Mixed Models (mixed + models)

Distribution by Scientific Domains

Kinds of Mixed Models

  • generalized linear mixed models
  • linear mixed models
  • semiparametric mixed models

  • Selected Abstracts

    Stress-induced dynamic adjustments of reproduction differentially affect fitness components of a semi-arid plant

    JOURNAL OF ECOLOGY, Issue 1 2008
    Cristina F. Aragón
    Summary 1Summer drought stress is considered the primary constraint to plant performance in Mediterranean ecosystems. However, little is known about the implications of summer stress for plant reproduction under real field conditions and, particularly, for the regulatory mechanisms of maternal investment in reproduction. 2The relationship between plant physiological status at different reproductive stages over the course of the summer drought period and final reproductive output was modelled in the Mediterranean semi-arid specialist Helianthemum squamatum. 3Plant physiological status, assessed by the chlorophyll fluorescence-based parameter Fv/Fm, and soil moisture content beneath each plant, were determined in the field at five key phenological moments in a total of 88 plants. We used Generalized Linear Mixed Models to evaluate the effect of plant physiological status at those different dates on several components of reproduction (number of flowers and seeds per plant, fruit-set and intra-fruit seed abortion). We included soil moisture as an additional predictor to statistically control its potential effect on reproduction. 4Fv/Fm measured at midday was a significant predictor of reproductive output, but its significance varied over time and with the specific reproductive response variable. Fv/Fm measured at the onset of flowering was positively related to the number of flowers and seeds per plant, whereas Fv/Fm at the fruiting peak positively affected fruit-set. Soil moisture content was only significant when measured before flowering, being positively related to total flowers and seeds. The effect of stress on reproductive output acted either at an early stage of the reproductive season, by varying the number of flowers produced and seed primordia initiated, or at a later stage, by adjusting the number or ripe fruits. 5Synthesis. Our results show a direct relationship between physiological status and reproduction, and highlight the importance of the timing of stress for reproductive success. They also show that small departures from the physiological optimum at specific reproductive stages may cause significant decreases in the reproductive output. We suggest that the dynamic adjustment of reproduction in response to stress is adaptive in fluctuating and unpredictable Mediterranean semi-arid environments, where an adequate temporal distribution of maternal resources determines the species' ability to withstand severe environmental conditions. [source]

    Linear Mixed Models: a Practical Guide using Statistical Software

    R. Allan Reese
    No abstract is available for this article. [source]

    Applied Mixed Models in Medicine, 2nd edn by H. Brown and R. Prescott

    Ana F. Militino
    No abstract is available for this article. [source]

    SAS for Mixed Models by R. C. Littell, G. A. Milliken, W. W. Stroup, R. D. Wolfinger and O. Schabenberger

    Derek Robinson
    No abstract is available for this article. [source]

    Social Exchange in Work Settings: Content, Process, and Mixed Models

    Lynn M. Shore
    abstract Social exchange theory has provided the dominant basis for understanding exchange relationships in organizational settings. Despite its predominance within the management field, there are a number of unaddressed issues. This special issue seeks to further social exchange research in work settings. We differentiate social from economic exchange and highlight the moderating role of cultural and individual differences in explaining the outcomes associated with social exchange relationships. We introduce the ideas of content, process, and mixed models of exchange to reflect the different emphases given to the amount and type of resources exchanged, the quality of the relationship, and a combination of both. The five papers in this special issue illustrate these models. We discuss the applicability of social exchange theory across cultural contexts and present suggestions for future research. [source]

    Linear Mixed Models , A Practical Guide Using Statistical Software.


    No abstract is available for this article. [source]

    Bayesian Inference in Semiparametric Mixed Models for Longitudinal Data

    BIOMETRICS, Issue 1 2010
    Yisheng Li
    Summary We consider Bayesian inference in semiparametric mixed models (SPMMs) for longitudinal data. SPMMs are a class of models that use a nonparametric function to model a time effect, a parametric function to model other covariate effects, and parametric or nonparametric random effects to account for the within-subject correlation. We model the nonparametric function using a Bayesian formulation of a cubic smoothing spline, and the random effect distribution using a normal distribution and alternatively a nonparametric Dirichlet process (DP) prior. When the random effect distribution is assumed to be normal, we propose a uniform shrinkage prior (USP) for the variance components and the smoothing parameter. When the random effect distribution is modeled nonparametrically, we use a DP prior with a normal base measure and propose a USP for the hyperparameters of the DP base measure. We argue that the commonly assumed DP prior implies a nonzero mean of the random effect distribution, even when a base measure with mean zero is specified. This implies weak identifiability for the fixed effects, and can therefore lead to biased estimators and poor inference for the regression coefficients and the spline estimator of the nonparametric function. We propose an adjustment using a postprocessing technique. We show that under mild conditions the posterior is proper under the proposed USP, a flat prior for the fixed effect parameters, and an improper prior for the residual variance. We illustrate the proposed approach using a longitudinal hormone dataset, and carry out extensive simulation studies to compare its finite sample performance with existing methods. [source]

