共查询到20条相似文献，搜索用时 62 毫秒
1.
David Fletcher Malcolm Faddy 《Journal of Agricultural, Biological & Environmental Statistics》2007,12(3):315-324
In many ecological research studies, abundance data are skewed and contain more zeros than might be expected. Often, the aim
is to model abundance in terms of covariates, and to estimate expected abundance for a given set of covariate values. An approach
that has been advocated recently involves the use of a conditional model. This allows one to separately model presence and abundance given presence, which should lead to a more complete understanding as to how the covariates influence abundance. The focus of this article
is on the calculation of confidence intervals for expected abundance given particular values of the covariates. The standard
Wald confidence interval is symmetric, and therefore unlikely to be of much use for skewed data, where reliable confidence
intervals for abundance will generally be asymmetric. The purpose of this article is to show how to calculate a profile likelihood
confidence interval for expected abundance using a conditional model. 相似文献
2.
David Fletcher Daniel Turek 《Journal of Agricultural, Biological & Environmental Statistics》2012,17(1):38-51
Model-averaging is commonly used as a means of allowing for model uncertainty in parameter estimation. In the frequentist
framework, a model-averaged estimate of a parameter is the weighted mean of the estimates from each of the candidate models,
the weights typically being chosen using an information criterion. Current methods for calculating a model-averaged confidence
interval assume approximate normality of the model-averaged estimate, i.e., they are Wald intervals. As in the single-model
setting, we might improve the coverage performance of this interval by a one-to-one transformation of the parameter, obtaining
a Wald interval, and then back-transforming the endpoints. However, a transformation that works in the single-model setting
may not when model-averaging, due to the weighting and the need to estimate the weights. In the single-model setting, a natural
alternative is to use a profile likelihood interval, which generally provides better coverage than a Wald interval. We propose
a method for model-averaging a set of single-model profile likelihood intervals, making use of the link between profile likelihood
intervals and Bayesian credible intervals. We illustrate its use in an example involving negative binomial regression, and
perform two simulation studies to compare its coverage properties with the existing Wald intervals. 相似文献
3.
Group testing, in which units are pooled together and tested as a group for the presence of an attribute, has been used in
many fields of study, including blood testing, plant disease assessment, fisheries, and vector transmission of viruses. When
groups are of unequal size, complications arise in the derivation of confidence intervals for the proportion of units in the
population with the attribute. We evaluate several asymptotic interval estimation methods for problems in which groups are
of different size. Each method is examined for its theoretical properties, and adapted or developed for group testing. In
an initial assessment using a study of virus prevalence in carnations, four methods are found to be satisfactory, and are
considered further—two based on the distribution of the MLE, one on the score statistic, and one on the likelihood ratio.
The performance of each method is then tested empirically on five realistic group testing procedures, with the evaluation
focusing on the coverage probability provided by the confidence intervals. A method based on the score statistic with a correction
for skewness is recommended, followed by a method in which the logit function is applied to the MLE. 相似文献
4.
Heritability quantifies the extent to which a physical characteristic is passed from one generation to the next. From a statistical perspective, heritability is the proportion of the phenotypic variance attributable to (additive) genetic effects and is equal to a function of variance components in linear mixed models. Relying on normal distribution assumptions, one can compute exact confidence intervals for heritability using a pivotal quantity procedure. Alternatively, large-sample properties of the restricted maximum likelihood (REML) estimator can be used to construct asymptotic confidence intervals for heritability. Exact and asymptotic intervals are compared loineye muscle area measurements and balanced one-way random effects models having groups of correlated responses. In some cases the two interval methods yield vastly different results and the REML-based confidence interval does not maintain the nomiral coverate value even for seemingly large sample sizes. For finite sample size applications, the validity of the REML-based procedure depends on the correlation structure of the data. 相似文献
5.
Exact confidence intervals for variance components in linear mixed models rely heavily on normal distribution assumptions. If the random effects in the model are not normally distributed, then the true coverage probabilities of these conventional intervals may be erratic. In this paper we examine the performance of nonparametric bootstrap confidence intervals based on restricted maximum likelihood (REML) estimators. Asymptotic theory suggests that these intervals will achieve the nominal coverage value as the sample size increases. Incorporating a small-sample adjustment term in the bootstrap confidence interval construction process improves the performance of these intervals for small to intermediate sample sizes. Simulation studies suggest that the bootstrap standard method (with a transformation) and the bootstrap bias-corrected and accelerated (BC a ) method produce confidence intervals that have good coverage probabilities under a variety of distribution assumptions. For an interlaboratory comparison of mercury concentration in oyster tissue, a balanced one-way random effects model is used to quantify the proportion of the variation in mercury concentration that can be attributed to the laboratories. In this application the exact confidence interval using normal distribution theory produces misleading results and inferences based on nonparametric bootstrap procedures are more appropriate. 相似文献
6.
