Missing data are a common problem in virtually all epidemiological research, especially when conducting longitudinal studies. In these settings, clinicians may collect biological samples to analyze changes in biomarkers, which often do not conform to parametric distributions and may be censored due to limits of detection. Using complete data from the BioCycle Study (2005-2007), which followed 259 premenopausal women over two menstrual cycles, we compared four techniques for handling missing biomarker data with non-Normal distributions. We imposed increasing degrees of missing data on two non-Normally distributed biomarkers under conditions of missing completely at random, missing at random, and missing not at random. Generalized estimating equations were used to obtain estimates from complete case analysis, multiple imputation using joint modeling, multiple imputation using chained equations, and multiple imputation using chained equations and predictive mean matching on Day 2, Day 13 and Day 14 of a standardized 28-day menstrual cycle. Estimates were compared against those obtained from analysis of the completely observed biomarker data. All techniques performed comparably when applied to a Normally distributed biomarker. Multiple imputation using joint modeling and multiple imputation using chained equations produced similar estimates across all types and degrees of missingness for each biomarker. Multiple imputation using chained equations and predictive mean matching consistently deviated from both the complete data estimates and the other missing data techniques when applied to a biomarker with a bimodal distribution. When addressing missing biomarker data in longitudinal studies, special attention should be given to the underlying distribution of the missing variable. As biomarkers become increasingly Normal, the amount of missing data tolerable while still obtaining accurate estimates may also increase when data are missing at random. Future studies are necessary to assess these techniques under more elaborate missingness mechanisms and to explore interactions between biomarkers for improved imputation models.

No description available.

This study examined community college students’ educational goals at the time of first enrollment in college and the status of attainment of those goals two and four years later. A comparison of the traditional definition of success for community college students - on time graduation or transfer to a four-year institution - and a definition reflecting Southern Association for Colleges and Schools criteria for institutional effectiveness was conducted to determine what effect changing success criteria would have numbers of students who are considered successful. Between 1985 and 1989, 11,553 student attending community colleges in southwestern Virginia were tracked to determine the degree to which they attained entry level educational goals. Students who enrolled for the purpose of attaining a credential were more likely to graduate and to exceed their educational goals than their non-degree-seeking counterparts. Non-degree-seekers were more likely to attain their educational goals exactly and then discontinue their community college enrollment. Differences among demographic groups are discussed and suggestions for policymakers are given. / Ph. D.

Leung Yin-Wa. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2001. / Includes bibliographical references (leaves 47-59). / Abstract and questionnaires in English and Chinese. / ABSTRACT-English --- p.i / ABSTRACT - Chinese --- p.ii / ACKNOWLEDGEMENT --- p.iii / TABLE OF CONTENTS --- p.iv / INTRODUCTION --- p.1 / METHOD --- p.17 / RESULTS --- p.23 / DISCUSSION --- p.32 / REFERENCES --- p.47 / APPENDIX A TO APPENDIX F --- p.60 / FOOTNOTES --- p.66 / TABLE 1 TO TABLE8 --- p.67 / FIGURE CAPTIONS --- p.75 / FIGURE1 / FIGURE2 / FIGURE3

