A variance-covariance structure to take into account repeated measurements and heteroscedasticity in growth modeling     

Mathieu Fortin  Gaétan Daigle  Chhun-Huor Ung  Jean Bégin  Louis Archambault 《European Journal of Forest Research》2007,126(4):573-585

This study proposes a within-subject variance-covariance (VC) structure to take into account repeated measurements and heteroscedasticity in a context of growth modeling. The VC structure integrates a variance function and a continuous autoregressive covariance structure. It was tested on a nonlinear growth model parameterized with data from permanent sample plots. Using a stand-level approach, basal area growth was independently modeled for red spruce (Picea rubens Sarg.) and balsam fir [Abies balsamea (L.) Mill.] in mixed stands. For both species, the implementation of the VC structure significantly improved the maximum likelihood of the model. In both cases, it efficiently accounted for heteroscedasticity and autocorrelation, since the normalized residuals no longer exhibited departures from the assumptions of independent error terms with homogeneous variances. Moreover, compared with traditional nonlinear least squares (NLS) models, models parameterized with this VC structure may generate more accurate predictions when prior information is available. This case study demonstrates that the implementation of a VC structure may provide parameter estimates that are consistent with asymptotically unbiased variances in a context of nonlinear growth modeling using a stand-level approach. Since the variances are no longer biased, the hypothesis tests performed on the estimates are valid when the number of observations is large.  相似文献   

3个家蚕品种部分产茧量及体质量性状指标的协方差分析     

邹凤竹  孙春光  张世友  谢清忠  黄慧明 《蚕业科学》2009,35(4)

养蚕生产中家蚕的桑叶食下量难以人为控制在同一水平,通常认为食下量会对蚕体生长发育及全茧量等产茧量指标产生显著甚至极显著影响。以山东省现行的3个春用蚕品种为材料,将食下量作为协变量进行协方差分析的结果表明,3个家蚕品种在3-5龄的蚕体质量增量、全茧量、茧层量性状指标上无统计学意义(P>0.125 2),仅个别指标均值间的最大差异在0.1水平下显著(P全茧量=0.066 9,P茧层量=0.056 1),因此该方法校正了用常规方差分析得出的上述性状指标在品种间存在极显著差异(P<0.000 1)的有偏结论,还明确了消化吸收量对上述性状指标影响不显著,可不作为协变量考虑。  相似文献   

协方差分析方法优化农用车悬架参数     

张孝祖  余军合  薛念文 《农业机械学报》2000,31(3):27-30

介绍了协方差分析随机振动的基本原理,建立了５自由度的农用车平面模型和状态方程,给出了评价农用车行驶平顺性的目标函数,通过协方差方法得出目标函数随悬架的阻尼和刚度变化的关系,用实例验证利用这种方法优化农用车悬架参数是可行的。  相似文献   

青藏高原3种主要植被类型的表观量子效率和最大光合速率的比较   总被引：1，自引：1，他引：0  

张法伟  李英年  李红琴  王勤学  杜明远  赵亮  汪诗平 《草地学报》2007,15(5):442-448

以海北高寒草甸生态系统定位站的涡度相关系统连续观测的CO₂通量数据为基础,分析了青藏高原的高寒矮嵩草(Kobresia humilis)草甸、高寒金露梅(Potentilla fruticosa)灌丛草甸和高寒藏嵩草(Kobresia tibetica)沼泽化草甸等3种主要植被类型在2005年植物生长季(6-9月)的表观量子产额(a)、最大光合速率(P_max)和呼吸速率(R_eco)的变化特征。结果表明:3种植被类型白天的净生态系统CO₂交换量(NEE)和光量子通量密度(PPFD)存在明显的直角双曲线关系(P<0.05),其a、P_max和R_eco呈现出相似的季节变化趋势,在生长季初期(6月)最小,在7月或8月份达到最大;高寒矮嵩草草甸的a、P_max和R_eco大于灌丛草甸和沼泽化草甸,而后两者差别不大。  相似文献   

双列杂交的遗传分析     

邹学校 《湖南农业大学学报(自然科学版)》1988,(1)

本文分析了以两个非真而不能验证的先决条件(亲本所具基因的分布彼此独立;非等位基因间无互作)为基础的双列杂交法在育种上的实用价值.并对Griffing四种方案估算的配合力效应进行了比较。此外,本文还建立了配合力协方差模型。  相似文献   