Tsai-Yu Lin Chen-Tuo Liao Hari K. Iyer 《Journal of Agricultural, Biological & Environmental Statistics》2008,13(2):221-241
Tolerance intervals are useful in practice to help determine limits for detection monitoring or assessment monitoring of factors that may impact the environment, ecological systems, or other biological processes. This article provides a procedure for construction of one-sided and two-sided tolerance intervals for a normally distributed random variable when the mean and variance of its distribution are estimated using data following an unbalanced one-way random effects model with covariates under heterogeneous error variances. The procedure developed here is based on the concept of a generalized pivotal quantity which has been frequently used to obtain confidence intervals in situations where conventional methods are difficult to apply or fail to provide s satisfactory solutions. For the one-sided case, the generalized pivotal quantity approach yields an exact solution. On the other hand, the method leads to good approximate intervals for the two-sided case. This is confirmed by a detailed simulation study, showing that the method may be recommended for practical use. Two real-data examples are given to illustrate the applicability of the proposed procedures. 相似文献
7.
In a capture-recapture analysis, uncertainty in the parameter estimates is usually expressed by presenting classical Wald-type
confidence intervals. This approach involves (1) the assumption that the maximum likelihood estimates are asymptotically normal
and (2) numerical computation of the variance-covariance matrix of these estimates. When the sample size is small or when
the estimates are on the boundary of their domain, a Wald confidence interval often performs badly. A natural alternative
is to use profile-likelihood confidence intervals. In general, these intervals require a greater amount of computation. We
propose a new implementation of this approach that is efficient, both in reducing the amount of computation and in coping
with boundary estimates. We also show how profile-likelihood confidence intervals can be adjusted for overdispersion. Simulations
were used to check whether nominal coverage levels were attained, and allowed us to compare this approach with the classical
Wald procedure. We illustrate this work by considering a multi-state model for a sooty shearwater (Puffinus griseus) population. 相似文献
8.
9.
Biodiversity determination at any organization level is a heuristic tool in environmental management. The Shannon biodiversity
index stands out among the various available indexes that combine both components of the concept: species number and their
relative abundance. Whatever the index used, an aspect that has received little attention in the study of biodiversity is
the uncertainty quantification, precluding hypothesis testing. Applying standard bootstrap methods to estimate the index allows
us to build confidence bounds, but it yields underestimated indexes due to unsampled species. This article explores the empirical
mean coverage of confidence intervals for the Shannon biodiversity index when the total number of species in the sample itself
is subject to random variation. I analyze their dependence on evenness, total abundance, and number of species within the
community. Very low empirical coverages of bootstrap confidence intervals were obtained using the standard and bias-corrected
techniques. I propose a new technique using the difference of the original sample and each bootstrap replication to build
an empirical, adjustment for the bias, and get an adjusted point estimator and corresponding confidence interval. An example
based on field data, is considered. 相似文献
10.
A statistical approach for predicting accuracies of soil properties measured by single, double and dual gamma beams 总被引：2，自引：0，他引：2
A statistical analysis is developed to describe the accuracies of soil properties measured through single, double and dual gamma-beam methods. The gamma sources are 137Cs for the high-energy beam (662 keV) and 241Am for the low-energy beam (60 keV). The theoretical confidence intervals of the thicknesses or water contents of samples are determined using a modification of the method of Gardner el al. (1972), who derived theoretical variances using standard error propagation formulae. The finding of Gardner et al. that, in the double-beam method, errors in the soil bulk density and water content measurements are primarily caused by random emission from the sources, is confirmed by the theoretical confidence intervals and is applied to the single-beam and dual-beam methods. The effects of interferences between gamma rays in the dual-beam method on the accuracies of estimations are examined. The minimum measurement time to attain a required accuracy can be predicted from the theoretical confidence intervals. 相似文献
11.
Marked fish placed above a smolt trap allow fisheries biologists to estimate the efficiency of the trap and hence to estimate
the number of smolts migrating past the trap. We compared stratified maximum likelihood estimators and confidence intervals
of total numbers of fish migrating to two other well-known estimators and associated confidence intervals using simulation.
The Bailey adjusted Lincoln-Petersen estimator and its bootstrap confidence interval performed well overall. 相似文献
12.
