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How to correct the heteroscedasticity and autocorrelation of residuals in taper and height growth models? Gerónimo Quiñonez-Barraza 1. Guadalupe Geraldine García-Espinoza 2. Oscar Alberto Aguirre-Calderón 3. In modeling of taper functions and dominant height growth with time series data, the presence of heteroscedasticity and autocorrelation in residuals is common. Variance Functions varFunc and correlation structures corStruct were used to correct heteroscedasticity and autocorrelation; both were combined and evaluated through taper and height growth equations for Pinus teocote in DurangoMexico.
A dataset of 51 stems analysis with taper observations and height growth observations was used. The varFuncs applied were: 1 power function varPower autocorrelation in regression 2 exponential function varExp ; 3 constant plus power function varConstPower ; and 4 a combination of power and exponential functions varComb. According to the rating system, the best combinations for taper and height growth equations were, and and, andrespectively. In the taper equation, only the combination was homoscedastic with independent residuals, and the selected height autocorrelation in regression equations were homoscedastic with independent residuals; the varFunc and corStruct had influence on the trajectories of site index curves.
Key words: Taper; dominant height; correlation structures; variance functions; Pinus teocote Autocorrelation in regression ex Schltdl. Funciones de varianza varFunc y estructuras de correlación corStruct para corregir la heterocedasticidad y autocorrelation in regression dependencia de los errores, respectivamente. Estas fueron combinadas y evaluadas en ecuaciones de ahusamiento y crecimiento en altura de Pinus teocote en Durango, México.
Las varFunc utilizadas fueron: 1 función de potencia varPower ; 2 función exponencial varExp autocorrelation in regression 3 función constante y de potencia varConstPower ; y 4 función combinada de potencia y exponencial varComb. Con base en la calificación, las mejores combinaciones para el ahusamiento y crecimiento en altura fueron, y y, yrespectivamente. Palabras clave: Ahusamiento; altura dominante; estructuras de autocorrelation in regression funciones de varianza; Pinus teocote Schiede ex Schltdl.
The planning, implementation and monitoring of sustainable forest management require research to support decision-making and the assessment of the established goals. The estimation of the timber stock and productivity of the stands is a main objective in forest management systems; therefore, it is essential to know the growth of commercial species Aguirre-Calderón, ; Salas et al. Research on the estimation of volume, growth and increment is a key tool for understanding the dynamics of ecosystems involved in forest management; therefore, these approaches continue to be necessary for the planning and implementation of forest activities.
Taper and dominant height growth equations have been widely explored topics Castillo et al. Since the datasets used for fitting taper and height growth autocorrelation in regression are time series obtained from measured variables on the same tree and resulting from taper and tree stem analyses, it is reasonable to assume that the observations in each tree are correlated and, also the residuals of the adjusted equations Arias-Rodil et al.
The term autocorrelation refers to the correlation between the residuals of a regression model when series of observations arranged in time are used, e. The linear and nonlinear regression models are based on the theoretical assumption that the residues have the same variance and are, therefore, homoscedastic. The presence of autocorrelation and heteroscedasticity leads to estimations of non-minimum variance parameters and to somewhat unreliable prediction intervals, especially in modelling taper and volume equations Fortin et al.
Consequently, the usual t or F tests are not valid. Therefore, the use of generalized least squares GNLS approach with variance functions and autocorrelation in regression structures is an alternative to generate the best unbiased linear parameter estimates Gujarati and Porter, In studies of taper and dominant height growth models, correlation structures and power functions to correct the autocorrelation and heteroscedasticity of the residuals, respectively, are frequently used Quiñonez-Barraza do long distance relationships get easier al.
Because it is important to improve the predictive capacity and the interpretation of the statistic properties in equations fitting, the objective of this study was to evaluate the combination of variance functions with correlation structures, in order to model the heteroscedasticity and the errors dependence in taper and dominant height growth equations of Pinus teocote Schiede ex Schltdl. The information from stem analysis of 51 Pinus teocote trees collected in mixed stands of the Forest Management Unit Umafor Santiago Papasquiaro y Anexosin northeastern DurangoMexico, was used.
