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181.
182.
A copula-based closed-form binary logit choice model for accommodating spatial correlation across observational units 总被引:4,自引:2,他引:2
This study focuses on accommodating spatial dependency in data indexed by geographic location. In particular, the emphasis
is on accommodating spatial error correlation across observational units in binary discrete choice models. We propose a copula-based
approach to spatial dependence modeling based on a spatial logit structure rather than a spatial probit structure. In this
approach, the dependence between the logistic error terms of different observational units is directly accommodated using
a multivariate logistic distribution based on the Farlie-Gumbel-Morgenstein (FGM) copula. The approach represents a simple
and powerful technique that results in a closed-form analytic expression for the joint probability of choice across observational
units, and is straightforward to apply using a standard and direct maximum likelihood inference procedure. There is no simulation
machinery involved, leading to substantial computation gains relative to current methods to address spatial correlation. The
approach is applied to teenagers’ physical activity participation levels, a subject of considerable interest in the public
health, transportation, sociology, and adolescence development fields. The results indicate that failing to accommodate heteroscedasticity
and spatial correlation can lead to inconsistent and inefficient parameter estimates, as well as incorrect conclusions regarding
the elasticity effects of exogenous variables.
相似文献
Ipek N. SenerEmail: |
183.
西格陵兰位于北极—北大西洋地区,其海洋环境演变受西格陵兰洋流中冷、暖水团的显著影响。本文运用主成分分析方法对西格陵兰Disko湾DA06-139G孔沉积物中主要硅藻属种进行研究,试图揭示5000a BP以来西格陵兰洋流中冷、暖水团强度变化对研究区海洋环境的影响。5000—3800a BP期间,东格陵兰寒流携带的极地冷水团强度较弱,而北大西洋暖水团势力较强,Disko湾地区海洋环境较为温暖。3800—2000a BP期间,西格陵兰洋流中极地冷水团势力呈现阶段性增强,此时Disko湾地区气候缓慢转冷,海冰覆盖面积逐渐增加。2000a BP以后,特别是650a BP之后,样品在主成分轴一上的得分显著增高,表明Disko湾气候明显变冷,来自东格陵兰寒流的极地冷水团强度显著增强。Disko湾中晚全新世以来的硅藻记录及样品在主成分轴一上的得分所指示的极地冷水团的强度变化与格陵兰冰芯温度及海冰变化等具有较好的一致性,说明研究区海流特征与大气温度、海冰等环境要素密切相关。 相似文献
184.
长江三角洲末次盛冰期的气候波动 总被引:2,自引:0,他引:2
通过对长江三角洲东部地区鹤鸣孔第一硬黏土层的孢粉、藻类分析,研究了该地区末次盛冰期的气候波动。结果表明,鹤鸣孔第一硬黏土层含种类较丰富的孢子、花粉和藻类化石,并可以划分为3个孢粉、藻类组合,代表了该地区末次盛冰期气候变化的3个阶段,反映了该地区末次盛冰期并不是一个持续的干冷状况,第一硬黏土层沉积的早期和晚期气候较温暖湿润,针阔叶混交林发育,而中期阶段气候偏冷,植被贫乏。 相似文献
185.
Ecosystems have been viewed both as chaotic, untamed nature, and as mechanical systems with predictable equilibrium states. A developing concept of ecosystems as “complex adaptive systems” lies between these extreme concepts, with recognizably patterned but not fully predictable behavior. Sustainability has also been redefined as humans have exploited and often depleted desirable natural resources. Fisheries management desires sustainable yield, but must rethink this concept within the ecosystem context. The most powerful union of “ecosystem” and “sustainability” acknowledges the defining characteristics of complex adaptive systems with the objective of identifying and sustaining healthy relationships within and between ecosystems, economies, and society. 相似文献
186.
Michael R. Landry Susan L. Brown Yoshimi M. Rii Karen E. Selph Robert R. Bidigare Eun Jin Yang Melinda P. Simmons 《Deep Sea Research Part II: Topical Studies in Oceanography》2008,55(10-13):1348
As part of E-Flux III cruise studies in March 2005, we investigated phytoplankton community dynamics in a cyclonic cold-core eddy (Cyclone Opal) in the lee of the Hawaiian Islands. Experimental incubations were conducted under in situ temperature and light conditions on a drift array using a two-treatment dilution technique. Taxon-specific estimates of growth, grazing and production rates were obtained from analyses of incubation results based on phytoplankton pigments, flow cytometry and microscopy. Cyclone Opal was sampled at a biologically and physically mature state, with an 80–100 m doming of isopycnal surfaces in its central region and a deep biomass maximum of large diatoms. Depth-profile experimentation defined three main zones. The upper (mixed) zone (0–40 m), showed little compositional or biomass response to eddy nutrient enrichment, but growth, grazing and production rates were significantly enhanced in this layer relative to the ambient community outside of the eddy. Prochlorococcus spp. dominated the upper mixed layer, accounting for 50–60% of its estimated primary production both inside and outside of Opal. In contrast, the deep zone of 70–90 m showed little evidence of growth rate enhancement and was principally defined by a 100-fold increase of large (>20-μm) diatoms and a shift from Prochlorococcus to diatom dominance (80%) of production. The intermediate layer of 50–60 m marked the transition between the upper and lower extremes but also contained an elevated biomass of physiologically unhealthy diatoms with significantly depressed growth rates and proportionately greater grazing losses relative to diatoms above or below. Microzooplankton grazers consumed 58%, 65% and 55%, respectively, of the production of diatoms, Prochlorococcus and the total phytoplankton community in Cyclone Opal. The substantial grazing impact on diatoms suggests that efficient recycling was the major primary fate of diatom organic production, consistent with the low export fluxes and selective export of biogenic silica, as empty diatom frustules, in Cyclone Opal. 相似文献
187.
