共查询到20条相似文献，搜索用时 15 毫秒
1.
The development of methods to identify, validate, and use surrogate markers to test for a treatment effect has been an area of intense research interest given the potential for valid surrogate markers to reduce the required costs and follow‐up times of future studies. Several quantities and procedures have been proposed to assess the utility of a surrogate marker. However, few methods have been proposed to address how one might use the surrogate marker information to test for a treatment effect at an earlier time point, especially in settings where the primary outcome and the surrogate marker are subject to censoring. In this paper, we propose a novel test statistic to test for a treatment effect using surrogate marker information measured prior to the end of the study in a time‐to‐event outcome setting. We propose a robust nonparametric estimation procedure and propose inference procedures. In addition, we evaluate the power for the design of a future study based on surrogate marker information. We illustrate the proposed procedure and relative power of the proposed test compared to a test performed at the end of the study using simulation studies and an application to data from the Diabetes Prevention Program. 相似文献
2.
Observations on larval development of Malacosoma neustrium were conducted both in a cork oak stand and in the laboratory by using leaves of different host trees (cork oak, holm oak and downy oak) as food source. Instars were determined using head capsule and frass measurements. In the field the larvae progressed up to the fifth instar before pupating, and the increase in head capsule width followed Dyars Rule with a rate of increase (R.I.) value of 1.74. The same number of instars was determined for the larvae reared with cork oak (R.I. = 1.73) and holm oak (R.I. = 1.70) leaves. The caterpillars reared with downy oak foliage completed larval development in five, six and seven instars and the R.I. values obtained were 1.60, 1.52 and 1.44 respectively. A lower mortality was recorded for the larvae reared on holm oak. Growth and feeding indices were determined for the larvae from the third up to the last instar. The highest leaf consumption was detected for the fifth instar larvae reared on holm oak. For the caterpillars which completed five instars before pupating, the relative consumption rate (RCR) decreased from the third up to the fifth instar: from 4.8 to 1.7 (cork oak), from 7.4 to 3.3 (holm oak) and from 14.3 to 2.1 (downy oak). The relative growth rate (RGR) was highest during the fourth stadium (0.24, 0.27 and 0.33 for larvae reared with cork oak, holm oak and downy oak leaves respectively) and decreased in the fifth instar (0.09, 0.14 and 0.14 for larvae reared with cork oak, holm oak and downy oak leaves respectively), probably because of greater expense of energy due to the approach of maturity. Feeding and growth indices could be useful to define a defoliation prediction model. 相似文献
3.
土地生态安全评价能高效引导土地合理利用,协调生态保护与区域经济发展之间的矛盾。以核密度估算法及克里金空间插值为技术支撑,构建基于突变级数法的土地生态安全评价指标体系,并采用空间变异系数法及灰色预测模型对新疆土地生态安全进行探索性测度分析。结果表明:(1)新疆土地生态安全总体水平不断提高。土地生态安全指数由2004年的0.76上升到2017年0.94,安全等级呈现\"不安全→较不安全→临界安全→较安全\"的发展趋势。(2)新疆各县市土地生态安全的不均衡性得到改善,空间差异缩小。新疆土地生态安全空间变异系数由2004年的0.51上升为2017年的0.84,呈持续上升趋势,其中2009年到2014年期间最快增加了0.29。(3)研究区人口增加了27.59%,同时未利用地和草地面积的减少量分别占到整个新疆面积的7.51‰和9.45‰。自然资源分布不均、社会经济发展对未利用地和草地的大量占用以及区域人口的急剧增加可能是新疆土地生态安全面临的主要问题。(4)通过灰色模型预测,2020年新疆土地生态安全将进一步得到改善,土地生态安全空间差异将进一步缩小。与2005年相比, 2020年空间变异系数将上升到0.85;安全区和临界安全区将分别增加2.06%和11.24%;较安全区、较不安全区、不安全区将分别下降2.98%、9.16%、1.15%。 相似文献
4.
We compare the performances of local and global rules for smoothingparameter choice, in terms of asymptotic mean squared errorsof the resulting estimators. In some instances there is surprisinglylittle to choose between local and global approaches; our analysisidentifies contexts where the differences are small or large.This work motivates development of smoothing rules that forma ‘half-way house’ between local and global smoothing.There, interpolation provides a basis for partial local smoothing.A key result shows that interpolation on even a coarse gridcan produce a very good approximation to full local smoothing.Our theoretical and numerical results lead us to suggest linearinterpolation of a bandwidth obtained by integral approximationson discrete intervals. 相似文献
5.
Ghosh D 《Functional & integrative genomics》2005,5(1):28-31
Due to the advent of high-throughput genomic technology, it has become possible to monitor cellular activities on a genomewide basis. With these new methods, scientists can begin to address important biological questions. One such question involves the identification of replication origins, which are regions in the chromosomes where DNA replication is initiated. One hypothesis is that their locations are nonrandom throughout the genome. In this article, we analyze data from a recent yeast study in which candidate replication origins were profiled using cDNA microarrays to test this hypothesis. We find no evidence for such clustering. 相似文献
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7.