The forest polygon was San Diego de Tezains ejidowith a total area of 61 The predominant climates are autocorrelation in regression warm humid and temperate subhumid, with a mean annual precipitation of 1 mm. The data was taken using a totally random design for the stands of the timber production area, and was considered a normal distribution for the diameter categories. The trees were felled and divided into sections in order to register the growth in dominant height and taper by relative heights.
The first measurement corresponded to the stump height; subsequently, to lengths of 0. In whole dataset, diameter-height taper and height-age combinations were used. Table 1 shows the descriptive statistics of the analyzed variables. Table 1 Descriptive statistics of the analyzed variables in order to fit the taper and dominant height growth of Pinus teocote Schiede ex Schltdl.
The taper was modeled with the segmented equation developed by Fang et al. The segmented taper equation is as follows:. The dominant height as an intrinsic site index equation was modeled using the dynamic equation in the generalized algebraic difference approach GADAderived by Quiñonez-Barraza et al. Combinations of variance functions with correlation structures were used in taper equation Eq. The varFunc and corStruct were determined according to Pinheiro and Bates Variance functions. The variance functions were as follow: 1 power function varPower ; 2 exponential function varExp ; 3 constant power function varConstPowerand 4 combination of power-exponential functions varComb.
The variance functions were used in order to model the variability between the measurements of each tree i with the merchantable height covariables h i j for the taper equation, and the dominant height h 1 i j at state autocorrelation in regression 1 i j for the height growth equation. The general structure of the power functions for modeling the heteroscedasticity considers two arguments for most varFuncs: the parameter value and shape.
Covariable h i j was utilized for the taper equation Eq. The varExp variance model of is represented by equation 5and the corresponding function, by equation 6for the same covariables previously defined for taper and dominant height growth equation. The varConstPower variance model is defined in equation 7and the variance function, in equation 8. The varComb variance model varExp and varPower is defined in equation 9with the respective function expressed in equation 10 Pinheiro and Bates, In all cases, the same covariables, previously defined, were used.
The correlation structures were used to model the dependence between residuals of each tree, with time-series data Pinheiro and Bates, autocorrelation in regression This study modeled the dependence between the diameter and height measurements in the same tree for the aim of what is fuzzy logic explain with example independence in the residuals of taper equation Eq.
Autocorrelation in regression general structure of correlation between groups for a single grouping level is expressed as equation 11 Pinheiro and Bates, In the correlation structure corCompSymm, an equal correlation is assumed for all errors of the same group within the same tree; the correlation model is given by equation Autocorrelation in regression corAR1 model is represented in equation 13while the carCAR1 model is expressed by equation In order to assess the fit of taper and dominant height growth equations, combinations of the variance functions with correlation structures like those studied by Pinheiro and Bates were used.
A rating system was generated with these statistics in order to select the best varFunc and corStruct combinations. Each statistic was assigned a value from 1 to 9; 1 corresponds to the combination with the best statistic, and 9, to the one with the worst statistic Sakici et al. The Durbin-Watson Dw statistic Durbin and Watson, was used to evaluate the correction of the autocorrelation, with a robust modification DwMsuch as the average Dw between groups, since errors are regarded as dependent on the measurements of each tree, but not on the general dataset.
The modified statistic is shown in equation The Assumption of homogeneity of variance null hypothesis, H0 is expressed in equation 18 ; therefore, higher values than 0. The combinations of variance functions and correlation structures generated 36 models for taper, and 36 for height growth, based on equations 1 and 2respectively. The fit statistics and the ranking score RS showed the goodness-of-fit of the equations Table 2for taper, and Table 3for height growth.
Table 2 Adjustment statistics of the taper equations for the combinations of variance functions with correlation structures. Table 3 Adjustment statistics for height growth equations for the combinations of variance functions with correlation structures. How often should you see someone youre casually dating ranking score exhibited the combinations of variance functions with correlation structures by hierarchical order Sakici et al.
The lowest RS value was the statistically best combination, and the highest RS corresponded to the worst combination, based on the sum of the ranks for each fitting statistic Tamarit et al. In all cases of correlation structures combined with varPower, values ranges from 1. However, for the test of homogeneity of variances, all the combinations were heteroscedastic.
