全文获取类型
收费全文 | 1028篇 |
免费 | 97篇 |
国内免费 | 34篇 |
专业分类
1159篇 |
出版年
2024年 | 4篇 |
2023年 | 23篇 |
2022年 | 17篇 |
2021年 | 31篇 |
2020年 | 37篇 |
2019年 | 47篇 |
2018年 | 35篇 |
2017年 | 28篇 |
2016年 | 33篇 |
2015年 | 42篇 |
2014年 | 50篇 |
2013年 | 64篇 |
2012年 | 41篇 |
2011年 | 43篇 |
2010年 | 37篇 |
2009年 | 55篇 |
2008年 | 66篇 |
2007年 | 58篇 |
2006年 | 42篇 |
2005年 | 34篇 |
2004年 | 27篇 |
2003年 | 26篇 |
2002年 | 27篇 |
2001年 | 24篇 |
2000年 | 16篇 |
1999年 | 21篇 |
1998年 | 12篇 |
1997年 | 9篇 |
1996年 | 21篇 |
1995年 | 5篇 |
1994年 | 12篇 |
1993年 | 14篇 |
1992年 | 15篇 |
1991年 | 12篇 |
1990年 | 6篇 |
1989年 | 5篇 |
1988年 | 9篇 |
1987年 | 9篇 |
1986年 | 7篇 |
1985年 | 8篇 |
1984年 | 13篇 |
1983年 | 6篇 |
1982年 | 14篇 |
1981年 | 8篇 |
1980年 | 8篇 |
1979年 | 6篇 |
1978年 | 8篇 |
1976年 | 4篇 |
1973年 | 3篇 |
1972年 | 7篇 |
排序方式: 共有1159条查询结果,搜索用时 15 毫秒
1.
Semiparametric Regression in Size-Biased Sampling 总被引:1,自引:0,他引:1
Ying Qing Chen 《Biometrics》2010,66(1):149-158
Summary . Size-biased sampling arises when a positive-valued outcome variable is sampled with selection probability proportional to its size. In this article, we propose a semiparametric linear regression model to analyze size-biased outcomes. In our proposed model, the regression parameters of covariates are of major interest, while the distribution of random errors is unspecified. Under the proposed model, we discover that regression parameters are invariant regardless of size-biased sampling. Following this invariance property, we develop a simple estimation procedure for inferences. Our proposed methods are evaluated in simulation studies and applied to two real data analyses. 相似文献
2.
The experimental study of the relationship between biodiversity and ecosystem function has mainly addressed the effect of species and number of functional groups. In theory, this approach has mainly focused on how extinction affects function, whereas dispersal limitation of ecosystem function has been rarely discussed. A handful of seed introduction experiments, as well as numerous observations of the effects of long‐distance dispersal of alien species, indicate that ecosystem function may be strongly determined by dispersal limitation at the local, regional and/or global scales. We suggest that it is time to replace biodiversity manipulation experiments, based on random draw of species, with those addressing realistic scenarios of either extinction or dispersal. Experiments disentangling the dispersal limitation of ecosystem function should have to take into account the probability of arrival. The latter is defined as the probability that a propagule of a particular species will arrive at a particular community. Arrival probability depends on the dispersal ability and the number of propagules of a species, the distance a species needs to travel, and the permeability of the matrix landscape. Current databases, in particular those in northwestern and central Europe now enable robust estimation of arrival probability in plant communities. We suggest a general hypothesis claiming that dispersal limitation according to arrival probability will have ecosystem‐level effects different from those arising due to random arrival. This hypothesis may be rendered more region‐, landscape‐ or ecosystem‐specific by estimating arrival probabilities for different background conditions. 相似文献
3.
Tests for change-points with epidemic alternatives 总被引:1,自引:0,他引:1
4.
Summary Voltage-dependent K channels could be identified in on-cell and excised patch-clamp records on membranes of isolated plant cell vacuoles. The current through a membrane patch is dominated by a channel population with a conductance of about 121 pS in symmetrical 250mm KCl solution. The single channel adopts at least two conducting levels the 121-pS state being most frequently observed. The channel shows outward rectification, representing a cation flux into the vacuoles. The rectification appears to be caused by a vanishing open probability and a short channel lifetime at hyperpolarizing voltages. A selectivity ratio of potassium over sodium of about 6 was derived as an estimate. Occasionally, an additional population of K channels with a single-channel conductance of approximately 18 pS is observed. This channel type exhibits outward rectification as well. 相似文献
5.