Andrew R. Solow 《Biometrical journal. Biometrische Zeitschrift》1993,35(5):627-633
Spectral estimation is commonly based on smoothing the periodogram or its logarithm. In such a smoothing exercise, it is not entirely sensible to use the same bandwidth for all frequencies, since the smoothness of the underlying spectrum may vary. This note describes the use of a variable span smoother in estimating the log spectrum. An application to annual lynx data is presented. 相似文献
8.
TODD D. STEURY JOHN E. MCCARTHY TIMOTHY C. ROTH II STEVEN L. LIMA DENNIS L. MURRAY 《The Journal of wildlife management》2010,74(3):539-548
ABSTRACT The kernel density estimator is used commonly for estimating animal utilization distributions from location data. This technique requires estimation of a bandwidth, for which ecologists often use least-squares cross-validation (LSCV). However, LSCV has large variance and a tendency to under-smooth data, and it fails to generate a bandwidth estimate in some situations. We compared performance of 2 new bandwidth estimators (root-n) versus that of LSCV using simulated data and location data from sharp-shinned hawks (Accipter striatus) and red wolves (Canis rufus). With simulated data containing no repeat locations, LSCV often produced a better fit between estimated and true utilization distributions than did root-n estimators on a case-by-case basis. On average, LSCV also provided lower positive relative error in home-range areas with small sample sizes of simulated data. However, root-n estimators tended to produce a better fit than LSCV on average because of extremely poor estimates generated on occasion by LSCV. Furthermore, the relative performance of LSCV decreased substantially as the number of repeat locations in the data increased. Root-n estimators also generally provided a better fit between utilization distributions generated from subsamples of hawk data and the local densities of locations from the full data sets. Least-squares cross-validation generated more unrealistically disjointed estimates of home ranges using real location data from red wolf packs. Most importantly, LSCV failed to generate home-range estimates for >20% of red wolf packs due to presence of repeat locations. We conclude that root-n estimators are superior to LSCV for larger data sets with repeat locations or other extreme clumping of data. In contrast, LSCV may be superior where the primary interest is in generating animal home ranges (rather than the utilization distribution) and data sets are small with limited clumping of locations. 相似文献
9.
K. SZPILA M. J. R. HALL K. L. SUKONTASON T. I. TANTAWI 《Medical and veterinary entomology》2013,27(2):181-193
10.
Nonparametric regression using local kernel estimating equations for correlated failure time data 总被引：1，自引：0，他引：1
We study nonparametric regression for correlated failure timedata. Kernel estimating equations are used to estimate nonparametriccovariate effects. Independent and weighted-kernel estimatingequations are studied. The derivative of the nonparametric functionis first estimated and the nonparametric function is then estimatedby integrating the derivative estimator. We show that the nonparametrickernel estimator is consistent for any arbitrary working correlationmatrix and that its asymptotic variance is minimized by assumingworking independence. We evaluate the performance of the proposedkernel estimator using simulation studies, and apply the proposedmethod to the western Kenya parasitaemia data. 相似文献
11.
Guillermo A. Bermejo G. Marius Clore Charles D. Schwieters 《Protein science : a publication of the Protein Society》2012,21(12):1824-1836
Statistical potentials that embody torsion angle probability densities in databases of high‐quality X‐ray protein structures supplement the incomplete structural information of experimental nuclear magnetic resonance (NMR) datasets. By biasing the conformational search during the course of structure calculation toward highly populated regions in the database, the resulting protein structures display better validation criteria and accuracy. Here, a new statistical torsion angle potential is developed using adaptive kernel density estimation to extract probability densities from a large database of more than 106 quality‐filtered amino acid residues. Incorporated into the Xplor‐NIH software package, the new implementation clearly outperforms an older potential, widely used in NMR structure elucidation, in that it exhibits simultaneously smoother and sharper energy surfaces, and results in protein structures with improved conformation, nonbonded atomic interactions, and accuracy. 相似文献
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13.
H. Schbe 《Biometrical journal. Biometrische Zeitschrift》1991,33(2):191-206
Kernel estimators are used to obtain new estimators for the tumour onset distribution and its density. Asymptotical properties of the estimators are studied and confidence tests are constructed. 相似文献
14.
Rosamarie Frieri William Fisher Rosenberger Nancy Flournoy Zhantao Lin 《Biometrics》2023,79(3):2565-2576
When there is a predictive biomarker, enrichment can focus the clinical trial on a benefiting subpopulation. We describe a two-stage enrichment design, in which the first stage is designed to efficiently estimate a threshold and the second stage is a “phase III-like” trial on the enriched population. The goal of this paper is to explore design issues: sample size in Stages 1 and 2, and re-estimation of the Stage 2 sample size following Stage 1. By treating these as separate trials, we can gain insight into how the predictive nature of the biomarker specifically impacts the sample size. We also show that failure to adequately estimate the threshold can have disastrous consequences in the second stage. While any bivariate model could be used, we assume a continuous outcome and continuous biomarker, described by a bivariate normal model. The correlation coefficient between the outcome and biomarker is the key to understanding the behavior of the design, both for predictive and prognostic biomarkers. Through a series of simulations we illustrate the impact of model misspecification, consequences of poor threshold estimation, and requisite sample sizes that depend on the predictive nature of the biomarker. Such insight should be helpful in understanding and designing enrichment trials. 相似文献
15.