    Variable Selection for Semiparametric Mixed Models in Longitudinal Studies

    BIOMETRICS, Issue 1 2010
    Xiao Ni
    Summary We propose a double-penalized likelihood approach for simultaneous model selection and estimation in semiparametric mixed models for longitudinal data. Two types of penalties are jointly imposed on the ordinary log-likelihood: the roughness penalty on the nonparametric baseline function and a nonconcave shrinkage penalty on linear coefficients to achieve model sparsity. Compared to existing estimation equation based approaches, our procedure provides valid inference for data with missing at random, and will be more efficient if the specified model is correct. Another advantage of the new procedure is its easy computation for both regression components and variance parameters. We show that the double-penalized problem can be conveniently reformulated into a linear mixed model framework, so that existing software can be directly used to implement our method. For the purpose of model inference, we derive both frequentist and Bayesian variance estimation for estimated parametric and nonparametric components. Simulation is used to evaluate and compare the performance of our method to the existing ones. We then apply the new method to a real data set from a lactation study. [source]

    Cluster Detection Based on Spatial Associations and Iterated Residuals in Generalized Linear Mixed Models

    BIOMETRICS, Issue 2 2009
    Tonglin Zhang
    Summary Spatial clustering is commonly modeled by a Bayesian method under the framework of generalized linear mixed effect models (GLMMs). Spatial clusters are commonly detected by a frequentist method through hypothesis testing. In this article, we provide a frequentist method for assessing spatial properties of GLMMs. We propose a strategy that detects spatial clusters through parameter estimates of spatial associations, and assesses spatial aspects of model improvement through iterated residuals. Simulations and a case study show that the proposed method is able to consistently and efficiently detect the locations and magnitudes of spatial clusters. [source]

    Diagnosis of Random-Effect Model Misspecification in Generalized Linear Mixed Models for Binary Response

    BIOMETRICS, Issue 2 2009
    Xianzheng Huang
    Summary Generalized linear mixed models (GLMMs) are widely used in the analysis of clustered data. However, the validity of likelihood-based inference in such analyses can be greatly affected by the assumed model for the random effects. We propose a diagnostic method for random-effect model misspecification in GLMMs for clustered binary response. We provide a theoretical justification of the proposed method and investigate its finite sample performance via simulation. The proposed method is applied to data from a longitudinal respiratory infection study. [source]

    Linear Model Theory: Univariate, Multivariate, and Mixed Models edited by Muller, K. E. and Stewart, P. W.

    BIOMETRICS, Issue 1 2007
    Article first published online: 16 APR 200
    No abstract is available for this article. [source]

    Bayesian Covariance Selection in Generalized Linear Mixed Models

    BIOMETRICS, Issue 2 2006
    Bo Cai
    Summary The generalized linear mixed model (GLMM), which extends the generalized linear model (GLM) to incorporate random effects characterizing heterogeneity among subjects, is widely used in analyzing correlated and longitudinal data. Although there is often interest in identifying the subset of predictors that have random effects, random effects selection can be challenging, particularly when outcome distributions are nonnormal. This article proposes a fully Bayesian approach to the problem of simultaneous selection of fixed and random effects in GLMMs. Integrating out the random effects induces a covariance structure on the multivariate outcome data, and an important problem that we also consider is that of covariance selection. Our approach relies on variable selection-type mixture priors for the components in a special Cholesky decomposition of the random effects covariance. A stochastic search MCMC algorithm is developed, which relies on Gibbs sampling, with Taylor series expansions used to approximate intractable integrals. Simulated data examples are presented for different exponential family distributions, and the approach is applied to discrete survival data from a time-to-pregnancy study. [source]

