排序方式: 共有35条查询结果,搜索用时 15 毫秒
1.
Lina Vitali Fabio Monforti Roberto Bellasio Roberto Bianconi Valentina Sachero Sonia Mosca Gabriele Zanini 《Atmospheric environment (Oxford, England : 1994)》2006,40(40):8020-8033
In this paper the inert version of a Lagrangian particle model named photochemical Lagrangian particle model (PLPM) is described and validated. PLPM implements four density reconstruction algorithms based on the kernel density estimator. All these methods are fully grid-free but they differ each other in considering local or global features of the particles distribution, in treating the Cartesian directions separately or together and in being based on receptors or particles positions in space. Each kernel has been shown to have both advantages and disadvantages, but the overall good performances of the model when compared with the well known Copenhagen and Kincaid data sets are very encouraging in view of its extension to fully chemically active simulations, currently under development. 相似文献
2.
Frank C. Curriero Michael E. Hohn Andrew M. Liebhold Subhash R. Lele 《Environmental and Ecological Statistics》2002,9(1):89-110
Geostatistics is a set of statistical techniques that is increasingly used to characterize spatial dependence in spatially referenced ecological data. A common feature of geostatistics is predicting values at unsampled locations from nearby samples using the kriging algorithm. Modeling spatial dependence in sampled data is necessary before kriging and is usually accomplished with the variogram and its traditional estimator. Other types of estimators, known as non-ergodic estimators, have been used in ecological applications. Non-ergodic estimators were originally suggested as a method of choice when sampled data are preferentially located and exhibit a skewed frequency distribution. Preferentially located samples can occur, for example, when areas with high values are sampled more intensely than other areas. In earlier studies the visual appearance of variograms from traditional and non-ergodic estimators were compared. Here we evaluate the estimators' relative performance in prediction. We also show algebraically that a non-ergodic version of the variogram is equivalent to the traditional variogram estimator. Simulations, designed to investigate the effects of data skewness and preferential sampling on variogram estimation and kriging, showed the traditional variogram estimator outperforms the non-ergodic estimators under these conditions. We also analyzed data on carabid beetle abundance, which exhibited large-scale spatial variability (trend) and a skewed frequency distribution. Detrending data followed by robust estimation of the residual variogram is demonstrated to be a successful alternative to the non-ergodic approach. 相似文献
3.
The United States Environmental Protection Agency's Environmental Monitoring and Assessment Program (EMAP) is designed to
describe status, trends and spatial pattern of indicators of condition of the nation's ecological resources. The proposed
sampling design for EMAP is based on a triangular systematic grid and employs both variable probability and double sampling.
The Horvitz-Thompson estimator provides the foundation of the design-based estimation strategy used in EMAP. However, special
features of EMAP designed to accommodate the complexity of sampling environmental resources on a national scale require modifications
of standard variance estimation procedures as well as development of new techniques. An overview of variance estimation methods
proposed for application to EMAP's sampling strategy for discrete resources is presented. 相似文献
4.
Stehman Stephen V. Nshinyabakobeje Sophonie 《Environmental and Ecological Statistics》2000,7(3):301-321
Practical considerations often motivate employing variable probability sampling designs when estimating characteristics of forest populations. Three distribution function estimators, the Horvitz-Thompson estimator, a difference estimator, and a ratio estimator, are compared following variable probability sampling in which the inclusion probabilities are proportional to an auxiliary variable, X. Relative performance of the estimators is affected by several factors, including the distribution of the inclusion probabilities, the correlation () between X and the response Y, and the position along the distribution function being estimated. Both the ratio and difference estimators are superior to the Horvitz-Thompson estimator. The difference estimator gains better precision than the ratio estimator toward the upper portion of the distribution function, but the ratio estimator is superior toward the lower end of the distribution function. The point along the distribution function at which the difference estimator becomes more precise than the ratio estimator depends on the sampling design, as well as the coefficient of variation of X and . A simple confidence interval procedure provides close to nominal coverage for intervals constructed from both the difference and ratio estimators, with the exception that coverage may be poor for the lower tail of the distribution function when using the ratio estimator. 相似文献
5.
