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1.
The importance of micro mammals from many points of view, mainly with an ecological approach was stressed. The study of the spatial-temporal distribution of parasites in their hosts may be carried out in several ways. Tests done in collaboration with the Parasitology Laboratory in the Faculty of Chemistry at the University of Barcelona, the NRCS and the Department of Ecological Studies in Cosenza, have contributed to an understanding of the Helminth communities as relating to several intrinsic variables of microteriofauna as well as extrinsic ones, particularly those concerning environment, climate and season. These comparisons were made using statistical means which compared the categorical and dichotomic variables which would highlight risk differences and their effects on the system. Quantitative dependent variables were also considered in relation to the aforementioned qualitative variables. One of the models studied is that of logistic regression, which estimates the function of regression, connecting the probability of the presence of Helminth as a dependent variable, with biological and ecological parameters (independent variables) such as: gender, age, season of capture, bioclimate, biotope and trapping section.  相似文献   

2.
Relationships between morphological features of human skeletal nasal protrusion, latitude, and climate were investigated. Craniofacial dimensions and indices determined by Woo and Morant (1934) on a world sample of 55 skeletal populations were used as dependent variables. Sample sizes were as low as 39 in some calculations because either skeletal or geographic data were missing. Thirteen climatically related averaged variables, for each population's provenience, were the independent variables. Multivariate techniques of bivariate correlation, multiple regression, and partial correlation were applied. A strong, statistically significant cline of increasing nose protrusion, with decreasing absolute humidity and with increasing latitude, was found. Cold climatic variables appeared to be of greater importance than warm measures. Similarly, absolute humidity was found to be a much better predictor of nose protrusion than was relative humidity.  相似文献   

3.
J M Neuhaus  N P Jewell 《Biometrics》1990,46(4):977-990
Recently a great deal of attention has been given to binary regression models for clustered or correlated observations. The data of interest are of the form of a binary dependent or response variable, together with independent variables X1,...., Xk, where sets of observations are grouped together into clusters. A number of models and methods of analysis have been suggested to study such data. Many of these are extensions in some way of the familiar logistic regression model for binary data that are not grouped (i.e., each cluster is of size 1). In general, the analyses of these clustered data models proceed by assuming that the observed clusters are a simple random sample of clusters selected from a population of clusters. In this paper, we consider the application of these procedures to the case where the clusters are selected randomly in a manner that depends on the pattern of responses in the cluster. For example, we show that ignoring the retrospective nature of the sample design, by fitting standard logistic regression models for clustered binary data, may result in misleading estimates of the effects of covariates and the precision of estimated regression coefficients.  相似文献   

4.
Abstract Much of biogeography, conservation and evolutionary biology, and ecology involves very large spatial and temporal extents. Direct manipulation to test hypotheses is usually almost impossible at appropriate scales so that multivariate modelling and especially regression are used to draw causal inferences about which ‘independent’ variables influence the distribution and abundances of species. Such inferences clearly are crucial for the successful management of biological resources and for conserving threatened species. A succession of regression approaches has arisen, many of which yield inconsistent implications. The main problem has been the quest for one (the ‘best’ or the ‘optimal’) regression model from which the impacts of independent variables are inferred. This note is to draw the attention of ecologists to a relatively recent method, hierarchical partitioning, that does not aim to identify a best regression model as such but rather uses all models in a regression hierarchy to distinguish those variables that have high independent correlations with the dependent variable. Such variables are likely to be most influential in controlling variation in the dependent variable. Hierarchical partitioning is not to be regarded as a substitute for experimental manipulation when that is appropriate, but it is likely to produce better deductions than common regression approaches in the many ecological situations in which manipulation is impossible or of doubtful value.  相似文献   

5.
Multivariate recurrent event data are usually encountered in many clinical and longitudinal studies in which each study subject may experience multiple recurrent events. For the analysis of such data, most existing approaches have been proposed under the assumption that the censoring times are noninformative, which may not be true especially when the observation of recurrent events is terminated by a failure event. In this article, we consider regression analysis of multivariate recurrent event data with both time‐dependent and time‐independent covariates where the censoring times and the recurrent event process are allowed to be correlated via a frailty. The proposed joint model is flexible where both the distributions of censoring and frailty variables are left unspecified. We propose a pairwise pseudolikelihood approach and an estimating equation‐based approach for estimating coefficients of time‐dependent and time‐independent covariates, respectively. The large sample properties of the proposed estimates are established, while the finite‐sample properties are demonstrated by simulation studies. The proposed methods are applied to the analysis of a set of bivariate recurrent event data from a study of platelet transfusion reactions.  相似文献   