    Bayesian Prediction of Spatial Count Data Using Generalized Linear Mixed Models

    BIOMETRICS, Issue 2 2002
    Ole F. Christensen
    Summary. Spatial weed count data are modeled and predicted using a generalized linear mixed model combined with a Bayesian approach and Markov chain Monte Carlo. Informative priors for a data set with sparse sampling are elicited using a previously collected data set with extensive sampling. Furthermore, we demonstrate that so-called Langevin-Hastings updates are useful for efficient simulation of the posterior distributions, and we discuss computational issues concerning prediction. [source]

    On Estimation and Prediction for Spatial Generalized Linear Mixed Models

    BIOMETRICS, Issue 1 2002
    Hao Zhang
    Summary. We use spatial generalized linear mixed models (GLMM) to model non-Gaussian spatial variables that are observed at sampling locations in a continuous area. In many applications, prediction of random effects in a spatial GLMM is of great practical interest. We show that the minimum mean-squared error (MMSE) prediction can be done in a linear fashion in spatial GLMMs analogous to linear kriging. We develop a Monte Carlo version of the EM gradient algorithm for maximum likelihood estimation of model parameters. A by-product of this approach is that it also produces the MMSE estimates for the realized random effects at the sampled sites. This method is illustrated through a simulation study and is also applied to a real data set on plant root diseases to obtain a map of disease severity that can facilitate the practice of precision agriculture. [source]

    Dynamic Conditionally Linear Mixed Models for Longitudinal Data

    BIOMETRICS, Issue 1 2002
    M. Pourahmadi
    Summary. We develop a new class of models, dynamic conditionally linear mixed models, for longitudinal data by decomposing the within-subject covariance matrix using a special Cholesky decomposition. Here ,dynamic' means using past responses as covariates and ,conditional linearity' means that parameters entering the model linearly may be random, but nonlinear parameters are nonrandom. This setup offers several advantages and is surprisingly similar to models obtained from the first-order linearization method applied to nonlinear mixed models. First, it allows for flexible and computationally tractable models that include a wide array of covariance structures; these structures may depend on covariates and hence may differ across subjects. This class of models includes, e.g., all standard linear mixed models, antedependence models, and Vonesh-Carter models. Second, it guarantees the fitted marginal covariance matrix of the data is positive definite. We develop methods for Bayesian inference and motivate the usefulness of these models using a series of longitudinal depression studies for which the features of these new models are well suited. [source]

    Modeling mood variation associated with smoking: an application of a heterogeneous mixed-effects model for analysis of ecological momentary assessment (EMA) data

    ADDICTION, Issue 2 2009
    Donald Hedeker
    ABSTRACT Aims Mixed models are used increasingly for analysis of ecological momentary assessment (EMA) data. The variance parameters of the random effects, which indicate the degree of heterogeneity in the population of subjects, are considered usually to be homogeneous across subjects. Modeling these variances can shed light on interesting hypotheses in substance abuse research. Design We describe how these variances can be modeled in terms of covariates to examine the covariate effects on between-subjects variation, focusing on positive and negative mood and the degree to which these moods change as a function of smoking. Setting The data are drawn from an EMA study of adolescent smoking. Participants Participants were 234 adolescents, either in 9th or 10th grades, who provided EMA mood reports from both random prompts and following smoking events. Measurements We focused on two mood outcomes: measures of the subject's negative and positive affect and several covariates: gender, grade, negative mood regulation and smoking level. Findings and conclusions Following smoking, adolescents experienced higher positive affect and lower negative affect than they did at random, non-smoking times. Our analyses also indicated an increased consistency of subjective mood responses as smoking experience increased and a diminishing of mood change. [source]

    An in vivo Study of the Relationship between Craving and Reaction Time during Alcohol Detoxification Using the Ecological Momentary Assessment

    ALCOHOLISM, Issue 12 2005
    M Lukasiewicz
    Abstract: Background: To study cognitive interference associated with craving for alcohol, the Ecological Momentary Assessment (EMA) method was used to measure the relationship between craving and reaction time. A secondary aim was the study of the predictive factors for craving during alcohol detoxification. The EMA enables both repeated measures of craving in a natural setting and the recording of reaction time without the patient being aware of this. Methods: Craving for alcohol, reaction time, sadness and anxiety were recorded 8 to 12 times a day, over three weeks of detoxification in 14 alcoholics (n= 1767 measures), on an electronic diary issuing random prompts. Mixed models were used for statistical analysis (,= 5%, 1-,= 88%). Results: Reaction time was significantly increased in univariate analysis when a craving episode occurred but this difference did not persist after multivariate analysis. Craving episodes were more frequent and intense than previously reported. Predictive factors of craving during detoxification were: age, gender, sadness, anxiety and the number of previous detoxifications. Antidepressants, anticraving medications but not benzodiazepines were negatively associated to craving. [source]

