Gary K. Grunwald  Richard H. Jones 《Environmetrics》2000,11(3):327-339

We consider modelling time series of amounts which may be zero using a stochastic first‐order Markov model with mixed transition density having a discrete component at 0 and a continuous component describing non‐zero amounts. The models extend chain‐dependent stochastic models in the literature on modelling rainfall. Under certain assumptions the Markov chain likelihood can be factored to allow model parameters to be estimated by maximum likelihood using standard Generalized Linear Models methods and software. The results give estimates of seasonal patterns in mean amounts and probability distributions of amounts. We illustrate with 30 years of daily rainfall data from Melbourne, Australia. Copyright © 2000 John Wiley & Sons, Ltd.  相似文献   

    

James E. Dunn 《Environmetrics》1999,10(5):531-546

Two case studies are presented in which vector‐valued, generalized additive models (VGAM) have played pivotal roles. In Case 1, each subject was sequentially exposed via face mask to five concentrations of butanol, with 60 s allowed between exposures to record sensations on six response scales (overall and odor intensity, eye, nose and throat irritation, and pleasantness). This was repeated immediately for a second replication, and then repeated twice again using five concentrations of pentanone. Preliminary data analysis indicated both a lack of trend in response with increasing exposure concentration and poor reproducibility between replications. Both problems were resolved by introducing two covariates, namely optimally scaled, monotone transforms of the previously recorded overall and odor intensity scores. This suggested that either (1) the mechanical system was not zeroing between exposure levels, or (more likely) (2) subjects retained either a residual of the actual stimulus or sensations of the stimulus from the previous exposure. This case illustrates that optimal transforms need not always be identified in terms of known functions. Case 2 is based on molecular structures of dioxin and furan, each of which admits the presence of 1–8 chlorine atoms. Measurements of these 16 homologues were reported for 24 test runs when burning refuse derived fuel (RDF) under varying conditions of HCl, coal, and lime injection. VGAM results, based on log‐ratios of homologue composition, suggested that the effects of [SO₂] and [SO₂]/[HCl] were implemented through Michaelis–Menten kinetics. All‐possible‐subsets selection among VGAMs was crucial to model selection, and is illustrated. Empirical transforms were sufficient for simulations under alternative test conditions. Copyright © 1999 John Wiley & Sons, Ltd.  相似文献   

    

Bruce Smith  Etsuo Miyaoka 《Environmetrics》1999,10(5):575-587

A seiche is a resonant response of a lake or harbour to external forces. Frequency domain methods are used in the identification of seiche like characteristics of sea‐level records collected at Sydney and Halifax harbours, Nova Scotia. The data exhibit structures predicted by simple physical seiche models, and there is some evidence that these seiches are responses to tidal forcing. Copyright © 1999 John Wiley & Sons, Ltd.  相似文献   

    

S. Hussain  A. Elbergali  A. Al‐Masri  G. Shukur 《Environmetrics》2004,15(2):155-171

Detecting and estimating long‐range dependence are important in the analysis of many environmental time series. This article proposes a periodogram roughness (PR) estimator and describes its uses for testing and estimating the dependence structure. Asymptotic critical values are generated for performing the test, and special attention is given to investigating the properties of the PR regarding size and power. The conventional short‐memory models, such as the autoregressive (AR), are shown to be less parsimonious. Forecasting errors of both fractional Gaussian noise (FGN) and fractional autoregressive moving average (FARMA) are investigated by conducting simulation studies. In addition to the PR, maximum likelihood (ML) and semi‐parametric (SP) estimators are used and evaluated. Our results have shown that more accurate forecasted points are obtained when using the fractional forecasting. The methods are illustrated using Swedish wind speed data. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

    

Chang‐Tai Chao 《Environmetrics》2004,15(8):757-775

For the prediction problem in survey sampling under a finite population, n sampling units are selected out of N population units and observed to predict the population quantity of interest. For a correlated spatial population, one can obtain lower prediction mean‐square error with careful sampling arrangement of the sampling sites. For example, a systematic design can be used to select samples for better prediction results. However, it is only effective under certain population covariance structures. For more general cases, the optimal sampling strategies proposed by different authors in the past can be used to select the optimal sample with which the mean‐square error can be minimized. Nevertheless, the computational load can be very intensive, and also the optimization algorithm used is not easy to implement. In addition, the exact population distribution has to be assumed. Two sampling methods, that are based on the eigensystem of the population covariance matrix, are proposed in this article. These sampling methods require fewer population assumptions and the sampling procedures are straightforward. No computationally intensive algorithm is required. Simulation study shows that they can be more efficient than simple random sampling. An example on the utilization of the proposed sampling methods in practice is also presented. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

    