Russell?Alpizar-JaraEmail author Kenneth?H.?Pollock Dawn?E.?Haines 《Environmental and Ecological Statistics》2005,12(2):155-168
The combined mark-recapture and line transect sampling methodology proposed by Alpizar-Jara and Pollock [Journal of Environmental and Ecological Statistics, 3(4), 311–327, 1996; In Marine Mammal Survey and Assessment Methods Symposium. G.W. Garner, S.C. Amstrup, J.L. Laake, B.F.J. Manly, L.L. McDonald, and D.C. Robertson (Eds.), A.A. Balkema, Rotterdam, Netherlands, pp. 99–114, 1999] is used to illustrate the estimation of population size for populations with prominent nesting structures (i.e., bald eagle nests). In the context of a bald eagle population, the number of nests in a list frame corresponds to a pre-marked sample of nests, and an area frame corresponds to a set of transect strips that could be regularly monitored. Unlike previous methods based on dual frame methodology using the screening estimator [Haines and Pollock (Journal of Environmental and Ecological Statistics, 5, 245–256, 1998a; Survey Methodology, 24(1), 79–88, 1998b)], we no longer need to assume that the area frame is complete (i.e., all the nests in the sampled sites do not need to be seen). One may use line transect sampling to estimate the probability of detection in a sampled area. Combining information from list and area frames provides more efficient estimators than those obtained by using data from only one frame. We derive an estimator for detection probability and generalize the screening estimator. A simulation study is carried out to compare the performance of the Chapman modification of the Lincoln–Petersen estimator to the screening estimator. Simulation results show that although the Chapman estimator is generally less precise than the screening estimator, the latter can be severely biased in presence of uncertain detection. The screening estimator outperforms the Chapman estimator in terms of mean squared error when detection probability is near 1 wheareas the Chapman estimator outperforms the screening estimator when detection probability is lower than a certain threshold value depending on particular scenarios. 相似文献
6.
稳健变异函数在土壤污染物来源识别中的应用:以某重金属污染场地为例 总被引:1,自引:0,他引:1
空间特异值可以提供污染源的位置信息.以Genton稳健估计量为基础的变异函数为研究工具,对某重金属冶炼场地土壤中Cu、Zn、Pb、Cd、As、Se和Hg含量进行了空间特异值分析,并根据空间特异值的地理位置及土地利用方式等信息指示污染源.结果表明,7种元素变异函数曲线的理论模型符合球状模型,7种元素都具有较小的块金方差,Cu、Zn和Pb只为0.000.与Matheron估计量相比,该研究得到的变异函数具有更小的块金效应,说明Genton估计量能够消除或减弱特异值对变异函数的影响.7种元素的空间特异值多集中在个别小范围内,以簇状形式存在,与该区域冶炼高炉及尾渣堆放处的位置具有很好的相关性,说明冶炼行为及其相关过程是该区重金属的重要来源. 相似文献
7.
Environmental and Ecological Statistics - The theory underlying line transect and variable circular plot surveys—distance sampling—begins with an assumed detectability function, giving... 相似文献
8.
Walter W. Piegorsch 《Environmental and Ecological Statistics》1994,1(2):153-162
Hierarchical models are considered for estimating the probability of agreement between two outcomes or endpoints from an environmental
toxicity experiment. Emphasis is placed on generalized regression models, under which the prior mean is related to a linear
combination of explanatory variables via a monotone function. This function defines the scale over which the systematic effects
are modelled as additive. Specific illustration is provided for the logistic link function. The hierarchical model employs
a conjugate beta prior that leads to parametric empirical Bayes estimators of the individual agreement parameters. An example
from environmental carcinogenesis illustrates the methods, with motivation derived from estimation of the concordance between
two species carcinogenicity outcomes. Based on a large database of carcinogenicity studies, the inter-species concordance
is seen to be reasonably informative, i.e. in the range 67–84%. Stratification into pertinent potency-related sub-groups via
the logistic model is seen to improve concordance estimation: for environmental stimuli at the extremes of the potency spectrum,
concordance can reach well above 90%. 相似文献
9.
Estimators for the population sizes of animal species are similar to Horvitz-Thompson estimators — they involve dividing counts of detected animals by the probabilities of detection. Knowing detection probabilities for different subpopulations allows one to estimate each sub-population size with such an estimator, and to add the results for an estimator of the total population. In the case where the proportions of animals belonging to the different sub-populations are also known, this paper shows that using those proportions to arrive at a common average detection probability will, when used in conjunction with the total number of animals detected, result in a better estimator. We provide two examples where the inferior estimator may seem sensible. 相似文献
10.
Hosung Ahn 《Journal of the American Water Resources Association》1998,34(3):583-593
ABSTRACT: The total phosphorous (TP) concentrations in the South Florida rainfall have been recorded in weekly intervals with a detection limit (DL) of 3.5 μg/L. As a large amount of the data is reported as below the DL, appropriate statistical methods are needed for data analysis. Thus, an attempt was made to identify an appropriate method to estimate the mean and variance of the data. In particular, a method to separate the statistics for the below DL portion from the estimated population statistics is proposed. The estimated statistics of the censored data are compared with the statistics of the uncensored data available from the recent years’ laboratory records. It was found that the one-step restricted maximum likelihood method is the most accurate for the wet TP data, and that the proposed method to combine the estimated statistics for TP < DL portion and the sample statistics for TP ≥ DL portion improves estimates compared to the conventional maximum likelihood estimates. 相似文献