    High dimensional multivariate mixed models for binary questionnaire data

    Steffen Fieuws
    Summary., Questionnaires that are used to measure the effect of an intervention often consist of different sets of items, each set possibly measuring another concept. Mixed models with set-specific random effects are a flexible tool to model the different sets of items jointly. However, computational problems typically arise as the number of sets increases. This is especially true when the random-effects distribution cannot be integrated out analytically, as with mixed models for binary data. A pairwise modelling strategy, in which all possible bivariate mixed models are fitted and where inference follows from pseudolikelihood theory, has been proposed as a solution. This approach has been applied to assess the effect of physical activity on psychocognitive functioning, the latter measured by a battery of questionnaires. [source]

    Limited effects of above- and belowground insects on community structure and function in a species-rich grassland

    Malcolm D. Coupe
    Abstract Question: Do above- and belowground insects differentially impact plant community structure and function in a diverse native grassland? Location: Rough fescue prairie in Alberta, Canada. Methods: Above- and belowground insects were suppressed with insecticides for 5 years using a randomised block design. During this experiment, a severe drought began in 2001 and ended in 2003. Aboveground plant growth was measured as cover and biomass from 2001 to 2005. Root demography was measured in 2002 using a minirhizotron. Mixed models and repeated measures ANOVA were used to determine treatment effects on a number of response variables. MRBP were used to test for treatment effects on community composition. Results: Five years of insect suppression had few significant effects on plant growth, species richness or community composition, and were limited primarily to the severe drought in 2002. During the drought, insect attack increased root mortality, reduced plant cover, and altered community composition. Following the drought, plant responses were unaffected by insecticide application for the remainder of the experiment. Conclusions: Five years of insect suppression had only minor effects on community structure and function in this diverse native grassland. There was no indication that these effects increased over time. The results are counter to studies conducted in productive old-field communities that revealed large effects of insects on community structure. We suggest that the unique features of this system, such as high species evenness, abundance of generalist herbivores, and a lack of competition for light among plants, limited the potential for insects to greatly impact community-level processes. [source]

    Double-blind randomized trial of risperidone versus divalproex in pediatric bipolar disorder

    BIPOLAR DISORDERS, Issue 6 2010
    Mani N Pavuluri
    Pavuluri MN, Henry DB, Findling RL, Parnes S, Carbray JA, Mohammed T, Janicak PG, Sweeney JA. Double-blind randomized trial of risperidone versus divalproex in pediatric bipolar disorder. Bipolar Disord 2010: 12: 593,605. © 2010 The Authors. Journal compilation © 2010 John Wiley & Sons A/S. Objective:, To determine the relative effects of risperidone and divalproex in pediatric mania. Methods:, This is a double-blind, randomized, outpatient clinical trial with 66 children and adolescents (mean age = 10.9 ± 3.3 years; age range = 8,18 years) with mania who were randomly assigned to either risperidone (0.5,2 mg/day, n = 33) or divalproex (60,120 ,g/mL, n = 33) for a six-week period. Measures included the Young Mania Rating Scale (YMRS) and Child Depression Rating Scale-Revised (CDRS-R). Results:, Mixed-effects regression models, with interaction between time and the active drug as predictors, found that the risperidone group had more rapid improvement than the divalproex group (p < 0.05), although final scores did not differ significantly between groups. Mixed models using only those subjects who completed the six-week study found similar results. The response rate on YMRS was 78.1% for risperidone and 45.5% for divalproex (p < 0.01). The remission rate for risperidone was 62.5%, compared with 33.3% for divalproex (p < 0.05). Improvement on the CDRS-R was significantly higher for the risperidone group relative to the divalproex group (p < 0.05). There were no significant differences between groups in safety, but subject retention was significantly higher at study endpoint in the risperidone group (p < 0.01). Dropout rate was 24% in the risperidone group and 48% in the divalproex group, with increased irritability being the most common reason for dropout in the latter. There was no significant weight gain in either group. Conclusion:, Results suggest that risperidone was associated with more rapid improvement and greater reduction in manic symptoms compared to divalproex. Although the results suggest that both drugs are safe, risperidone's lower attrition rate and lower rate of adverse events may suggest better toleration. Clinical trials with larger samples are required to confirm these preliminary findings. [source]