多变量时间剩余协克立格模型理论及其在水文数据分析中的应用     

刘小燕  刘廷玺  刘晓革 《内蒙古农业大学学报(自然科学版)》2001,22(3):93-99

本文将随机水文学和多变量地质统计学相结合，提出了多变量时间剩余协克立格模型。该模型将水文变量的周期性成分和趋势性成分视为确定性分量从原始各测站水文时间序列中剔除，剔除剩下的剩余值属于时空随机函数，从而以这种剩余值作为研究变量去刻划原水文变量的时空相关特征。该模型可用于水文变量的插补延展和短期预报，具有较高的精度。  相似文献   

Effect of probability-distance based Markovian texture extraction on discrimination in biological imaging     

S.N. Ondimu  H. Murase 《Computers and Electronics in Agriculture》2008,63(1):2-robotics

The effect of probability-distance based re-weighting of image texture features on their discrimination ability was evaluated in this study. A quadratic surface smoothing function was developed from the Bhattacharyya probability distance between two classes of images. The function was used as a re-weighting function for standard grey level co-occurrence matrix (GLCM) textural features in two cases. Case 1 involved 42 images of health and smokers’ lungs. Case 2 involved 144 images of well watered; partially water stressed and dry Rhacomitrium canescens plants. Multilayer perception (MLP) neural networks (NN) classifiers based on standard weighted and probabilistic-distance based re-weighted GLCM textural features were developed for each case. The classifiers were trained and tested using the leave-one-out and the cross-validation evaluation strategies.Probability-distance based re-weighting of GLCM textural features resulted in 37% and 39.22% reduction in true classification error for cases 1 and 2, respectively. It was concluded that re-weighting image texture features based on the probability distance between the classes involved, makes the GLCM texture analysis technique more discriminative and adaptive. This modification will help to overcome the fixed nature of GLCM features which is an area of weakness open to further improvement. Although GLCM features were used in this study, the method can be extended to other co-occurrence-based methods.  相似文献   

海南西北地区像胶芽接树冬季不同停割期研究     

何定全 《热带作物学报》1995,(2)

海南西北部地区橡胶芽接树按常规割制，入冬后以黄叶量占树位总叶量１０％时停割为宜；停割时间不早于１２月２３日，比１２月１日停割极显著高产，且死皮发展、干胶含量下降、树围生长等差异皆不显著，对次年的产胶能力影响极微。其中，ＰＲ１０７品系表现最好，ＰＢ８６次之，国内外混杂品系较差。  相似文献   

试验数据程序化协方差分析     

谢振宇  冯文喜 《计算机与农业》2008,(4):135-136

笔者利用VB6．0编成语言设计了协方差分析计算机程序,用该程序对用玉米秸秆饲喂生长鹅的试验数据进行协方差分析。试验过程是将80只鹅分成5组,在前4组每组分别在常规配合料中加入10％、15％、25％和35％的玉米秸秆粉,观察生长鹅对玉米秸秆的消化利用能力。第5组（处理35％组）除在配合料加入35的玉米秸秆粉外,还加入秸秆分解剂对添加的秸秆进行处理,观察其试验效果（数据见表1）。该程序可以把协方差分析的计算过程、中间数据、计算公式以及各数据、公式之间的关系分步列出,可使数据处理人员,能够全面地了解协方差处理数据的全过程。  相似文献   

试验数据程序化协方差分析     

谢振宇  冯文喜 《农业网络信息》2008,(4):135-137

笔者利用VB6.0编成语言设计了协方差分析计算机程序,用该程序对用玉米秸秆饲喂生长鹅的试验数据进行协方差分析.试验过程是将80只鹅分成5组,在前4组每组分别在常规配合料中加入10%、15%、25%和35%的玉米秸秆粉,观察生长鹅对玉米秸秆的消化利用能力.第5组(处理35%组)除在配合料加入35的玉米秸秆粉外,还加入秸秆分解剂对添加的秸秆进行处理,观察其试验效果(数据见表1).该程序可以把协方差分析的计算过程、中间数据、计算公式以及各数据、公式之间的关系分步列出,可使数据处理人员,能够全面地了解协方差处理数据的全过程.  相似文献