The main objectives of this master thesis are to explore the effectiveness of nonlinear mixed effects model for longitudinal data. Mixed effect models allow to investigate the nature of relationship between the time-varying covariates and the response while also capturing the variations of subjects. I investigate the robustness of the longitudinal models by building up the complexity of the models starting from multiple linear models and ending up with additive nonlinear mixed models. I use a dataset where firms’ leverage are explained by four explanatory variables in addition to a grouping factor that is the firm factor. The models are compared using comparison statistics such as AIC, BIC and by a visual inspection of residuals. Likelihood ratio test has been used in some nested models only. The models are estimated by maximum likelihood and restricted maximum likelihood estimation. The most efficient model is the nonlinear mixed effects model which has lowest AIC and BIC. The multiple linear regression model failed to explain the relation and produced unrealistic statistics

A Thesis submitted to The University of Arizona College of Medicine - Phoenix in partial fulfillment of the requirements for the Degree of Doctor of Medicine. / End‐of‐life (EOL) care and decision‐making in pediatrics is a challenging and complex aspect of patient care experienced by residents and physician attendings. Previous studies have evaluated determinants that contribute to physicians’ attitudes towards EOL care as well as preparedness of students and residents in EOL decision‐making. However, the determinants contributing to a physician’s ability to make such decisions and feel confident in addressing EOL issues are dynamic. Recognizing that decision‐making changes over time, identifying when these changes occur may demonstrate the need for educational interventions for medical students and residents early in their career to help prepare them for EOL decision‐making. A longitudinal assessment of changes in attitudes and knowledge of EOL discussions and how they impact EOL decision‐making was not previously evaluated. This preliminary study establishes a baseline for medical student, resident, and attendings for EOL decision‐making and those factors that contribute to their decisions. This preliminary data has demonstrated a difference amongst attendings compared to residents and students. Despite low probability of survival, residents and students are more likely to select more aggressive management options when compared to attendings. Data obtained after completion of future surveys will show when decision‐making changes, which factors contribute to these changes and their significance in making decisions, and when participants are comfortable addressing EOL care.

Introduction. The relationships among military service, combat intensity and long-term health effects were investigated in a cohort of 6,355 Vietnam-era American Legionnaires who were recruited in 1984 and followed through 1998. First, the effect of Vietnam service on coronary heart disease (CHD) risk was assessed among 3,781 veterans who responded to both questionnaires. Next, the effect of serving in Vietnam and combat exposure was investigated as risk factors for the mortality of the cohort. Finally, potential threats to the validity of this study were evaluated.Methods. Military service and lifestyle factors were assessed by questionnaires in 1984 and 1998. Vital status in 1998 was determined and causes of death were ascertained through the National Death Index. Cox proportional hazards modeling was used to calculate hazard ratios (HR) and 95% confidence intervals (CI) for CHD incidence and mortality in relation to service location and combat exposure, adjusting for age, smoking, alcohol consumption, body mass, and hypertension. Response bias and reliability of self-reported data were examined.Results. Serving in Vietnam was associated with an increased hazard of developing heart disease (HR=1.37, 95% CI: 1.09, 1.73), after controlling for independent risk factors. Vietnam veterans experienced a 50% higher mortality than non-Vietnam veterans during 14-year follow-up (HR=1.48, 95% CI= 1.13 - 1.93), which increased with combat intensity after adjustment for other risk factors, (low combat: HR 1.17, 95% CI 0.79 - 1.73; medium combat HR=1.51, 95% CI 1.05 - 2.17; high combat HR=1.82, 95% CI 1.20 - 2.76). A stronger relationship was observed by level of combat for CHD mortality (low combat: HR =1.48, 95% CI 0.75 - 2.95; medium combat HR= 2.01, 95% CI 1.06 - 3.79; high combat HR= 2.27, 95% CI 1.08 - 4.79). Results showed that non-respondents differed only slightly from respondents for important variables potentially related to exposures and chronic disease outcomes. Furthermore, veteran self-report was moderately to highly reliable when measured over a 14 year period.Conclusions. Vietnam veterans are still experiencing higher rates of adverse health effects, even more than thirty years after their military service. These data support a long term and independent adverse effect of military service in Vietnam on cardiovascular health.