    A randomized controlled study of paroxetine and cognitive-behavioural therapy for late-life panic disorder

    G.-J. Hendriks
    Hendriks G-J, Keijsers GPJ, Kampman M, Oude Voshaar RC, Verbraak MJPM, Broekman TG, Hoogduin CAL. A randomized controlled study of paroxetine and cognitive-behavioural therapy for late-life panic disorder. Objective:, To examine the effectiveness of paroxetine and cognitive-behavioural therapy (CBT) in elderly patients suffering from panic disorder with or without agoraphobia (PD(A)). Method:, Forty-nine patients aged 60+ years with confirmed PD(A) were randomly assigned to 40 mg paroxetine, individual CBT, or to a 14-week waiting list. Outcomes, with avoidance behaviour and agoraphobic cognitions being the primary measures, were assessed at baseline and at weeks 8, 14 (conclusion CBT/waiting list), and at week 26 (treated patients only) and analysed using mixed models. Results:, All outcome measures showed that the patients having received CBT and those treated with paroxetine had significantly better improvement compared with those in the waiting-list condition. With one patient (1/20, 5%) in the CBT and three (3/14, 17.6%) in the paroxetine condition dropping out, attrition rates were low. Conclusion:, Patients with late-life panic disorder respond well to both paroxetine and CBT. Although promising, the outcomes warrant replication in larger study groups. [source]

    Maternal prenatal anxiety, postnatal caregiving and infants' cortisol responses to the still-face procedure

    Kerry-Ann Grant
    Abstract This study prospectively examined the separate and combined influences of maternal prenatal anxiety disorder and postnatal caregiving sensitivity on infants' salivary cortisol responses to the still-face procedure. Effects were assessed by measuring infant salivary cortisol upon arrival at the laboratory, and at 15-, 25-, and 40-min following the still-face procedure. Maternal symptoms of anxiety during the last 6 months of pregnancy were assessed using clinical diagnostic interview. Data analyses using linear mixed models were based on 88 women and their 7-month-old infants. Prenatal anxiety and maternal sensitivity emerged as independent, additive moderators of infant cortisol reactivity, F (3, 180),=,3.29, p,=,.02, F (3, 179),=,2.68, p,=,.05 respectively. Results were independent of maternal prenatal depression symptoms, and postnatal symptoms of anxiety and depression. Infants' stress-induced cortisol secretion patterns appear to relate not only to exposure to maternal prenatal anxiety, but also to maternal caregiving sensitivity, irrespective of prenatal psychological state. © 2009 Wiley Periodicals, Inc. Dev Psychobiol 51: 625,637, 2009 [source]

    Post-challenge glucose predicts coronary atherosclerotic progression in non-diabetic, post-menopausal women,

    DIABETIC MEDICINE, Issue 10 2007
    P. B. Mellen
    Abstract Aims, We sought to determine whether fasting or post-challenge glucose were associated with progression of coronary atherosclerosis in non-diabetic women. Methods, We performed a post-hoc analysis of 132 non-diabetic women who underwent 75-g oral glucose tolerance testing. The primary outcome of interest was progression of atherosclerosis determined by baseline and follow-up coronary angiography, a mean of 3.1 ± 0.9 years apart. We analysed the association of change in minimal vessel diameter (,MD) by quartile of fasting and post-challenge glucose using mixed models that included adjustment for age, systolic blood pressure, total : high-density lipoprotein cholesterol ratio, current smoking, lipid-lowering and anti-hypertensive medication use and other covariates. Results, At baseline, participants had a mean age of 65.7 ± 6.7 years and a mean body mass index of 27.9 ± 8.5 kg/m2. Although there were no significant differences in atherosclerotic progression by fasting glucose category (P for trend across quartiles = 0.99), there was a significant inverse association between post-challenge glucose and ,MD (in mm) (Q1 : 0.01 ± 0.03; Q2 : 0.08 ± 0.03; Q3 : 0.13 ± 0.03; Q4 : 0.11 ± 0.03; P for trend = 0.02). Conclusions, In post-menopausal women without diabetes, post-challenge glucose predicts angiographic disease progression. These findings suggest that even modest post-challenge hyperglycaemia influences the pathogenesis of atherosclerotic progression. [source]

    Testing for trends in the violation frequency of an environmental threshold in rivers

