全文获取类型
收费全文 | 6035篇 |
免费 | 703篇 |
国内免费 | 142篇 |
出版年
2023年 | 152篇 |
2022年 | 77篇 |
2021年 | 169篇 |
2020年 | 247篇 |
2019年 | 299篇 |
2018年 | 271篇 |
2017年 | 269篇 |
2016年 | 267篇 |
2015年 | 265篇 |
2014年 | 330篇 |
2013年 | 442篇 |
2012年 | 258篇 |
2011年 | 291篇 |
2010年 | 239篇 |
2009年 | 305篇 |
2008年 | 341篇 |
2007年 | 358篇 |
2006年 | 255篇 |
2005年 | 241篇 |
2004年 | 220篇 |
2003年 | 165篇 |
2002年 | 181篇 |
2001年 | 152篇 |
2000年 | 122篇 |
1999年 | 114篇 |
1998年 | 79篇 |
1997年 | 61篇 |
1996年 | 63篇 |
1995年 | 59篇 |
1994年 | 56篇 |
1993年 | 47篇 |
1992年 | 44篇 |
1991年 | 65篇 |
1990年 | 32篇 |
1989年 | 34篇 |
1988年 | 32篇 |
1987年 | 35篇 |
1986年 | 26篇 |
1985年 | 29篇 |
1984年 | 28篇 |
1983年 | 13篇 |
1982年 | 47篇 |
1981年 | 22篇 |
1980年 | 16篇 |
1979年 | 12篇 |
1978年 | 8篇 |
1975年 | 5篇 |
1974年 | 6篇 |
1973年 | 11篇 |
1972年 | 12篇 |
排序方式: 共有6880条查询结果,搜索用时 15 毫秒
91.
A robust method for selection of variables with the greatest discriminatory power is presented in the paper. The method deals with the two groups of data problem. An application of the method to some respiratory disease data and comparisons with classical procedures are given, also. 相似文献
92.
93.
94.
Based on a simple stochastic model the influence of the incubation period of the HIV-infection on some qualitative aspects of the spread of AIDS is studied. A critical ‘nfection rat’ is calculated in dependence on parameters of the incubation period density function. 相似文献
95.
R F Sturrock 《International journal for parasitology》1973,3(2):165-174
Growth curves, calculated for field populations of B. glabrata, were not materially affected by habitat, altitude or season. A mean growth curve was therefore used to estimate the age-frequency distribution of snails in successive field samples. These data permitted the construction of ecological life tables and the estimation of r, the intrinsic rate of natural increase (or decrease) of the different populations. The calculated values of r were inserted in a simple model of unlimited population growth but the resultant curves poorly represented the observed data. A model for unlimited growth was more satisfactory for pond and marsh populations but, apparently, immigration made it less satisfactory for stream and banana drain populations. Nevertheless, r may still be of value in predicting repopulation rates in certain habitats after a mollusciciding which does not kill the entire snail population. 相似文献
96.
G. H. Morrey 《Biometrical journal. Biometrische Zeitschrift》1989,31(5):589-598
Problems with carry-over effects in the simple two-period cross-over have lead to interest in more complex cross-over designs. A method for analysing the optimum two-treatment three-period design with binary response variables is given by making a simple extension to Gart's logistic model. The method gives independent tests for, and estimates of the difference in treatment and first-order carry-over effects. An example of the analysis is given, using the loglinear models facility in GLIM. 相似文献
97.
98.
99.
100.
Mauricio Cantor Leonardo L. Wedekin Fábio G. Daura‐Jorge Marcos R. Rossi‐Santos Paulo C. Simões‐Lopes 《Marine Mammal Science》2012,28(1):63-83
This study represents the first attempt to study the population dynamics of Guiana dolphins (Sotalia guianensis), by evaluating a set of demographic parameters. The population of the Caravelas River estuary, eastern Brazil, was systematically monitored through a long‐term mark‐recapture experiment (2002–2009). Abundance estimates revealed a small population (57–124 dolphins), comprised of resident dolphins and individuals that temporarily leave or pass through the study area. Temporary emigration from the estuary to adjacencies (γ″= 0.33 ± 0.07 SE) and return rate (1 ?γ′= 0 .67) were moderate and constant, indicating that some dolphins use larger areas. Survival rate (?= 0.88 ± 0.07 SE) and abundance were constant throughout the study period. Power analysis showed that the current monitoring effort has high probability of detecting abrupt population declines (1 ?β= 0.9). Although the monitoring is not yet sensitive to subtle population trends, sufficient time to identify them is feasible (additional 3 yr). Despite such apparent stability, this population, as many others, inhabits waters exposed to multiple human‐related threats. Open and closed population modeling applied to photo‐identification data provide a robust baseline for estimating several demographic parameters and can be applied to other populations to allow further comparisons. Such synergistic efforts will allow a reliable definition of conservation status of this species. 相似文献