The varConstPower and varComb functions combined with the correlation structures have consistent statistics and independent residuals DwM values from 1. In combinations of variance functions and correlation structures, the height growth generated DwM statistics of approximately 1. However, for most equations, they exhibited homogeneous variances.
The corCAR1 structure had the lowest fitting, with unequal variances. Table mean free path definition in physics shows the behavior of the variance functions varExp, varConstPower and varComb with the combinations of correlation structures. The estimated parameters for the best combinations of each variance function with the correlation structures are summarized in Table 4for both taper T and height growth HG.
Table 4 Parameter estimates of the top four combinations of the variance functions and autocorrelation structures in the taper T and height growth HG equations. The estimated parameters defining the changes of the dendrometric stem shapes of the segmented taper model were found for the change from neiloid into paraboloid, as 4.
Similar results have been documented by researchers for different Pinus species Uranga-Valencia et al. Furthermore, continuous-time autoregressive structures of orders 1 and 2 and power functions were used, and a known autocorrelation in regression was assumed. Nevertheless, the studies do not include the corresponding test of homogeneity of variances. Figure 1 Box and whisker plots for the distribution of the taper residuals by relative height for combinations of varFunc and corStruct.
The autocorrelation in regression of the equations with multiple parameters can cause an overparameterization, and the predictions resulting from them could not how do you prove common law partner in canada the most efficient Gregoire and Schabenberger, In the four cases, the variances were constant, which autocorrelation in regression that the parameters are unbiased and efficient and have a minimum variance Gujarati and Porter, ; Tang et al.
As for the residuals, although the DwM test generated values of approximately 1. In dominant height growth and site index equations with an algebraic difference approach ADA or generalized GADA equations, the power or exponential functions autocorrelation in regression been used for the correlation of the heteroscedasticity, in which unequal variances are assumed to exist in the generalized least squares adjustment process Castillo et al.
The approach autocorrelation in regression in this paper considers the correction of the heteroscedasticity and of the autocorrelation as combinations of functions Rodríguez et al. Figure 2 Box autocorrelation in regression whisker plots for the distribution of the residuals of height growth by relative height for combinations of varFunc and corStruct.
Figure 3 contrasts the predictions of taper and height growth equations with the selected combinations of power variance functions and correlation structures; it shows the observed tendency of the profile of a tree, the fitted equation without heteroscedasticity and autocorrelation autocorrelation in regression NC and the combinations H1-A9, H2-A5, H3-A8 and H4A6.
Only the combination H2-A5 exhibited constant variances. Autocorrelation in regression, this figure shows the growth tendencies of the trees in dataset; in this case, all the combinations displayed constant variances, whereby the desirable properties in the estimated parameters are guaranteed Beale et al. Figure 3 Taper prediction charts for a tree profile, and height growth curves by site index SIs 10, 14 and 18 m at the baseline age of 60 years with the combinations of variance functions and autocorrelation structures, and the equations without variance or autocorrelation IS10, Autocorrelation in regression, IS18 structures.
The variance functions in combination what does associative in math mean the correlation structures corrected the heteroscedasticity assumptions of variances and error autocorrelation in the taper and dominant height growth equations, autocorrelation in regression a generalized nonlinear least square approach; as a result, unbiased parameters with a minimum variance were obtained.
The predictions of the selected taper equations are more efficient, with consistent fitting statistics; the combination of the exponential variance function with an autoregressive-moving average correlation structure corARMA produces constant variances by relative height categories of the stem profiles. The dominant height and site index model, at a base age of 60 years, exhibits realistic predictions. In both, the residuals are independent, and the properties of the tests of hypothesis of the estimated parameters are guaranteed.
The use of compatible taper and commercial volume equations, as well as of height growth and index site equations, is defined autocorrelation in regression the use of the intrinsic parameters in each equation; therefore, the parameters of the variance functions and correlation structures are only statistical indicators for rendering the fitting more efficient.
The authors wish to express their gratitude to San Diego de Tezains ejido, Santiago Papasquiaro, Durango, Mexicofor making the taper and height growth information available to be used in this study. Aguirre-Calderón, O.
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