    ENVIRONMETRICS, Issue 1 2009
    Lieven Clement
    Abstract Nutrient pollution in rivers is a common problem. It can provoke algae blooms which are related to increased fish mortality. To restore the water status, the regulator recently has promulgated more restrictive regulations. In Flanders for instance, the government has introduced several manure decrees (MDs) to restrict nutrient pollution. Environmental regulations are commonly expressed in terms of threshold levels. This provides a binary response to the decision maker. To handle such data, we propose the use of marginalised generalised linear mixed models. They provide valid inference on trends in the exceedance frequency. The spatio-temporal dependence of the river monitoring network is incorporated by the use of a latent variable. The temporal dependence is assumed to be AR(1) and the spatial dependence is derived from the river topology. The mean model contains a term for the trend and corrects for seasonal variation. The model formulation allows an assessment on the level of individual sampling locations and on a more regional scale. The methodology is applied to a case study on the river Yzer (Flanders). It assesses the impact of the MDs on the violation probability of the nitrate standard. A trend change is detected after the introduction of the second MD. Copyright © 2008 John Wiley & Sons, Ltd. [source]

    Quantification of surface EMG signals to monitor the effect of a Botox treatment in six healthy ponies and two horses with stringhalt: Preliminary study

    I. D. Wijnberg
    Summary Reasons for performing the study: Therapeutic options for stringhalt in horses are limited, whereas medical experiences with botulinum toxin type A (Botox) have been positive. To evaluate its effectiveness in horses, surface electromyography (sEMG) signals before and after injection need to be quantified. Hypothesis: Treatment of healthy ponies and cases with Botox should reduce muscle activity in injected muscles and reduce spastic movements without adverse side effects. Methods: Unilaterally, the extensor digitorum longus, extensor digitorum lateralis and lateral vastus muscles of 6 healthy mature Shetland ponies and 2 talented Dutch Warmblood dressage horses with stringhalt were injected (maximum of 400 iu per pony and 700 iu per case; 100 iu in 5 ml NaCl divided into 5 injections) with Botox under needle EMG guidance. Surface EMG data were evaluated using customised software, and in the individuals gait was analysed using Proreflex. Statistical analysis was performed using mixed models and independent sample t test (P<0.05). Results: Surface EMG signals were quantified using customised software. The area under the curve (integrated EMG) in time was used as variable. It became significantly reduced in injected muscles after injection of Botox in normal ponies (P<0.05). This effect was present from Day 1 until Day 84 after injection. In the 2 cases, after injection of 3 muscles, the integrated EMG in time became significantly reduced in all 3 muscles. Kinematic measurements confirmed reduction of frequency and amplitude of hyperflexing or hyperabducting strides of the affected hindlimbs. The duration of effect was also seen in the cases until around 12 weeks after injection. Conclusions and potential relevance: After EMG guided injections of Botox, sEMG signals recorded from injected muscle were reduced, which proves this to be a useful tool in statistically evaluating a treatment effect. The positive results of this pilot study encourage further research with a larger group of clinical cases. [source]

    An echocardiographic and auscultation study of right heart responses to training in young National Hunt Thoroughbred horses

    Summary Reasons for performing study: There are few data available to determine the effect of training on cardiac valve function. Objectives: To investigate the effect of commercial race training on right ventricular (RV) and tricuspid valve function in an untrained group of National Hunt Thoroughbreds (TB). Material and methods: Cardiac auscultation, guided M-mode echocardiography of the RV, and colour flow Doppler (CFD) tricuspid valve and right atrium were performed in 90 TB horses (age 2,7 years) 1998,2003. Forty horses were examined at least once and 48 horses were examined on at least 2 occasions. Examinations were then classified as: i) before commencement of race training, ii) after cantering exercise had been sustained for a period of 8,12 weeks and iii) at full race fitness. Tricuspid valve regurgitation (TR) murmurs were graded on a 1,6 scale and CFD echocardiography TR signals were graded on a 1,9 scale. Right ventricular internal diameter (RVID) in diastole and systole (RVIDd and RVIDs) was measured by guided M-mode. Associations between continuous RVID and TR measures and explanatory covariates of weight, age, heart rate, yard and stage of training were examined using general linear mixed models with horse-level random effects. Results: On average, RVIDd and RVIDs increased by 0.08 and 0.1 cm, respectively, per year increase in age (P=0.1 and 0.02) and by 0.3 and 0.4 cm, respectively between pre-training and race fitness (P = 0.07 and 0.005). Tricuspid regurgitation score by colour flow Doppler increased by 0.6/year with age (P<0.0001) and by 1.8 between pre-training and race fitness (P< 0.0001). No significant associations were found between any outcomes and weight, heart rate and training yard. Due to the high level of co-linearity between age and training, multivariable models including both terms were not interpretable. Conclusions and clinical relevance: Athletic training of horses exerts independent effects on both severity and prevalence of tricuspid valve incompetence. This effect should therefore be taken into account when examinations are performed. Dimensions of RV increase with age and training in TB horses in a manner that appears to be similar to that of the LV. [source]

