Partially and fully time-unstructured residual variance-covariance matrices in growth curve modeling: Consequences of ignoring variability in times of assessment

In a longitudinal study, the data can be either time-structured, where the times of assessment are the same across participants, or time-unstructured, where the times of assessment vary across participants. Currently, growth curve modeling can accommodate time-unstructured data when modeling average change over time (fixed effects) and variability in change over time (random effects), but not residual variances and covariances. Through Monte Carlo simulation, a first goal of this study was to determine the effects of ignoring variability in times of assessment when modeling the residual variances and covariances in linear growth curve models in terms of convergence, parameter bias, power to detect change over time, and model fit. A second goal was to evaluate a novel method to construct partially\u27 time-unstructured matrices for residual variances and covariances. The simulation factors manipulated in this study are type of analysis (time-structured, partially time-unstructured, fully time-unstructured), residual matrix (heterogeneous diagonal, first-order autoregressive), sample size (50, 200, 500), and number of time points (3, 6, 9). Results showed that convergence was generally high when the matrix was autoregressive, whereas when the matrix was heterogeneous diagonal, the time-structured and partially time-unstructured analyses converged most often; in many conditions, the fully time-unstructured analysis never converged. Fixed effects were generally spared from bias across conditions, as were the random effects when the matrix was heterogeneous diagonal. With the time-structured autoregressive matrix and only 3 time points, the intercept and slope variances were overestimated, and the intercept-slope correlation and residual variance were underestimated. There was a large effect of type of analysis on autocorrelation bias, with only the fully time-unstructured analysis yielding unbiased estimates, and the partially time-unstructured analysis yielding less bias than the time-structured analysis. Power to detect change over time was high across conditions. In terms of model fit, all fit indexes examined favored time-unstructured analyses when the matrix was autoregressive, whereas only deviance favored time-unstructured analyses when the matrix was heterogeneous diagonal. Overall, this study shows that accommodating time-unstructured data when modeling residual variances and covariances can be important, perhaps especially when residuals are autocorrelated. Moreover, when the fully time-unstructured matrix cannot be used, the partially time-unstructured matrix provides an improvement over the standard time-structured matrix under certain conditions

Ignoring individual differences in times of assessment in growth curve modeling

Researchers often collect longitudinal data so as to model change over time in a phenomenon and for a population of interest. Inevitably, there will be some variation across individuals in specific time intervals between assessments. By necessity or by choice, a researcher can decide to ignore these individual differences in times of assessments. In this simulation study of growth curve modeling, I investigate how ignoring individual differences in time points when modeling change over time relates to convergence and admissibility of solutions, bias in estimates of parameters, power to detect change over time, and, when there is no change over time, Type I error rate. The simulation factors that I manipulate in this study are magnitude of the individual differences in assessment times that are ignored, magnitude of change over time, number of time points, and sample size. Results show that, in contrast to the correct analysis, ignoring individual differences in time points frequently led to inadmissible solutions, especially with few time points and small samples, regardless of the specific magnitude of individual differences that were ignored. Mean intercept and slope were generally estimated without bias. With few time points and small samples, ignoring individual differences in time points yielded overestimated intercept and slope variances and underestimated intercept-slope covariance and residual variance, more so than when using the correct analysis. When there were more than 3 time points, or when there were 3 time points and sample size was 500, ignoring individual differences in time points yielded overestimated residual variance, but only if individual differences were large. Power and Type I error rate for the linear slope were unaffected by the type of analysis. Overall, this study suggests that it is advisable to account for individual differences in time points whenever possible

Pratiques, objets et finalités de collaboration en lien avec l’intégration des tablettes numériques dans une école secondaire

Bien que les technologies de l’information et de la communication (TIC) occupent une place de plus en plus prépondérante, tant au sein de la société que de l’école (Anderson, 2010), on remarque que leur intégration en contexte scolaire représente encore un défi de taille (Underwood et Dillon, 2011). Selon certains auteurs (Portelance, 2011; Desgagné, 1997), la collaboration constituerait un vecteur pour transformer les pratiques éducatives. Comment la collaboration peut-elle contribuer à ce changement que représente l’intégration des TIC? Les résultats d’une recherche-action, menée depuis trois ans de concert avec une école secondaire ayant pour objectif d’intégrer la tablette numérique, permet de mettre en lumière des pratiques, des objets et des finalités de collaboration qui ont émergé entre les différents acteurs, favorisant ainsi l’implantation de cet outil technologique. Although Information and Communication Technologies (ICT) play an increasingly predominant role (Anderson, 2010), their integration in school context is still a challenge (Underwood and Dillon, 2011). According to Portelance (2011) and Desgagné (1997), collaboration is one vector that can be used to transform educational practices. How can collaboration contribute to the ICT integration process into our schools? The results of an action research, conducted for three years along with a high school that aims to integrate digital tablet, allows to highlighting practices, objects and collaboration purposes that emerged between actors, therefore facilitating the implementation of this technological tool

Appréciation et perception du projet d’intégration des tablettes numériques au secondaire par les finissants de 2014-2015 : rapport de recherche

Présentation des données issues d'une série d'entrevues avec les finissants d'une école secondaire qui intègre massivement les tablettes numériques

Prise de notes numériques au secondaire : comparaison de la qualité des notes numériques avec celle des notes manuscrites