In the post-harvest area, a common approach to quantify the average color of fruits peel over time is the sampling of small number of points generally on its equatorial region using a colorimeter. However, when we use a colorimeter to classify an uneven-colored fruit misclassification may occur because points in the peel region may not be representative of average color of fruit. The main problem when we use this method is to determine the number of points to be sampled as well as the location of these points on the fruit\'s surface. An alternative method to evaluate measure of color is digital image analysis because it covers whole of the object surface, by using a sample of pixels taken from the image. As the colorimeter approach is faster and easier than image analysis, it may not be suitable for assessing the overall mean color of the papaya\'s peel and its performance will depend on the number of measured points and choice of sampled region. In this sense, the comparison between these approach is still necessary because we need to know if a sample on the equatorial region can reproduce a sample over the whole region, and if the colorimeter can compete with a scanner or digital camera in measuring the mean hue of papaya peel over time. Thus, we proposed a longitudinal concordance correlation (LCC) based on polynomial mixed-effects regression model to evaluate the extent of agreement among methods. The results show that ideally image analysis of whole fruit\'s region should be used to compute the mean hue and that the topography and curved surface of papaya fruit did not affect the mean hue obtained by the scanner. Since there are still no packages available to estimate the LCC in the free software environment R, we are developing a package called lcc, which provides functions for estimating the longitudinal concordance correlation (LCC) among methods based on variance components and fixed effects of polynomial mixed-effects model. Additionally, we implemented arguments in this function to estimating the longitudinal Pearson correlation (LPC), as precision measure, and longitudinal bias corrector factor (LA), as accuracy measure. Moreover, these components can be estimated using different structures for variance- covariance matrices of random effects and variance functions to model heteroscedasticity among within-group errors using or not the time as variance covariate. / No setor de pós-colheita é muito comum a utilização de colorímetros para avaliar a cor média da casca de frutos ao longo do tempo. No entanto, muitas vezes as técnicas de amostragem utilizando esse equipamento podem levar a medidas tendenciosas da média amostral. Alternativamente, a utilização de imagens digitais pode levar a um menor viés, uma vez que toda a região da casca do fruto é amostrada de forma sistemática. No entanto, ainda é necessária a comparação de ambas abordagens, pois o colorímetro tem vantagens em relação a facilidade de utilização e menor tempo para realizar a amostragem em cada fruto quando comparado a um scanner de mesa. Assim, no caso de variáveis respostas medidas em uma escala contínua, a reprodutibilidade das medidas tomadas por ambos equipamentos pode ser avaliada por meio do coeficiente de correlação de concordância. Dessa forma, para avaliar o perfil da concordância entre métodos, nós propomos uma correlação de concordância longitudinal (LCC), baseada em um modelo de regressão polinomial com efeitos mistos. Os resultados sugeriram que as técnicas por meio de imagens digitais devem ser utilizadas para a quantificação da tonalidade média de frutos. Adicionalmente, a partir do perfil de concordância estimado notamos que existe um período em que ambos os equipamentos podem ser utilizados. A performance do coeficiente de concordância longitudinal foi avaliada por meio de um estudo de simulação, o qual sugeriu que nossa metodologia é robusta a dados desbalanceados (\"dropout\") e que a probabilidade de convergência é aceitavel para uma amostra de 20 frutos e ideal para amostras a partir de 100 frutos. Uma vez que ainda não existem pacotes disponibilizados no ambiente computacional R para a estimação da correlação de concordância longitudinal, nós estamos desenvolvendo um pacote intitulado lcc, o qual será submetido ao \"Comprehensive R Archive Network\" (CRAN). Nesse pacote nós implementamos procedimentos para estimação da correlação de concordância longitudinal, da correlação de Person longitudinal e de uma medida de acurácia longitudinal. Além disso, nosso pacote foi desenvolvido para dados balanceados e desbalanceados, permitindo modelar a heteroscedasticidade entre erros dentro do grupo usando ou não o tempo como covariável, e, também, permitindo a inclusão de covariáveis no preditor linear para controlar variações sistemáticas na variável resposta.

This thesis investigates the development of intonation in questions and L1 influence. It is a longitudinal study using data from classroom interaction over six ten-week terms. The data was from video recordings at the National Labsite for Adult ESOL at Portland State University.Yes-no/and wh-/questions from one Russian speaking learner of English were analyzed over time and by language support level. Both acoustic and perceptual analysis was done. The yes-no/questions showed a clear pattern of target-like boundary tones more often without language support than with language support. A much smaller percentage of wh-/questions were target-like. The influence of L1 on L2 intonation was evident in both the yes-no/and wh-questions, although more so in the wh-questions. There were some aspects of interlanguage observed and there was no change in intonation patterns over time to become more target-like. Implications for this study include the importance in teaching intonation explicitly and how classroom exercises may or may not facilitate the development of L2 intonation.