    Airway inflammation in Michigan pleasure horses: prevalence and risk factors

    N. E. Robinson
    Summary Reasons for performing study: Although subclinical airway inflammation is thought to be common in horses, there is little information on its prevalence and none on risk factors. Objective: To determine the prevalence and risk factors for an increased number of inflammatory cells and for mucus accumulation in the trachea of pleasure horses. Methods: Horses (n = 266) in stables (n = 21) in Michigan were examined endoscopically, once in winter and once in summer 2004. Visible tracheal mucoid secretions were graded 0,5 and inflammatory cell numbers counted in a tracheal lavage sample. Information collected about each horse included age, gender, presence of cough, percent time indoors and source of roughage. The repeated measures were analysed by generalised estimating equations and linear mixed models. Results: Horses eating hay, especially from round bales, had the most neutrophils, whereas horses feeding from pasture had the fewest. Being female and being outdoors in winter were associated with increased numbers of inflammatory cells. Older horses had fewer macrophages than young horses. More than 70% of horses had >20% neutrophils in tracheal lavage. Twenty percent of horses had a mucus accumulation score >1; 17% had both a mucus score >1 and >20% neutrophils. The significant risk factors for mucus accumulation >1 were age >15 years, feeding on hay as compared to pasture, and being outdoors for more than 80% time in winter. Even though mucus accumulation score >1 was a risk factor for cough, only half of such horses coughed. Cough and mucus accumulation were associated with increased number of neutrophils. Conclusions: In comparison to pasture feeding, hay feeding, particularly from round bales, was associated with an increased number of neutrophils in the airway. Being outdoors in winter was associated with increased numbers of inflammatory cells and with mucus accumulation. Because 70% of horses have >20% neutrophils, this value should not be used as the sole indicator of airway inflammation. Potential relevance: The study reinforces the importance of hay feeding and older age as risk factors for inflammatory airway disease. Horses that do not have ,heaves' may be best kept indoors when winters are cold. [source]

    Summer drought: a driver for crown condition and mortality of Norway spruce in Norway

    FOREST PATHOLOGY, Issue 2 2004
    S. Solberg
    Summary Summer drought, i.e. unusually dry and warm weather, has been a significant stress factor for Norway spruce in southeast Norway during the 14 years of forest monitoring. Dry and warm summers were followed by increases in defoliation, discolouration of foliage, cone formation and mortality. The causal mechanisms are discussed. Most likely, the defoliation resulted from increased needle-fall in the autumn after dry summers. During the monitoring period 1988,2001, southeast Norway was repeatedly affected by summer drought, in particular, in the early 1990s. The dataset comprised 455 ,Forest officers' plots' with annual data on crown condition and mortality. Linear mixed models were used for estimation and hypothesis testing, including a variance,covariance structure for the handling of random effects and temporal autocorrelation. Résumé La sécheresse estivale, c'est à dire un temps exceptionnellement sec et chaud, a été un facteur significatif de stress pour l'Epicéa commun dans le sud-est de la Norvège au cours de 14 années de surveillance. Les étés secs et chauds ont été suivis d'une augmentation de la défoliation, des colorations anormales du feuillage, de la formation de cônes et de la mortalité. Les mécanismes causaux sont discutés. La défoliation peut probablement s'expliquer par une chute automnale des aiguilles après les étés secs. Pendant la période de suivi de 1988 à 2001, le sud-est de la Norvège a été affecté de façon répétée par des sécheresses estivales, en particulier au début des années 1990. La base de données comprend 455 ,parcelles d'agents forestiers' avec des données annuelles sur l'état des houppiers et la mortalité. Des modèles linéaires mixtes ont été utilisés pour tester les hypothèses et faire les estimations, en incluant une structure de variance-covariance pour prendre en compte les effets aléatoires et les auto-corrélations temporelles. Zusammenfassung Sommertrockenheit, d.h. ungewöhnlich trockenes und warmes Wetter, war ein wesentlicher Stressfaktor für die Fichte (Picea abies) in Südwestnorwegen während der 14 Jahre, in denen der Waldzustand bisher erfasst wurde. Nach trockenen und warmen Sommern nahmen der Nadelverlust, die Nadelverfärbung, die Zapfenbildung und die Mortalität zu. Die ursächlichen Mechanismen hierfür werden diskutiert. Am wahrscheinlichsten ist der Blattverlust das Ergebnis eines erhöhten Nadelfalles im Herbst nach einem trockenen Sommer. Während der Beobachtungsperiode von 1988 bis 2001 traten in Südwestnorwegen wiederholt trockene Sommer auf, insbesondere zu Beginn der 90er Jahre. Das Datenset umfasste 455 Stichprobeflächen mit jährlichen Angaben zum Kronenzustand und zur Mortalität. Für die statistische Analyse wurden lineare Modelle mit gemischten Effekten verwendet, einschliesslich einer Varianz-Kovarianzstruktur für die zeitreihenbedingten Autokorrelationen. [source]