Guosheng Yin Jianwen Cai Jinheum Kim 《Biometrical journal. Biometrische Zeitschrift》2003,45(5):602-617
Quantiles, especially the medians, of survival times are often used as summary statistics to compare the survival experiences between different groups. Quantiles are robust against outliers and preferred over the mean. Multivariate failure time data often arise in biomedical research. For example, in clinical trials, each patient in the study may experience multiple events which may be of the same type or distinct types, while in family studies of genetic diseases or litter matched mice studies, failure times for subjects in the same cluster may be correlated. In this article, we propose nonparametric procedures for the estimation of quantiles with multivariate failure time data. We show that the proposed estimators asymptotically follow a multivariate normal distribution. The asymptotic variance‐covariance matrix of the estimated quantiles is estimated based on the kernel smoothing and bootstrap techniques. Simulation results show that the proposed estimators perform well in finite samples. The methods are illustrated with the burn‐wound infection data and the Diabetic Retinopathy Study (DRS) data. 相似文献
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17.
On extended partially linear single-index models 总被引：2，自引：0，他引：2
18.
Jorgen Holm Petersen 《Biometrical journal. Biometrische Zeitschrift》2009,51(4):697-709
A conceptually simple two‐dimensional conditional reference curve is described. The curve gives a decision basis for determining whether a bivariate response from an individual is “normal” or “abnormal” when taking into account that a third (conditioning) variable may influence the bivariate response. The reference curve is not only characterized analytically but also by geometric properties that are easily communicated to medical doctors – the users of such curves. The reference curve estimator is completely non‐parametric, so no distributional assumptions are needed about the two‐dimensional response. An example that will serve to motivate and illustrate the reference is the study of the height/weight distribution of 7–8‐year‐old Danish school girls born in 1930, 1950, or 1970. 相似文献
19.
精确实时的大型野生食草动物和家畜调查数据是开展野生动物保护、发展现代畜牧业和草原生态文明建设的基础。研究通过文献方式总结和归纳了目前主要的大型野生食草动物和家畜调查方法,包括地面调查法、卫星调查法、有人机调查法和无人机调查法涉及的设备、数据类型、分辨率、覆盖范围、调查物种;分析了现有大型食草动物智能识别算法、区域种群数量估算方法的优势和缺点;探讨了大型野生食草动物和家畜调查所涉及的不同调查平台、智能识别算法、区域种群密度估算方法等方面研究存在的问题,并对未来研究方向进行了展望。研究认为融合多平台、多传感器数据对构建大尺度、长时序动物数据集至关重要。未来有必要针对食草动物分布密集、目标小的问题研发高精度识别模型,发展基于机器学习的区域种群密度估算方法,揭示区域种群密度与气象、地形等环境因子之间的复杂关联关系。 相似文献
20.
掌握野生动物栖息地适宜性及活动节律对科学保护森林生态系统具有重要意义。狍(Capreolus pygargus)作为森林生态系统中重要组成部分,其生物学习性对于植被更新及同域物种分布具有指示作用,掌握其栖息地适宜性及活动节律是恢复生物多样性的基础。于2022年6月-2023年5月在北极村国家级自然保护区采用样线调查和相机监测相结合的方法收集狍活动信息,通过随机布设方法在保护区内共布设35条样线和140台红外相机,并利用最大熵(MaxEnt)栖息地建模及核密度估计法分析狍的栖息地适宜性与活动节律,结果表明:(1) MaxEnt模型的ROC曲线评价结果的平均AUC值为0.839,能够较好的反映研究区域狍栖息地适宜性的实际分布情况;(2)研究利用的14个变量中,距离河流越远出现概率越低是影响狍栖息地选择的关键变量,贡献率达34%。其次针阔混交林、常绿针叶林、林间小路、海拔、草地和居民区是影响狍栖息地质量的次要变量;(3)保护区内狍适宜栖息地和次适宜栖息地面积分别为240.44km2和490.64 km2,占研究区域总面积的17.48%和35.67%,主要分布于保护区中部和东部,西部适宜栖息地面积较少且破碎化较为严重;(4)活动节律分析表明,狍在9:00左右达到日活动高峰;不同季节狍活动节律存在差异,暖季狍活动节律具有晨昏性,冷季狍上午和下午的活动高峰期分别延迟和提前。基于研究结果提出以下保护建议:(1)加大保护区周边人类活动的管控力度,根据不同人为干扰强度针对性地对狍种群进行保护;(2)通过加强栖息地连通性管理、构建栖息地廊道等方法使破碎的栖息地连接成整体,促进保护区狍个体间交流。研究获得的结果为分布于我国最北端狍种群的保护与恢复提供了基础数据,对于推动该地区生物多样性保护具有重要意义。 相似文献