The neurophysiological basis of cognition changes with age is relatively unexplained, with most studies reporting weak relationships between cognition and measures of brain function, such as event related potentials, brain size and cerebral blood flow. Proton magnetic resonance spectroscopy (1H-MRS) is an in vivo method used to detect metabolites within the brain that are relevant to certain brain processes. Recent studies have shown that these metabolites, in particular N-acetyl aspartate (NAA), which is associated with neuronal viability, correlate with performance on neuropsychological tests or other measures of cognitive function in patients with a variety of cognitive disorders associated with ageing and in normal ageing subjects. We have studied the relationship between metabolites and cognitive function in elderly patients 3 months and 3 years after a stroke or transient ischemic attack (TIA) and in an ageing comparison group. Metabolites were no different between stroke/TIA patients and elderly controls, however, there were significant metabolite differences between stroke/TIA patients with cognitive impairment (Vascular Cognitive Impairment and Vascular Dementia) and those without. Frontal measures of NAA and NAA/Cr predicted cognitive decline over 12 months and 3 years in stroke/TIA patients and elderly controls, and these measures were superior predictors than structural MRI measures. Longitudinal stability of metabolites in ageing over 3 years was associated with stability of cognitive function. The results indicate that 1H-MRS is a useful tool in differentiating stroke/TIA patients with and without cognitive impairment, with possibly superior predictive ability than structural MRI for assessing future cognitive decline. The changes in 1H-MRS that occur with ageing and cognitive decline have implications for the neurophysiological mechanisms and processes that are occurring in the brain, as well as application to clinical diagnosis, the early detection of pathology and the examination of longitudinal change.

Conselho Nacional de Desenvolvimento Científico e Tecnológico / This dissertation aimed at comparing the performance of five (5) fertilizer metering mechanism, utilized at seed and fertilizer drill in lines, relating to its longitudinal distribution of fertilizers, its uniformity of flow according to its longitudinal function and its transverse inclinations and different levels of the reservoir. This research was developed in the Agriculture Machinery Laboratory of Research and Development (LASERG), in the Universidade Federal de Santa Maria (UFSM). Experiments were conduct in the lab, using a counter for tests of fertilizer metering mechanism . Three metering mechanism with auger-type element have been evaluated, one with a transverse discharge through overflow, one with overflow and discharge to the side and the other with gravity discharge of fertilizers, and also the mechanisms of type fluted cilinder and star wheel. Essays were performed with three fertilizers, granule mixture 02-20-20, 16-16-16 granulated mixture and blending mash 00-15-30. It was observed that in the longitudinal distribution of fertilizers, the coefficients of variation of the mechanisms were evaluated over 30%, and the metering mechanism, auger-type transverse discharge through overflow and fluted cilinder, who had a more uniform distribution. In relation to the longitudinal inclination, all feeders had significant change in their dosages, however, the coefficient of variation of the auger-type with overflow and discharge to the side and star wheel, were lower than 10%. In tests with different slopes transverse changes were observed in less than 5%, with the auger-type transverse discharge through overflow, auger-type with discharge by gravity and fluted cilinder those who achieved the best performance. Depending on the different levels of the reservoir, the average coefficient of variation was less than 3%, and the auger-type type were more uniform in relation to this factor. / Este trabalho teve por objetivo comparar o desempenho de cinco mecanismos dosadores de fertilizantes, utilizados em semeadoras-adubadoras em linhas, quanto a sua distribuição longitudinal de fertilizantes, uniformidade de vazão em função de inclinações longitudinais e transversais e diferentes níveis de reservatório. A pesquisa foi realizada no Laboratório de Pesquisa e Desenvolvimento de Máquinas Agrícolas (LASERG), da Universidade Federal de Santa Maria. Os experimentos foram conduzidos em laboratório, utilizando-se uma bancada para ensaios de mecanismos dosadores de fertilizantes. Foram avaliados três dosadores com elemento transportador sem-fim, um com descarga por transbordo transversal, um com transbordo e descarga lateral e outro com descarga de fertilizantes por gravidade, e ainda, os mecanismos do tipo rotor acanalado e disco estrela. Os ensaios foram realizados com três fertilizantes, mistura de grânulos 02-20-20, mistura granulada 16-16-16 e mistura farelada 00-15-30. Foi possível observar que na distribuição longitudinal de fertilizantes, os coeficientes de variação dos mecanismos avaliados foram superiores a 30%, sendo os dosadores sem-fim com transbordo transversal e rotor acanalado, os que obtiveram maior uniformidade de distribuição. Em relação às inclinações longitudinais, todos os dosadores tiveram alterações significativas em sua dosagem, no entanto, o coeficiente de variação médio dos dosadores sem-fim com transbordo e descarga lateral, sem-fim com transbordo transversal e disco estrela, foram inferiores a 10%. Nos ensaios com diferentes inclinações transversais, foram observadas variações na vazão, inferiores a 5%, sendo os dosadores sem-fim com transbordo transversal, sem-fim com descarga por gravidade e rotor acanalado aqueles que alcançaram os melhores desempenhos. Em função dos diferentes níveis de reservatório, os coeficientes de variação médios foram inferiores a 3%, e os dosadores do tipo sem-fim foram os mais uniformes em relação a este fator.