    Modelling the hydraulic preferences of benthic macroinvertebrates in small European streams

    FRESHWATER BIOLOGY, Issue 1 2007
    Summary 1. Relating processes occurring at a local scale to the natural variability of ecosystems at a larger scale requires the design of predictive models both to orientate stream management and to predict the effects of larger scale disturbances such as climate changes. Our study contributes to this effort by providing detailed models of the hydraulic preferences of 151 invertebrate taxa, mostly identified at the species level. We used an extensive data set comprising 580 invertebrate samples collected using a Surber net from nine sites of second and third order streams during one, two or three surveys at each site. We used nested non-linear mixed models to relate taxon local densities to bed shear stresses estimated from FliesswasserStammTisch hemisphere numbers. 2. An average model by taxon, i.e. independent from surveys, globally explained 25% of the density variations of taxa within surveys. A quadratic relationship existed between the average preferences and the niche breadth of taxa, indicating that taxa preferring extreme hemisphere numbers had a reduced hydraulic niche breadth. A more complete model, where taxa preferences vary across surveys, globally explained 38% of the variation of taxa densities within surveys. Variations in preferences across surveys were weak for taxa preferring extreme hemisphere numbers. 3. There was a significant taxonomic effect on preferences computed from the complete model. By contrast, season, site, average hemisphere number within a survey and average density of taxa within a survey used as covariates did not consistently explain shifts in taxon hydraulic preferences across surveys. 4. The average hydraulic preferences of taxa obtained from the extensive data set were well correlated to those obtained from two additional independent data sets collected in other regions. The consistency of taxon preferences across regions supports the use of regional preference curves for estimating the impact of river management on invertebrate communities. By contrast, the hydraulic niche breadths of taxa computed from the different data sets were not related. [source]

    Use of longitudinal data in genetic studies in the genome-wide association studies era: summary of Group 14

    Berit Kerner
    Abstract Participants analyzed actual and simulated longitudinal data from the Framingham Heart Study for various metabolic and cardiovascular traits. The genetic information incorporated into these investigations ranged from selected single-nucleotide polymorphisms to genome-wide association arrays. Genotypes were incorporated using a broad range of methodological approaches including conditional logistic regression, linear mixed models, generalized estimating equations, linear growth curve estimation, growth modeling, growth mixture modeling, population attributable risk fraction based on survival functions under the proportional hazards models, and multivariate adaptive splines for the analysis of longitudinal data. The specific scientific questions addressed by these different approaches also varied, ranging from a more precise definition of the phenotype, bias reduction in control selection, estimation of effect sizes and genotype associated risk, to direct incorporation of genetic data into longitudinal modeling approaches and the exploration of population heterogeneity with regard to longitudinal trajectories. The group reached several overall conclusions: (1) The additional information provided by longitudinal data may be useful in genetic analyses. (2) The precision of the phenotype definition as well as control selection in nested designs may be improved, especially if traits demonstrate a trend over time or have strong age-of-onset effects. (3) Analyzing genetic data stratified for high-risk subgroups defined by a unique development over time could be useful for the detection of rare mutations in common multifactorial diseases. (4) Estimation of the population impact of genomic risk variants could be more precise. The challenges and computational complexity demanded by genome-wide single-nucleotide polymorphism data were also discussed. Genet. Epidemiol. 33 (Suppl. 1):S93,S98, 2009. © 2009 Wiley-Liss, Inc. [source]