Eucalypt trees are one of tree species used for the manufacturing of papers in South Africa. The manufacturing of paper consists of cooking the wood with chemicals until obtaining a pulp. The wood is made of different cells. The shape and structure of these cells, called wood anatomical characteristics are important for the quality of paper. In addition, the anatomical characteristics of wood are influenced by environmental factors like climatic factors, soil compositions etc…. In this study we investigated the effects of the climatic factors (temperature, rainfall, solar radiation, relative humidity, and wind speed) on wood anatomical characteristics of two Eucalyptus clones, a GC (Eucalyptus grandis × camuldulensis) and a GU (Eucalyptus grandis × urophylla). Nine trees per clone have been selected. Two sets of data have been collected for this study. The first set of data was eleven anatomical characteristics of the wood formed daily over a period of five years. The second set of data was the daily measurement of temperature, rainfall, solar radiation, relative humidity and wind speed in the experimental area. Wood is made of two kinds of cell, the fibres and the vessels. The fibres are used for the strength and support of the tree and the vessels for the nutrition. Eleven characteristics related to those cells have been measured (diameter, wall thickness, frequency). These characteristics are highly correlated. To reduce the number of response variables, the principal component analysis was used and the first four principal components accounts for about 95% of the total variation. Based on the weights associated with each component the first four principal components were labelled as vessel dimension (VD), fibre dimension (FD), fibre wall (FW) and vessel frequency (VF). The longitudinal linear mixed model with age, season, temperature, rainfall, solar radiation, relative humidity and wind speed as the fixed effects factors and tree as random effect factor was fitted to the data. From time series modelling result, lagged order of climatic variables were identified and these lagged climatic variables were included in the model. To account for the physical characteristic of the trees we included the effect of diameter at breast height, stem radius, daily radial increment, and the suppression or dominance of the tree in the model. It was found that wood anatomical characteristics of the two clones were more affected by climatic variables when the tree was on juvenile stage as compared to mature stage. / Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2010.

Studies over the past couple of decades have depicted loneliness as a significant concern to physical health, although its meaning for overall health outcomes is still unclear. The precise impact of loneliness on life expectancy and on specific disease processes remains unknown. With regression modeling techniques, this thesis uses data from the Alameda County Health and Ways of Living Study to characterize the impact of loneliness on self-rated health, mortality, and fatalities from specific diseases. A key hypothesis is that loneliness as a health problem hinges on its persistence over time. This hypothesis is also tested by examining the reliability of the loneliness measure across the full 34 years of the survey. A second test is to examine its interplay with marital status as a mutable social circumstance. Results show that loneliness is a risk factor for poor self-rated health, non-ischemic cardiovascular diseases, cerebrovascular diseases, infections, and overall mortality. Results also show that loneliness need not be a stable problem across the life span in order to pose health risks. The reliability of the loneliness measure fades across time and levels of loneliness also vary with changes in marital status. Loneliness did not clearly mediate the impact of marital status on self-rated health, mortality, or specific causes of death.

This dissertation investigates the effects and value of instructional activities for improving second language use of English articles. After reviewing a number of issues concerning pedagogical, linguistic, psycholinguistic, and internal validity, this study presents the results of eight longitudinal time-series case studies of adult Japanese learners of English residing in Vancouver, Canada, four of whom received grammatical explanations, input processing activities, and output practice activities regarding English article use. Learner development was assessed on three different narrative retelling tasks (spoken, written, and cloze) and the production was analysed with reference to specific contexts of use, indicating the form-function mappings that comprised the learners' interlanguage knowledge. The results indicated that the learners' interlanguage production exhibited (a) the anticipated task variation, with greater suppliance of the on tasks that allowed greater attention to form, and (b) the anticipated discoursal variation, with the supplied more consistently when it was primed as a redundant element on the written task and with the supplied less consistently when it was efficiently deleted as a redundant element on the spoken task. The results also indicated the variable nature of individual development and the value of assessing development longitudinally on different tasks. Importantly, the results indicated that the learners improved or continued improving after instruction, and strongly suggested that instruction can cause automatization of interlanguage knowledge. This finding suggests that form-focused instruction may be valuable for second language learning, and that pedagogical positions opposing form-focused instruction may need to be revised or abandoned.