6.
Population variation in species of Campanula Section Quinqueloculares and related taxa has been studied. 19 species and 48 populations from the Aegean region have been sampled. There are few distinct character states available for analysis so that metric variables have been scored. 11 leaf variables and 19 flower variables have been measured in up to 683 plants. Regressions of variables with overall size have been calculated and where significant allometric variation has been detected residual scores have been used as variables. Hierarchical analysis of variance has indicated that a large part of the total variation is found between populations within species rather than within populations. Between the 10 most closely related species there is a considerable overlap in population means of variables. Multivariate analysis also fails to identify distinct species clusters. In particular principal components analysis provides additional evidence of overlap between species. It is suggested that a detailed study of genetic variation within and between species is required in order to establish the presence of any distinct phylogenetic lineages.  相似文献   

7.
Janowski M  Kunert P 《PloS one》2012,7(4):e35634

Background

The treatment of chronic subdural hematoma (cSDH) is still charged of significant risk of hematoma recurrence. Patient-related predictors and the surgical procedures themselves have been addressed in many studies. In contrast, postoperative management has infrequently been subjected to detailed analysis. Moreover variable intravenous fluid administration (IFA) was not reported in literature till now in the context of cSDH treatment.

Methodology/Principal Findings

A total of 45 patients with cSDH were operated in our department via two burr hole craniostomy within one calendar year. Downward drainage was routinely left in hematoma cavity for a one day. Independent variables selected for the analysis were related to various aspects of patient management, including IFA. Two dependent variables were chosen as measure of clinical course: the rate of hematoma recurrence (RHR) and neurological status at discharge from hospital expressed in points of Glasgow Outcome Scale (GOS). Univariate and multivariate regression analyses were performed. Hematoma recurrence with subsequent evacuation occurred in 7 (15%) patients.Univariate regression analysis revealed that length of IFA after surgery influenced both dependent variables: RHR (p = 0.045) and GOS (p = 0.023). Multivariate regression performed by backward elimination method confirmed that IFA is a sole independent factor influencing RHR. Post hoc dichotomous division of patients revealed that those receiving at least 2000 ml/day over 3 day period revealed lower RHR than the group with less intensive IFA. (p = 0.031).

Conclusions/Significance

IFA has been found to be a sole factor influencing both: RHR and GOS. Based on those results we may recommend administration of at least 2000 ml per 3 days post-operatively to decrease the risk of hematoma recurrence.  相似文献   

8.
The interpretive benefits of employing multivariate analysis methods on experimental data with more than one dependent variable are described heuristically and illustrated on a set of data from a simply designed experiment in physiological psychology. Multivariate analysis of variance (MANOVA) is performed on the 9 dependent variables contained in the sample data and on the four composites derived from a principal components analysis (PCA) of the variability of the nine. A linear discriminant analysis (LDA) is conducted following both MANOVA results, and 5 methods of determining the "important" dependent variables in the experimental-control group difference are presented and discussed in terms of the data at hand.  相似文献   

9.
Hsu CH  Li Y  Long Q  Zhao Q  Lance P 《PloS one》2011,6(10):e25141
In colorectal polyp prevention trials, estimation of the rate of recurrence of adenomas at the end of the trial may be complicated by dependent censoring, that is, time to follow-up colonoscopy and dropout may be dependent on time to recurrence. Assuming that the auxiliary variables capture the dependence between recurrence and censoring times, we propose to fit two working models with the auxiliary variables as covariates to define risk groups and then extend an existing weighted logistic regression method for independent censoring to each risk group to accommodate potential dependent censoring. In a simulation study, we show that the proposed method results in both a gain in efficiency and reduction in bias for estimating the recurrence rate. We illustrate the methodology by analyzing a recurrent adenoma dataset from a colorectal polyp prevention trial.  相似文献   

