共查询到20条相似文献，搜索用时 31 毫秒
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Mariola Moeyaert Maaike Ugille John M. Ferron S. Natasha Beretvas Wim Van den Noortgate 《Journal of Experimental Education》2016,84(3):473-509
The impact of misspecifying covariance matrices at the second and third levels of the three-level model is evaluated. Results indicate that ignoring existing covariance has no effect on the treatment effect estimate. In addition, the between-case variance estimates are unbiased when covariance is either modeled or ignored. If the research interest lies in the between-study variance estimate, including at least 30 studies is warranted. Modeling covariance does not result in less biased between-study variance estimates as the between-study covariance estimate is biased. When the research interest lies in the between-case covariance, the model including covariance results in unbiased between-case variance estimates. The three-level model appears to be less appropriate for estimating between-study variance if fewer than 30 studies are included. 相似文献
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Xue-cheng Wu Qin-hui Wang Chen Tian Zhong-yang Luo Meng-xiang Fang Ke-fa Cen 《浙江大学学报(A卷英文版)》2008,9(7):922-931
Particle rotation plays an important role in gas-solid flows. This paper presents an experimental investigation on the spatial distribution of average rotation speed for glass beads in the upper dilute zone of a cold circulating fluidized bed （CFB） riser. It is shown that in the horizontal direction, the average rotation speed in the near-wall area is larger than that in the center area, while in the vertical direction, it decreases as the height increases. The reason resulting in this distribution is analyzed by considering several factors including particle size, particle shape, particle number density, particle collision behavior, and the surrounding flow field, etc. The effects of CFB operation conditions on the spatial distribution of average rotation speed are also studied. The results show that the increasing superficial gas velocity increases the average rotation speed of particles in the near wall area but takes nearly no effect on that in the center area. The external solids mass flux, however, takes the opposite effect. It is found that the average rotation speeds of particles in both areas are increased as the total amount of bed material increases. 相似文献
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直接从随机变量的数学期望与方差的定义出发,运用组合恒等式,给出了超几何分布和负二项分布的数学期望与方差的求解过程.该方法简洁明了,容易理解和掌握. 相似文献
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姚宇新 《辽宁科技学院学报》2005,7(1):19-21
利用分形和分维数的基本概念,推导出粉碎最终产品的分维数.利用能耗与粉碎最终产品颗粒表面积相关的观点,推导出单位体积粉碎能耗的分维数.利用分形的自相似性,推导出能耗的无标度域与粉碎粒度分布指数的相关性. 相似文献
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首先给出一阶移动平均型式的自相关及其扰动项的均值、方差、协方差,并给出扰动项的协方差矩阵,Ω证明Ω是正定矩阵;然后由此推得回归模型Y=Xβ+μ中β的LS估计值■,给出了■的均值、方差,最后给出了σ2的无偏估计量■2及在正态分布的场合下■与■2的分布。 相似文献
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龙姝明 《陕西理工学院学报(社会科学版)》2003,21(6):27-30
实物粒子能量E大于势能的最大值时，粒子的运动状态为游离态，游离态的能量是连续的，处于游离态的粒子在势场的非均匀界面上，必有反射和透射现象发生。透射系数一般小于1，研究发现，当游离态粒子的入射能量E取一系列特殊离散(量子化)值，会出现完全透射现象(即透射系数T=1)，导出了粒子在周期势阱上完全透射时的能级公式。 相似文献
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Richard T. W. Arthur 《Science & Education》2012,21(10):1461-1473
In this paper it is argued that virtual processes are dispensable fictions. The argument proceeds by a comparison with the phenomenon of quantum tunnelling. Building on an analysis of Lévy-Leblond and Balibar, it is argued that, although the phenomenon known as quantum tunnelling certainly occurs and is at the basis of many paradigmatic quantum effects, the implied conceptualization of it as a free particle burrowing through a potential barrier is flawed. An alpha particle, for example, does not exist as a free particle inside a uranium nucleus and then ??burrow through?? the massive potential barrier of the repulsive Coulomb potential: rather, it can be interpreted as existing in a bound state which gives it a corresponding (absolutely tiny, but) finite probability of appearing on the other side of the barrier. If the part of the state function representing the transmission through the barrier is conceived as representing a particle trajectory, the particle will have imaginary momentum and negative kinetic energy. A similar analysis then applies to virtual processes. For example, if (as in Hawking??s conception of black hole radiation) one imagines a pair of particles created at the Schwarzschild radius, one of which drops into the black hole, at its creation that particle will have imaginary momentum and negative kinetic energy; so will the pion that is imagined as mediating the nuclear exchange force on the standard model. In each case, it is argued, the phenomenon can be understood in terms of a finite probability of transmission predicted by quantum theory, without appealing to particle trajectories. The idea that a particle ??penetrates?? a barrier that it does not have the energy to surmount, or that a pair of particles is ??virtually?? produced one on either side of the Schwarzschild radius, in defiance of energy conservation, should be discarded as unphysical. 相似文献