College student retention has been described as a puzzle because retention rates have stagnated, and in some cases declined, despite over seventy years of research into the problem. The magnitude of the problem is that 50 percent of college students will leave their institution before obtaining a degree (Braxton, Hirschy, & McClendon, 2011). In an effort to improve retention rates, colleges and universities have concentrated their attention on first year students. But this concentrated strategy may have simply transferred the retention problem into the second year where retention rates for many schools are as low as first year rates (Amaury, Barlow, & Crisp, 2005). While advising practices have been identified as one of the three top contributors to increasing retention, major gaps exist about the role academic advising might play in the retention of second year students. The present correlational study was undertaken to fill gaps in the mostly conceptual second year literature base which implies second year students differ from first year and upper division students. Advising formed the focus of the study because advising has been identified as one of the most important methods for putting students into a mentoring relationship with college staff and faculty, a practice with strong ties to retention (Habley & McClanahan, 2004; Kuh, 2008). Six research questions were posed in the study which asked whether second year students differed from first year and upper division students and whether retained second year students differed from not retained second year students in their attitudes toward and experiences with advising. Using simultaneous and logistic regression models, and controlling for confounding variables, statistically significant differences were found between second year students and their first year and upper division peers as well as between retained second year students and not retained second year students. The findings of difference between second year and other students provide the growing second year retention literature with an empirical basis to support previously held assumptions about difference between class years which had also formed the basis for presumptions about practice for second year success and retention. Many of the findings in this study also support present retention and second year research and prescriptions for practice provided by that research.

The purpose of this study was to test a conceptual model for assessing the effects of student aid on community college student persistence. A sample consisting of all students who had entered a public community college during the 1980 fall term was drawn from the database of High School and Beyond, 1980 Senior Cohort. Omitting transfer students brought the sample size to 1,364 students. The model of student persistence was based on Tinto's theory of student integration and prior research that suggested student aid may be related to the persistence of community college students. Persistence was defined as the number of terms of enrollment over a two year period (1980-81 and 1981-82). Receipt of aid was associated with lower socioeconomic status, higher tuition charges, above average high school grades, and an ethnic background other than Asian or non-Hispanic white. Aid recipients considered college costs and the availability of aid more important factors in their college choice. A model of student persistence composed of eight exogenous and five endogenous variables was tested through path analysis. It was found that the receipt of student aid did not have significant effects on any of the subsequent variables in the model. Estimation of a reduced path model omitting the aid variable did not result in a significant reduction in explained variance. Degree goals, initial expectation regarding higher education, encouragement to attend college, academic integration, and full-time work were the most important influences on persistence. These findings validated the importance of some of the major constructs in the theory of student integration, but they did not support the research hypothesis that student aid recipients would have a higher rate of persistence than nonrecipients when other factors were held constant. Since encouragement from significant others had a strong and positive association with student persistence, it was suggested that future research consider the role of encouragement on persistence. / Ph. D.