Sourabh Bhattacharya 《Environmetrics》2006,17(7):763-776

Reconstruction of past environment based on fossil data is an important scientific problem and much work has been done to address this. One important issue in this regard is the relationship between species and environment. The available statistical approaches, both classical and Bayesian, make the assumption that species abundances occur symmetrically around a single preferable environmental condition. In other words, it is generally assumed that the relationship between species and environment is unimodal. However, this may not be true in reality. In this paper, we propose a Bayesian hierarchical model, extending previous work, to incorporate multiple preferences (or, multiple modes) of each species. In particular, we propose to use a mixture of Gaussian response curves to relate species to environment. A main novelty of our approach is the use of Dirichlet process to learn about the number of preferences of each species. We demonstrate that our multimodal modeling approach performs better than unimodal model. For implementation of our approach, we use a recently developed efficient computational procedure. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

    

Luis Guillermo Coca Velarde  Hlio S. Migon  Basilio de B. Pereira 《Environmetrics》2004,15(6):561-576

Climate variables assume non‐negative values and are often measured as zero. This is just the case when the rainfall level, in the dry season, is measured in a specified place. Then, the stochastic modeling demands the inclusion of a probability mass point at the zero level, and the resulting model is a mixture of a continuous and a Bernoulli distribution. In this article, spatial conditional autoregressive effects dealing with the idea that neighbors present similar responses is considered and the response level is modeled in two stages. The aim is to consider spatial interpolation and prediction of levels in a Bayesian context. Data on weekly rainfall levels measured in different stations at the central region of Brazil, an area with two well‐marked seasons, will be used as an example. A method for comparing models, based on the deviance function, is also implemented. The main conclusion is that the use of space–time models improves the modeling of hydrological and climatological variables, allowing the inclusion of real life considerations such as the influence of other covariates, space dependence and time effects such as seasonality. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

    

Peter Rothery  Suzanne J. Clark  Joe N. Perry 《Environmetrics》2003,14(7):711-717

The Farm‐Scale Evaluations (FSE) is a three‐year experiment to study the effects on farmland wildlife of the commercial management of four genetically modified herbicide‐tolerant (GMHT) crops—beet, maize, spring oil‐seed rape and winter oil‐seed rape. The experiment records abundance of a wide range of weed and invertebrates in the arable system. The FSE is a paired comparison arranged as a randomized block design with two treatments (GMHT and conventional crop management) and fields as blocks. The choice of the half‐field as the experimental unit is based on field availability, the primary focus on the vegetation, the need to reduce variability and gains in sampling effort. Farms and fields are selected to reflect the range of geographical variation and farming intensity for each crop. A power analysis shows that using about 60 fields per crop over the three years will be sufficient to detect differences of ecological significance. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

    

L. Barabesi  L. Fattorini  M. Marcheselli  C. Pisani  L. Pratelli 《Environmetrics》2015,26(3):202-215

The estimation of diversity indexes is considered when species abundance is estimated by means of plots, points or transects placed onto the study region in accordance with probabilistic schemes. Under uniform random sampling–that is when plots, points or transects are uniformly and independently located on the study region–the consistency and the large‐sample normality of diversity index estimators follow from the usual limit theorems as the sampling effort increases. In addition, variance estimation and finite‐sample bias reduction are achieved by means of standard results on the jackknife method. Despite its theoretical simplicity, uniform random sampling may lead to uneven coverage of the study region. In order to avoid this shortcoming, the use of one‐per‐stratum stratified sampling is usually considered. This sampling scheme involves partitioning the study region into subsets of equal size and uniformly selecting a plot, a point or a transect in each of these subsets. The present paper aims to show the consistency and the large‐sample normality of the diversity index estimators under one‐per‐stratum stratified sampling, as well as to deal with variance estimation and bias reduction. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

    

Fontina Petrakopoulou  Diego Iribarren  Javier Dufour 《Greenhouse Gases: Science and Technology》2015,5(3):268-276

CO₂ capture and storage involves technologies that separate, capture, and store CO₂ from large facilities, such as fossil fuel power plants. Although it is a promising measure to meet environmental standards on carbon pollution, proposed technologies in power plants are energy demanding and decrease the energy generated per unit of input fuel when compared to business‐as‐usual scenarios. In this paper, we evaluate the environmental performance of two similarly structured combined‐cycle power plants with pre‐combustion capture. The first power plant performs methane steam reforming in an autothermal reformer, while the second plant uses a reactor that includes a hydrogen‐separating membrane. The two plants are compared both to one another and to a business‐as‐usual scenario using six environmental impact potentials (abiotic depletion, global warming, ozone layer depletion, photochemical oxidant formation, acidification, and eutrophication). The goal is to pinpoint environmental weaknesses and strengths of the two capture technologies. We find that the two plants result in similar impacts, decreasing the contribution to global warming of conventional operation but, at the same time, increasing other impacts, such as ozone layer depletion and photochemical oxidant formation. Additionally, the two capture plants result in higher cumulative non‐renewable and total energy demands, as well as in lower life‐cycle energy balances and efficiencies. The most direct measure to decrease the environmental impacts of the examined techniques would be to increase their efficiency, by decreasing the requirements of the processes in natural and energy resources.© 2014 Society of Chemical Industry and John Wiley & Sons, Ltd  相似文献