Torsten Mattfeldt 《Journal of mathematical biology》1987,25(6):685-695
The absolute volume of biological objects is often estimated stereologically from an exhaustive set of systematic sections. The usual volume estimator
is the sum of the section contents times the distance between sections. For systematic sectioning with a random start, it has been recently shown that
is unbiased when m, the ratio between projected object length and section distance, is an integer number (Cruz-Orive 1985). As this quantity is no integer in the real world, we have explored the properties of
in the general and realistic situation m . The unbiasedness of
under appropriate sampling conditions is demonstrated for the arbitrary compact set in 3 dimensions by a rigorous proof. Exploration of further properties of
for the general triaxial ellipsoid leads to a new class of non-elementary real functions with common formal structure which we denote as np-functions. The relative mean square error (CE
2) of
in ellipsoids is an oscillating differentiable np-function, which reduces to the known result CE
2= 1/(5m
4) for integer m. As a biological example the absolute volumes of 10 left cardiac ventricles and their internal cavities were estimated from systematic sections. Monte Carlo simulation of replicated systematic sectioning is shown to be improved by using m instead of m . In agreement with the geometric model of ellipsoids with some added shape irregularities, mean empirical CE was proportional to m
–1.36 and m–1.73 in the cardiac ventricle and its cavity. The considerable variance reduction by systematic sectioning is shown to be a geometric realization of the principle of antithetic variates. 相似文献
6.
I. Hoeschele 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1988,76(2):311-319
Summary Statistical techniques for detection of major loci and for making inferences about major locus parameters such as genotypic frequencies, effects and gene action from field-collected data are presented. In field data, major genotypic effects are likely to be masked by a large number of environmental differences in addition to additive and nonadditive polygenic effects. A graphical technique and a procedure for discriminating among genetic hypotheses based on a mixed model accounting for all these factors are proposed. The methods are illustrated by using simulated data.Journal Paper No. J-12733 of the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa. Project No. 1901 相似文献
7.
Summary A Ca and potential-dependent K channel of large unit conductance was detected in the apical membrane of JTC-12.P3 cells, a continuous epithelial cell line of renal origin. The open probability of the channel is dependent on membrane potential and cytoplasmic-free Ca concentration. At cell-free configuration of the membrane patch, the open probability shows a bell-shaped behavior as function of membrane potential, which decreases at larger depolarization. With increasing Ca concentration, the width of the bell-shaped curve increases and the maximum shifts into the hyperpolarizing direction. For the first time the kinetics of this channel was analyzed under cell-attached conditions. In this case the kinetics could sufficiently be described by a simple open-closed behavior. The channel has an extremely small open probability at resting potential, which increases exponentially with depolarization. The low probability induces an uncertainty about the actual number of channels in the membrane patch. The number of channels is estimated by kinetic analysis. It is discussed that this K channel is essential for the repolarization of the membrane potential during electrogenic sodium-solute cotransport across the apical membrane. 相似文献
8.
Probabilistic models of the cell cycle maintain that cell generation time is a random variable given by some distribution function, and that the probability of cell division per unit time is a function only of cell age (and not, for instance, of cell size). Given the probability density, f(t), for time spent in the random compartment of the cell cycle, we derive a recursion relation for
n(x), the probability density for cell size at birth in a sample of cells in generation n. For the case of exponential growth of cells, the recursion relation has no steady-state solution. For the case of linear cell growth, we show that there exists a unique, globally asymptotically stable, steady-state birth size distribution,
*(x). For the special case of the transition probability model, we display
*(x) explicitly.This work was supported by the National Science Foundation under grants MCS8301104 (to J.J.T.) and MCS8300559 (to K.B.H.), and by the National Institutes of Health under grant GM27629 (to J.J.T.). 相似文献
9.
Estimating functionals of particle size distributions 总被引:1,自引:0,他引:1
10.
A multivariate t probability integral 总被引:2,自引:0,他引:2