When appropriate data from regional reference locations are available, tolerance-interval bounds can be computed to provide criteria or limits distinguishing reference from nonreference conditions. If the limits are to be to applied to locations and times beyond the original data, the data should include temporal and spatial variation and the tolerance interval calculations should utilize a random crossed or nested ANOVA statistical design. Two computational methods for such designs are discussed and evaluated with simulations. Both methods are shown to perform well, and the adverse effect of using an improper design model is demonstrated. Three real-world applications are shown, where tolerance intervals are used to (1) establish a reference threshold for a benthic community pollution index, (2) set criteria for chemicals in sediments, and (3) establish background thresholds for survival rates in sediment bioassay tests. Some practical considerations in the use of the tolerance intervals are discussed. 相似文献
13.
Gary Jackson Yuk W. Cheng 《Journal of Agricultural, Biological & Environmental Statistics》2001,6(2):243-257
Snapper (Pagrusauratus) stocks found inside Shark Bay, Western Australia, are the basis of important recreational fisheries. An evaluation of the daily egg production method for annual snapper stock assessment commenced in 1997. Initial spawning biomass estimates were inconclusive due to wide confidence intervals around egg production in particular. In 1998, more comprehensive surveys were conducted in an effort to improve the accuracy and precision of the estimated parameters. Egg survey design used information on the location of the main spawning areas and spawning times identified in 1997 and employed a systematic stratified adaptive sampling approach. Further refinements to the daily egg production method included use of nonlinear regression and nonparametric bootstrapping techniques. Results are discussed in relation to the precision of parameter estimates, with reference to egg production in particular, and the appropriateness of analytical methods used. 相似文献
14.
Mauricio Parra-Quijano José M. Iriondo Elena Torres 《Genetic Resources and Crop Evolution》2012,59(2):205-217
Information on plant adaptation can be very useful in agrobiodiversity studies. Ecogeographical land characterization (ELC)
maps constitute a new tool in this direction with great potential. To assess the usefulness of this approach, an ELC map of
Spain was created through multivariate methods. Its performance to characterize plant habitat preferences was compared with
existing ecological regions and land cover maps. Collecting sites and seed weight from eight plant species were used to test
the ELC map. Categories from each map were assigned to accessions using collecting sites. Chi-square tests were applied to
test if category frequency distributions for each species followed a distribution proportional to the relative frequency of
categories in each map. The tests found significant differences in the eight species studied. Thus, Bonferroni confidence
intervals (BCI) classified categories from maps in preferred, neutral or avoided habitats. Seed weight was used as a proxy
for plant adaptation. Comparison between observed and expected ranking of BCI and quartile classes in terms of seed weight
means, and GLM and post-hoc tests carried out to test the effect of these classes upon seed weight showed consistently better
results for the ELC map. Species results and applications of ecogeographic maps in plant genetic resources conservation are
discussed. 相似文献
15.
Sample adequately to estimate variograms of soil properties 总被引：17，自引：0，他引：17
The variogram is central in the spatial analysis of soil, yet it is often estimated from few data, and its precision is unknown because confidence limits cannot be determined analytically from a single set of data. Approximate confidence intervals for the variogram of a soil property can be found numerically by simulating a large field of values using a plausible model and then taking many samples from it and computing the observed variogram of each sample. A sampling distribution of the variogram and its percentiles can then be obtained. When this is done for situations typical in soil and environmental surveys it seems that variograms computed on fewer than 50 data are of little value and that at least 100 data are needed. Our experiments suggest that for a normally distributed isotropic variable a variogram computed from a sample of 150 data might often be satisfactory, while one derived from 225 data will usually be reliable. 相似文献
16.
Series of sighting records - the years in which a species has been recorded - can be used to infer whether species have gone extinct, and when extinctions occurred. We compiled sighting records for 52 rare bird species, subspecies, and distinct island populations from North America and Hawaii, 38 of which proved adequate for such analyses. Using a data set that combines non-controversial sight records with those for which physical evidence exists, no populations were judged likely to be extant, including those that have not been declared extinct. The ‘alalā was the only species with a 95% confidence interval around the extinction date that extended beyond 2009, suggesting that this population is the least unlikely to be extinct. Although all are probably extinct, populations were ranked according to their likelihood of persistence, so that any future searches can be prioritized to minimize the risk that resources are spent on extinct species. Estimated extinction dates spanned the period from the 1840s-2000s, with evidence for a peak in the early 1900s. On average, only about 4 years passed between a species’ last sighting and its estimated extinction date, and the 95% confidence intervals around extinction dates extended 9-26 years after the last sighting. Long gaps between sightings were very rare. Mean and median gap sizes between consecutive sightings within sighting records were 2.5 and 0 years, respectively. Gaps between the last and penultimate sightings were smaller than average gap sizes earlier in sighting records. Finally, a non-parametric method that can be calculated with more limited data proved a weak substitute for using more complete sighting records. 相似文献
17.
