved in this process.  相似文献   
96.
Blastomyces dermatitidis, a dimorphic broad-based budding yeast endemic to the Mississippi River Valley region, is responsible for morbidity in humans via inhalation and dissemination. The response of acute lung injury, which produces an illness with serious morbidity and an approximately 50% mortality, uncommonly occurs. Diagnosis can be difficult, and a high index of suspicion should be maintained in endemic regions for patients with acute lung injury of uncertain etiology, especially if their condition deteriorates on broad-spectrum antimicrobial and antitubercular therapy and they have a previous insidious respiratory complaint and constitutional symptoms. Diagnosis should be aggressively pursued and treatment with amphotericin B (0.6 to 0.8 mg/kg/day) initiated as early as possible.  相似文献   
97.
98.
The 9th International Congress on Genes, Gene Families, and Isozymes marked a historic transition in the series formerly known as the International Congress on Isozymes. The name of the congress was changed to reflect the broadened scope of this field and the new directions in which it is moving. To recognize and promote this transition, a number of new features were incorporated into this congress. Accordingly, the broad-based program featured preeminent biologists from 29 different countries. A total of 350 people attended the congress. A special new feature of this congress was the Student/Fellow Program, which was designed to enhance participation by advanced undergraduate and graduate students and postdoctoral fellows. This congress celebrated the progress that has occurred during the past 40 years beginning with studies of isozymes and leading into studies of specific genes and gene families. As we move into the next millennium, it is clear that our field is strongly positioned and will continue to be the focus of exciting and important new research.  相似文献   
99.
Reward information is processed in a limited number of brain structures, including fronto-basal ganglia systems. Dopamine neurons respond phasically to primary rewards and reward-predicting stimuli depending on reward unpredictability but without discriminating between rewards. These responses reflect 'errors' in the prediction of rewards in correspondence to learning theories and thus may constitute teaching signals for appetitive learning. Neurons in the striatum (caudate, putamen, ventral striatum) code reward predictions in a different manner. They are activated during several seconds when animals expect predicted rewards. During learning, these activations occur initially in rewarded and unrewarded trials and become subsequently restricted to rewarded trials. This occurs in parallel with the adaptation of reward expectations by the animals, as inferred from their behavioral reactions. Neurons in orbitofrontal cortex respond differentially to stimuli predicting different liquid rewards, without coding spatial or visual features. Thus, different structures process reward information processed in different ways. Whereas dopamine neurons emit a reward teaching signal without indicating the specific reward, striatal neurons adapt expectation activity to new reward situations, and orbitofrontal neurons process the specific nature of rewards. These reward signals need to cooperate in order for reward information to be used for learning and maintaining approach behavior.  相似文献   
100.
The relative risk of death by calendar date of diagnosis was investigated in a population-based incident cohort of 845 (463 males:382 females) IDDM diagnosed in Leicestershire before the age of 17 years between 1940 and 1989. The mortality status of 844 (99.9%) patients was determined as of the 31 December 1991, representing 14,346 person-years of risk. Trends in relative risk of death were investigated using Cox proportional hazards modelling for within cohort comparisons and age/sex and calendar time adjusted standardized mortality ratios (SMR) using generalized linear modelling for external comparisons. Median age at diagnosis was 10 years (range 3 months to 16 years); median duration of diabetes 15 years (range 1-51 years). Forty-four patients had died (5.2%; median age at death 31 years, range 11-51 years). A further four patients died at presentation (within 24 h) from ketoacidosis and are excluded from all analyses. Calendar date of diagnosis was found to be an important predictor of mortality. Adjusting for attained age there was evidence of a decline in relative risk of death with calendar date of diagnosis of 3.4% (95% CI, 0.005-6.9%) per annum, equivalent to a 32% fall per decade (95% CI, 5-51%), or 84% (95% CI, 21-97) from 1940 to 1989. The data are consistent with a large fall in mortality between the 1940s and 1950s representing over 50% of the total reduction in mortality between 1940 and 1991. Neither sex nor age at diagnosis were significant predictors of mortality. Over the study period 1940-89 the SMR (male and female combined) fell from 981 (541-1556) to 238 (60-953) relative to the general population. This population-based study shows that the prognosis for Type 1 (insulin-dependent) diabetes mellitus has improved markedly over the period 1940-1991.  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