10.
Multiple imputation (MI) is used to handle missing at random (MAR) data. Despite warnings from statisticians, continuous variables are often recoded into binary variables. With MI it is important that the imputation and analysis models are compatible; variables should be imputed in the same form they appear in the analysis model. With an encoded binary variable more accurate imputations may be obtained by imputing the underlying continuous variable. We conducted a simulation study to explore how best to impute a binary variable that was created from an underlying continuous variable. We generated a completely observed continuous outcome associated with an incomplete binary covariate that is a categorized version of an underlying continuous covariate, and an auxiliary variable associated with the underlying continuous covariate. We simulated data with several sample sizes, and set 25% and 50% of data in the covariate to MAR dependent on the outcome and the auxiliary variable. We compared the performance of five different imputation methods: (a) Imputation of the binary variable using logistic regression; (b) imputation of the continuous variable using linear regression, then categorizing into the binary variable; (c, d) imputation of both the continuous and binary variables using fully conditional specification (FCS) and multivariate normal imputation; (e) substantive-model compatible (SMC) FCS. Bias and standard errors were large when the continuous variable only was imputed. The other methods performed adequately. Imputation of both the binary and continuous variables using FCS often encountered mathematical difficulties. We recommend the SMC-FCS method as it performed best in our simulation studies.  相似文献   

11.
The relationship is examined between vegetation and climate using climatic variables collected from 644 meteorological stations located throughout China. Multivariate methods are applied directly to the raw climatic data in order to define climatic clusters and to examine the relationship between the clusters and vegetation types. This approach is based on the concept of multidimensional climatic space defined by the combination of climatic variables. Phytoclimatic classes are defined on the basis of the distribution of vegetation types in climatic clusters and a new phytoclimatic classification of China is proposed. Patterns of climatic changes between neighbouring phytoclimatic classes are described. Two indexes of the influence of climate on vegetation are proposed based on discriminant analysis.  相似文献   

12.
Multiple Linear Regression (MLR) was used to estimate past salinity and ionic composition in sediments of a paleolake in the southernmost Bolivian Altiplano. Modern diatom assemblages were sampled from the subsurface sediments of 14 saline lagoons, and compared quantitatively with fossil assemblages. Only one application of the method is reported: to a Quaternary outcrop located on the border of one of them (Laguna Ballivian). Multivariate analysis was performed on diatom data to check the range of variation of both the fossil and modern flora. It also allowed a reduced set of explanatory variables for regression analysis. Finally, a number of transformations of the chemical variables was computed prior to the regression process in order to stabilize the residuals variance and obtain a good error estimate. Only ten chemical components were studied, but the method could be applied to other palaoenvironmental parameters.  相似文献   

13.
Liu M  Taylor JM  Belin TR 《Biometrics》2000,56(4):1157-1163
This paper outlines a multiple imputation method for handling missing data in designed longitudinal studies. A random coefficients model is developed to accommodate incomplete multivariate continuous longitudinal data. Multivariate repeated measures are jointly modeled; specifically, an i.i.d. normal model is assumed for time-independent variables and a hierarchical random coefficients model is assumed for time-dependent variables in a regression model conditional on the time-independent variables and time, with heterogeneous error variances across variables and time points. Gibbs sampling is used to draw model parameters and for imputations of missing observations. An application to data from a study of startle reactions illustrates the model. A simulation study compares the multiple imputation procedure to the weighting approach of Robins, Rotnitzky, and Zhao (1995, Journal of the American Statistical Association 90, 106-121) that can be used to address similar data structures.  相似文献   

14.
If a dependent variable in a regression analysis is exceptionally expensive or hard to obtain the overall sample size used to fit the model may be limited. To avoid this one may use a cheaper or more easily collected “surrogate” variable to supplement the expensive variable. The regression analysis will be enhanced to the degree the surrogate is associated with the costly dependent variable. We develop a Bayesian approach incorporating surrogate variables in regression based on a two‐stage experiment. Illustrative examples are given, along with comparisons to an existing frequentist method. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

15.
The amount of 3Fe clusters in Thermodesulfobacterium commune ferredoxin is strongly dependent upon the presence of oxygen during the purification. An average of one 3Fe cluster per monomer can be found when the purification is not strictly anaerobic. These clusters are converted into |4Fe-4S| clusters by adding dithionite at usual pH and without adjunction of Fe2+. The EPR potentiometric titration reveals the existence of several types of 3Fe clusters with negative midpoint potentials differing by more than 100 mV. When the |4Fe-4S| clusters are partially reduced the EPR signal is composed of two different rhombic components. The component with gz = 2.04 could be related to a site implicated in the interconversion processes. In the fully reduced state, the spectrum presents the typical features of two interacting |4Fe-4S| clusters as those observed in two |4Fe-4S| bacterial ferredoxins. From the redox titration curves the midpoint potentials of these clusters are estimated at −395 and −435 mV.  相似文献   