Data from longitudinal surveys give rise to many statistical challenges. They often come from a vast, heterogeneous population and from a complex sampling design. Further, they are usually collected retrospectively at intermittent interviews spaced over a long period of time, which gives rise to missing information and loss to follow-up. As a result, duration data from this kind of surveys are subject to dependent censoring, which needs to be taken into account to prevent biased analysis. Methods for point and variance estimation are developed using Inverse Probability of Censoring (IPC) weights. These methods account for the random nature of the IPC weights and can be applied in the analysis of duration data in survey and non-survey settings. The IPC estimation techniques are based on parametric estimating function theory and involve the estimation of dropout models. Survival distributions without covariates are estimated via a weighted Kaplan-Meier method and regression modeling through the Cox Proportional Hazards model and other models is based on weighted estimating functions. The observational frameworks from Statistics Canada's Survey of Labour and Income Dynamics (SLID) and the UK Millenium Cohort Study are used as motivation, and durations of jobless spells from SLID are analyzed as an illustration of the methodology. Issues regarding missing information from longitudinal surveys are also discussed.

Longitudinal data, which is also known as repeated measures, has grown increasingly within the past years because of its ability to monitor change both within and between subjects. Statisticians in many fields of study have chosen this way of collecting data because it is cost effective and it minimizes the number of subjects required to produce a meaningful outcome. This thesis will explore the world of longitudinal studies to gain a thorough understanding of why this type of collecting data has grown so rapidly. This study will also describe several methods to analyze repeated measures using data collected on the behavior of both adolescent and adult rats. The question of interest is to see if there is a change in the mean response over time and if the covariates (age, bodyweight, gender, and time) influence those changes. After much testing, our data set has a positive nonlinear change in the mean response over time within the age and gender groups. Using a model that included random effects proved to be a better method than models that did not use any random effects. Taking the log of the response variable and using day as the random effect was overall a better fit for our dataset. The transformed model also showed all covariates except for age as being significant.

The individual growth model is a relatively new statistical technique. It is now widely used to examine the trajectories of individuals and groups in repeated measures data. This study examines the association of the receptive vocabulary over time and characteristics of children who were treated for brain tumors. The children undertook different types of treatment from one to any combinations of surgery, radiation and chemotherapy. The individual growth model is used to analyze the longitudinal data and to address the issues behind the data. Results of this study present several factors' influences to the rate of change of PPVT scores. The conclusions of this thesis indicate that the decline in the PPVT scores is associated with gender, age at diagnosis, socioeconomic status, type of treatment and Neurological Predictor Scale.

To address the pervasive concerns of educators about the social and emotional effects of mathematics acceleration on students and the paucity of findings on those issues, 6 students who had participated in 6 years of accelerated mathematics courses were purposefully identified and interviewed in this longitudinal study. Through a qualitative research design, using phenomenological methods, and accompanying descriptive statistics, the author elicited the students’ descriptions of their learning experiences. Major findings in this study were that all students described great benefits from the experience, negative effects were minor, and key factors contributing to success were work ethic, motivation, parents and teachers. The researcher examines a subset of able and promising students who experienced increased mathematics expectations, and she gives parents, educators and policymakers insight into how that population responded to those challenges. In the ever-shifting arena of higher learning expectations for all students under No Child Left Behind legislation and the poor showing of U.S. students on international tests, these results provide information about the possible responses that other students, those struggling and unmotivated, might have to those demands.