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Lattice Boltzmann method was used to numerically investigate the motion and orientation distribution of cylindrical and cubic particles in pipe flow with high concentration and high particle to pipe size ratio. The transient impulse model of 3D collisions between particles and between particle and wall is proposed. The numerical results are qualitatively in agreement with and quantitatively comparable to the experiment data. The results show that the increases of both the cylindrical particle to pipe size ratio and the particle aspect ratio decrease the rotation about all axes. All rotations of cubic particles decrease with increasing the particle concentration. The cubic particles, rotating more drastically in the flow with large Reynolds number, rotate faster than the cylindrical particles with the same size. The cylindrical particles align with the flow direction more obviously with decreasing Reynolds numbers. However, the orientations of cubic particles are spread all over the range with no significant difference in magnitude, and the Reynolds numbers have no obvious effect on the orientations of cubic particles. 相似文献
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正态总体方差和标准差的无偏估计 总被引：1，自引：0，他引：1
在正态总体分布下,给出了方差及标准差的矩估计量和极大似然估计量,讨论了两者之间的关系,得出两类估计量相同,并进一步给出无偏估计量。 相似文献
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龙姝明 《陕西理工学院学报(社会科学版)》1997,(6)
实物粒子能量E大于势能的最大值时，粒子的运动状态为游离态，游离态的能量是连续的．处于游离态的粒子在势场的非均匀界面（或点）上，必有反射和透射现象发生，而且一般地透射系数T小于1．笔者导出实物粒子以能量E（＞0）入射到一维势阱上时的完全透射条件，完全透射时，透射系数T＝1．我们称粒子在有限深势阱上的完全透射为共振散射，共振散射的能量也是量子化的． 相似文献
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刘谢进 《楚雄师范学院学报》2006,21(12):47-49
在给定的权回归模型下,讨论了最小二乘估计、最优加权最小二乘估计和线性无偏最小方差估计的性能比较,得出了在随机误差方差矩阵可逆条件下,可算出最优加权最小二乘估计与线性无偏最小方差估计误差方差阵的差表达式,并在一定条件下,两者趋于一致。 相似文献
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李汶龙 《湖南广播电视大学学报》2013,(2):16-24
人们对法治普遍怀有美好的期待,但法治究竟能够给人们提供什么,法治的实现是否就意味着某些人们珍视的价值的实现,人们对这些问题的答案是模糊且含混的。本文试图以人们对法治的期待为视角,对法治进行概念分析。首先提供一幅相对完整的期待系谱,这其中从"薄"到"厚"依次对"法治而非人治"的理念、形式法治以及实质法治铺陈开来,试图解释人们含混的期待背后的法治涵义的复杂性。并在此基础上探究如下问题:我们能否对什么样的期待是相对理性的这个问题达成共识?我们的期待在这个系谱中应该处于什么位置?满足了要求的法治究竟能给人们带来什么价值,或者其本身具有什么价值,以及我们"理性的期待"的具体内容是什么?最后,我们还需要对法治能在多大程度上实现作一番探讨,对理性期待的探索不仅需要对法治内涵展开分析,且对实践中可能遭遇到的困难也要有所重视。 相似文献
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建筑垃圾对城市雨水径流中Pb的吸附特性(英文) 总被引：2，自引：0，他引：2
以砖混建筑垃圾为研究对象,采用人工模拟雨水,通过静态和动态吸附实验研究了不同粒径粒级建筑垃圾对雨水径流中Pb的吸附效果.实验结果表明:准二级动力学模型比准一级动力学模型能更好地描述建筑垃圾对Pb的吸附过程;Freundlich等温模型能较好地拟合其等温吸附过程;不同粒径粒级建筑垃圾均对雨水径流中的Pb具有较好的净化效果,去除率高达99%,粒径粒级越小,对Pb的净化效果越好,但其渗透性能越差;建筑垃圾的粒径粒级对Pb的吸附平衡时间、吸附速率和吸附量具有重要影响,粒径2.36~4.75mm的建筑垃圾对Pb的平衡吸附速率和平衡吸附量最大,分别为18.1μg/min和5.5μg/g.上述研究结果为城市雨水径流中Pb污染的有效控制以及建筑垃圾资源化提供了科学依据. 相似文献
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WVD和XWVD的统计特征分析 总被引：4，自引：0，他引：4
1 Introduction TheWigner Villedistribution (WVD) [1 7] hasbeenshowntobeefficientintheestimationofinstanta neousfrequency (IF)ofasignal.BecauseWVDisofbettertime frequencyresolutionandabletolocateen ergybasedontheIFlaw ,especiallyforalinearfre quencymodulated(LFM )signalwithaconstantampli tude,WVDwillreducetoarowofdeltafunctions .ThereforeonecanobtainanunbiasedestimateofIFofthesignalusingthepeakofWVD .Butperformanceofthepeakestimatorisdegradedsignificantlywithesti mationvariance quic… 相似文献
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服从几何分布的多个独立离散型随机变量其最小值和最大值是一个含有多参数的离散型随机变量．本文证明了其最小值随机变量仍服从几何分布，并给出了最大值随机变量的概率函数、数学期望和方差． 相似文献
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有向网络是管理科学研究的重要工具.有向网络中,活动与结点均为随机变量,大量的研究表明,活动服从正态分布、β-分布或三角分布.在此基础上研究了活动服从正态分布的情况下节点的分布,并进一步求出了数学期望与方差. 相似文献
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E. A. Gaffan 《Learning & behavior》1992,20(3):240-252
The performance measures in many experiments on animal memory are expected to have an underlying binomial distribution, with additional variance contributed, for example, by between- subject differences. This paper examines whether the data from published studies of serial position effects (primacy and recency) in animals’ working memory conform to that expectation. In most cases, the variance, when it can be estimated, is consistent with those statistical assumptions, but in certain studies, it is significantly smaller than expected. This is usually a sign of faulty procedure or analysis, and possible causes are discussed. The conclusion is that much of the evidence for primacy in animals is unsatisfactory, on statistical or other methodological grounds. The analytic approach outlined here might usefully be applied to detect potential problems with other experiments of a similar type, especially when manually operated apparatus is employed, and to improve their statistical power. 相似文献