16.
《Anthrozo?s》2013,26(2):95-108
ABSTRACT

This study explores two explanations for well-being among older adults: social network analysis and human–animal relations. The data are based on a stratified probability sample of community living adults, aged 60 years and over, located in several rural communities in north-eastern Oregon. The dependent variables include three distinct measures of psychological well-being: life satisfaction, locus of control, and self-esteem. The independent variables include pet ownership and various measures of the personal network of respondents, including network structural variables (range, density, and frequency of contact) and network content variables (both instrumental and expressive). Control variables include socio-demographic characteristics and a measure of physical health. Multiple regression analysis is used to estimate the effects of the independent and control variables. Pet ownership was related to two of the well-being measures – self-esteem and locus of control – but only for men. Some methodological and theoretical implications are discussed.  相似文献   

17.
18.
Bondell HD  Reich BJ 《Biometrics》2008,64(1):115-123
Summary .   Variable selection can be challenging, particularly in situations with a large number of predictors with possibly high correlations, such as gene expression data. In this article, a new method called the OSCAR (octagonal shrinkage and clustering algorithm for regression) is proposed to simultaneously select variables while grouping them into predictive clusters. In addition to improving prediction accuracy and interpretation, these resulting groups can then be investigated further to discover what contributes to the group having a similar behavior. The technique is based on penalized least squares with a geometrically intuitive penalty function that shrinks some coefficients to exactly zero. Additionally, this penalty yields exact equality of some coefficients, encouraging correlated predictors that have a similar effect on the response to form predictive clusters represented by a single coefficient. The proposed procedure is shown to compare favorably to the existing shrinkage and variable selection techniques in terms of both prediction error and model complexity, while yielding the additional grouping information.  相似文献   

19.
An analysis of the relationships between lotic macroinvertebrates and environmental variables was earned out on material from 60 riffle sites in streams in northern Sweden The approach involved the use of TWINSPAN classification and canonical correspondence analysis on presence/absence data from two seasons (spring and autumn) Variables most strongly associated with distribution patterns of assemblages were drainage area, elevation, water quality in terms of alkalinity, colour and phosphate and the presence of macrophytes The significance of affinities of different species to these variables are discussed The eight clusters resulting from the TWINSPAN analysis could biologically be interpreted as classes of taxa related to stream size, chemical conditions and algae A multiple regression analysis for predicting species nchness using three independent variables, viz drainage area amount or organic matter, and discharge was constructed The results of the study could be used as a starting point for further work on the community organization of benthie stream assemblages  相似文献   

20.
Contamination, by pollen, from neighbouring accessions is a risk during Vicia faba seed stock multiplication. In insect‐pollinated plants, floral traits are expected to play an important role determining pollen‐mediated gene flow variation. The aim of this study was to identify floral traits associated with interplot gene flow that could be used as effective predictors of gene flow potential to improve between‐plot isolation strategies in the field for seed multiplication. The isolation strategies tested were a barren zone, two trap crops, a faba bean male sterile variety and a tetraploid genotype and a non‐pollinated crop, a Vicia narbonensis population. Four genotypes, fixed for alternative isozyme alleles allowed identification of between‐plot hybrids through progeny testing. G‐test was used to analyse the independence of isolation zone and genotype. Principal component analysis was used to obtain information on the usefulness of faba bean traps as barriers to prevent interplot gene flow. Paternity analysis in conjunction with multiple regression allowed the determination of floral traits that accounted for the largest proportion of variation in pollen‐mediated gene flow across each of the four isolation strategies. The floral traits studied were grouped into functional categories and were used as independent variables with gene flow as the dependent variable in a forward stepwise multiple regression procedure. There were large variations in gene flow level and patterns among the genotypes. It was shown that gene flow was isolation zone and genotype dependent. Multivariate regression models showed that differences in floral traits accounted for more than 70% of the variation in interplot gene flow. The relative importance of various floral traits depends on the isolation zone. Flower advertisement appeared important in predicting gene flow in plots surrounded by a barren zone. With regard to plots surrounded by a V. narbonensis population, the role of a reward trait, pollen production, was established. In contrast, in plots surrounded by faba bean trap crops, ovary length played the most important and consistent role in accounting for variation in gene flow. Our results show that interplot gene flow is largely dependent on floral traits, and thus confirm the utility of floral traits for generating hypotheses about cultivar outcrossing or inbreeding behaviour. Thus, it could help to devise more efficient procedures for seed multiplication.  相似文献   

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