S. Mischke U. Herzschuh C. Zhang J. Bloemendal F. Riedel 《Global and Planetary Change》2005,46(1-4):337
Two sediment cores (length 13.94 and 12.93 m) have been drilled from the small alpine Lake Luanhaizi in the eastern central Qilian Mountains and correlated by means of magnetic susceptibility (MS). This paper focuses on the lithology and chronology of the longer core, on the results of loss on ignition (LOI), element concentration, thermomagnetic as well as magnetic hysteresis loop measurements, and on the ostracod record.The recovered sediments represent three types of depositional environment: a shallow intermittent lake, a deeper permanent lake and a true playa lake. Three stages of a higher lake level and permanence of the water body are reconstructed. The lowermost stage of a permanent lake and inferred favourable environmental conditions occurred probably about 45 14C ka BP. The second stage of a deeper permanent lake occurred either shortly before or, more likely, following the LGM. Most favourable environmental conditions and highest water levels were reconstructed for the uppermost stage comprising the Holocene. Considering the lake record, glaciers have not reached the lake site at 3200 m altitude during the LGM, providing further evidence against a large ice sheet on the Tibetan Plateau. 相似文献
188.
Benony K. Kortatsi 《Aquatic Geochemistry》2007,13(1):41-74
Hydrochemical and stable isotope (18O and 2H) analyses of groundwater samples were used to establish the hydrochemistry of groundwater in the Ankobra Basin. The groundwater
was generally mildly acidic, low in conductivity and undersaturated with respect to carbonate phases. Major ions except bicarbonate
were low and dissolved silica was moderately high. Silicate minerals weathering is probably the main process through which
major ions enter the groundwater. Groundwater samples clustered tightly along the Global Meteoric Water Line suggesting integrative,
smooth and rapid recharge from meteoric origin. The majority of the boreholes and a few hand dug wells cluster towards the
Ca–Mg–HCO3 dominant section of the phase diagram, in conformity with the active recharge and short residence time shown by the isotope
data. Aluminium, arsenic, manganese, iron and mercury were the only trace metals analysed with concentrations significantly
above their respective detection limits. Approximately 20%, 5%, 40% and 25% respectively of boreholes had aluminium, arsenic,
iron and manganese concentrations exceeding the respective WHO maximum acceptable limits for drinking water. The relatively
large percentage of boreholes with high concentration of aluminium reflects the acidic nature of the groundwater. 相似文献
189.
Takamitsu Yamanaka Yutaka Komatsu Hironori Nomori 《Physics and Chemistry of Minerals》2007,34(5):307-318
Many of ilmenites ABO3 compounds bearing transition elements have semiconductive, ferroelectric and antiferromagnetic properties. The high-pressure
diffraction studies of FeTiO3 have been conducted up to 8.2 GPa using synchrotron radiation in KEK at Tsukuba with diamond anvil cell. The compression
mechanism of FeTiO3 ilmenite has been investigated by the structure refinements converged to the reliable factors R = 0.05. The deformations of the FeO6 and TiO6 octahedra were reduced with increasing pressure. In order to elucidate the electric conductivity change with pressure, electron
density distribution of ilmenite have been executed by maximum entropy method (MEM) using single-crystal diffraction intensity
data. MEM based on F
obs(hkl) of FeTiO3 clearly shows electron density in comparison with the difference Fourier synthesis based on F
obs(hkl) − F
calc(hkl). The radial distribution of the electron density indicates electron localization around the cation positions. The bonding
electron density found in bond Fe–O and Ti–O is lowered with pressure. The isotropic temperature factors B
iso become smaller with increasing pressure. Nevertheless the thermal vibration is considerably restrained by the compression,
the electric conductivity is enhanced with pressure. Neither charge transfer nor electron hopping between Fe and Ti along
the c axis in FeTiO3 is plausible under high pressure. But the electric conductivity due to electron super-exchange in Fe–Fe and Ti–Ti has been
clarified by the MEM electron density distribution. The anisotropy in the electric conductivity has been clarified. 相似文献
190.
神秘的109.4°——共轭变形带的夹角 总被引:5,自引:2,他引:5
塑性力学的滑移线理论、Watterson零伸长度理论和最大有效力矩准则均获得共轭变形带的夹角为109.4°.该值与黄金规则相容,然而,滑移线理论的预测值面对伸长方向,与实际不符.零伸长度理论所预测的109.4°,不能解释实际观察到的平面共轭剪切带.根据最大有效力矩准则理论,预测韧性变形域共轭变形带面对主压应力方向或瞬时最小伸长度方向的夹角为109.4°.迄今获得的全部野外观测值和岩石力学实验结果均位于该预测值的±20°范围内,证明最大有效力矩准则的有效性.最大有效力矩准则可解释或求解:1)折劈理的形成,2)大型低角度正断层和高角度逆冲断层的形成,3)地震反射剖面中的鳄鱼嘴构造,4)变质结晶基底的基本构造型式--菱网状韧性剪切带,5)拆离褶皱的形成,6)古主应力和相关的运动学涡度. 相似文献