Poison operations to control or eradicate exotic mammals are a key component of conservation management in the South Pacific. They also result in by-kill of native species. It is therefore important to develop reliable methods for estimating by-kill and assessing its impact. The North Island saddleback (Philesturnus carunculatus rufusater), a rare New Zealand forest bird, was reintroduced to Mokoia Island in 1992, and 4 years later there was an aerial drop of cereal pellets containing Brodifacoum aimed at eradicating mice. We used mark-recapture analysis on resighting data collected from 1992 to 1997 to estimate the by-kill of saddlebacks attributable to this poison drop. We nominated a set of candidate models to explain the data, and compared these using Akaike's Information Criterion. The analysis showed that saddleback survival was substantially lower than expected in the 6-week interval after the poison drop, taking age, density dependence, season and random variation into account. Taking expected survival rates into account, the probability of an adult being killed was estimated to be 0.45 (95% CI=0.34-0.56), and the probability of a juvenile being killed was estimated to be 0.35 (95% CI=−0.05-0.75). We then used a simulation model developed for the population to assess the longer-term impact of this mortality. While the mortality set back the expansion of the population by 1-2 years, we predicted that the population would have recovered fairly quickly and ultimately reach the same carrying capacity. Mark-recapture analysis permits precise estimates of background survival rates unconfounded by seasonal or random variation. 相似文献
18.
Soil erodibility measurements for detachment by raindrop impact were made on samples collected at monthly intervals from forest, pasture and farmland sites. All sites exhibited strong seasonal variations in erodibility with maximum values occurring during the winter and minimum values in the summer. The relative erodibility of the different soils is described and the field and laboratory methods used are compared. The best time for sampling the soil for soil erodibility determinations is indicated. Erodibility is estimated from laboratory simulation experiments on undisturbed samples, from aggregate stability determinations and from field measurements with splash boards. 相似文献
19.
Michaelis‐Menten kinetic parameters (Imax and KM) are useful for describing nutrient uptake by plants. This paper compares two methods for estimating the kinetics of P uptake. Both methods employed a steady‐state hydroponic system to measure P uptake by wheat (Triticum aestivum L.) seedlings. In one method, uptake was measured from two P concentrations in nutrient solution, with Imax and KM estimated by direct linear plot (DLP). In an alternate, multiple concentration (MC) method, uptake was measured from five P concentrations, and kinetic parameters were estimated by either nonlinear regression or the Hanes plot. The Imax and KM, estimates obtained by the DLP method were compared to those obtained by the MC method. The MC method offered practical advantages. Unlike the DLP, it allowed estimation of the external P concentration at which net influx = 0 (Cmin), and did not require a priori estimates of KM and Cmin. The MC method provided more precise median parameter estimates as indicated by smaller nonparametric confidence intervals. Using the median Cmin value of 1.9 μM, the best estimates of Imax and KM (and 96% confidence intervals) derived by nonlinear regression were 2.2 (1.6 to 2.8) nmol P g‐1s‐1, and 11 (10.6 to 12.9) μM, respectively. 相似文献
20.
Chris Gennings W. Hans Carter Richard A. Carchman Michael J. DeVito Jane Ellen Simmons Kevin M. Crofton 《Journal of Agricultural, Biological & Environmental Statistics》2007,12(1):96-111
When an interaction has been detected among the chemicals in a mixture, it may be of interest to predict the interaction threshold.
A method is presented for estimation of an interaction threshold along a mixture ray which allows differences in the shapes
of the dose-response curves of the individual components (e.g., mixtures of full and partial agonists with differing response
maxima). A point estimate and confidence interval for the interaction threshold may be estimated. The methods are illustrated
with data from a study of a mixture of 18 polyhalogenated aromatic hydrocarbons (PHAHs) in rats exposed by oral gavage for
four consecutive days. Serum total thyroxine (T4) was the response variable. Previous analysis of these data demonstrated
a dose-dependent interaction among the 18 chemicals in the mixture, with additivity suggested in the lower portion of the
dose-response curve and synergy (greater than additive response) in the higher portion of the dose-response curve. The present
work builds on this analysis by construction of an interaction threshold model along the mixture ray. This interaction threshold
model has two components: an implicit additivity region and an explicit region that describes the departure from additivity;
the interaction threshold is the boundary between the two regions. Estimation of the interaction threshold within the observed
experimental region suggested evidence of additivity in the low dose region. Total doses of the mixture that exceed the upper
limit of the confidence interval on the interaction threshold were associated with a greater-than-additive interaction. 相似文献