La prise de notes est une activité complexe, mais très importante pour l’apprentissage. C’est la raison pour laquelle cette activité occupe une très grande place dans la classe : elle est utilisée presque tous les jours et dans plusieurs cours. La plus grande accessibilité des technologies, comme l’iPad, offre maintenant aux élèves une nouvelle possibilité : prendre des notes numériques. Comment organiser la prise de notes avec une tablette ? Quels sont les effets de la tablette sur la prise de notes ? Dans la présente recherche, la comparaison de notes manuscrites et de notes numériques d’élèves de troisième secondaire d’une école où l’intégration du iPad s’étend à toutes les classes depuis 2014 a révélé certaines similitudes et différences. Ces dernières ont trait à l’organisation et au contenu des notes, à la qualité de la langue et à l’utilisation de procédés linguistiques de réduction

Stratégies de prise de notes à l'aide d'une tablette électronique chez des étudiants du secondaire | Digital Tablet Note-Taking Strategies among High School Students

Both media and digital literacies are essential for the 21st century. Consequently, several governments have integrated technologies with school curriculums. Recently, ICT integration efforts, particularly with the digital tablet, have multiplied. Several academic uses of the tablet are explored, and some raise questions. This is the case of note taking as viewed in a single Quebec high school. Which application should students use, and should it be the same one for all subjects? Is it effective? Which strategies should be promoted? These are not trivial questions where educational success is concerned. Whereas note taking using certain technological tools has already been studied, little research has focused on with the use of a digital tablet. Students who use a tablet daily were asked a series of questions related to note taking. Our analysis suggests that teachers should play an active role in the appropriation of this tool. Les littératies médiatique et numérique sont essentielles pour le 21e siècle. Plusieurs gouvernements les intègrent par conséquent aux curriculums scolaires. Actuellement, les efforts d’intégration de la tablette numérique se multiplient. Plusieurs usages scolaires sont cités et certains soulèvent des interrogations. C’est le cas de la prise de notes qui soulève des questions auprès de plusieurs enseignants d’une école secondaire québécoise. Avec quelle application? Une seule? Toujours la même? Dans toutes les matières ? Est-ce efficace? Quelles stratégies de prise de notes doit-on favoriser? Ces questions sont importantes pour la réussite scolaire. La prise de notes à l’aide d’outils technologiques a déjà fait l’objet d’études lors de l’intégration d’autres technologies mais peu de recherches se sont intéressées à cette stratégie d’apprentissage avec les tablettes numériques. Un questionnaire a été rempli par des étudiants du secondaire qui utilisent la tablette quotidiennement. L’analyse des résultats montre que les enseignants doivent jouer un rôle actif dans l'appropriation de cet outil

Study of the wettability of coke by different pitches and their blends

The properties of coal tar pitch, which is used as the binder material for carbon anode production, strongly affect the anode properties. Pitches have significant differences in their chemical compositions depending on their origin. In this study, four different coal tar pitches and their blends were studied with the aim of understanding the wettability of a calcined petroleum coke by pitch using the sessile-drop test. In this test, contact angle, which is an indication of wettability, is measured. Contact angles decrease with increasing time, and smaller contact angle means better wettability. The chemical properties of pitches and coke were studied using XPS to investigate their interactions and, consequently, the wetting mechanism. The results showed that blending different pitches influences the wettability. The presence of acidic, basic, and heteroatom containing functional groups in pitch might cause acid–base/condensation reactions when they are blended and, thus, influence the wetting behavior of the pitch blend

Evolution of anode properties during baking

Carbon anodes are used in electrolytic production of aluminum. Good quality anodes decrease carbon and energy consumption, greenhouse gas emissions, and increase anode production as well as anode life. Many factors such as quality of raw materials, anode recipe and operational parameters of different units (mixing, compacting and baking) affect anode properties, consequently, anode quality. One of the important anode properties is density. High density anodes have longer life and they have to be changed less often during the electrolysis. Too low or too high density is the indication of the presence of cracks and pores which increase the electrical resistivity, thus, energy required to produce the anodes. In this work, the evolution of anode structure (development of pores/cracks), consequently, anode properties (density, electrical resistivity) during baking was studied by stopping the anode baking at the intermediate temperatures and measuring the anode properties. In addition, an apparatus was developed for continuously monitoring the development of anode electrical resistivity during anode baking. This knowledge can be helpful for improving the anode quality

Effect of coke granulometry on the properties of carbon anodes based on experimental study and ANN analysis

The available anode-quality petroleum coke is not sufficient to cover the need created by the increase in the world aluminum production. Understanding the consequences of varying calcined coke quality is necessary to possibly compensate for the reduction in coke quality and adjust the anode paste recipe in the subsequent use of coke in order to obtain economically viable production of aluminum. Different fractions of coke particles were mixed to optimize the anode recipe; however, it was laborious to find experimentally the suitable percentage of each fraction in anode paste which would give good anode properties. In this study, Artificial Neural Network (ANN) model was developed for adjusting the granulometry of the raw materials for anode production. Tapped bulk density of dry aggregates was used to predict the anode paste recipe using the ANN method. A new anode recipe (by adjusting the medium fraction in the paste) was proposed based on the predictions of an ANN model, which resulted in improved anode properties

Investigation on the structure of carbonized pitch and calcined coke-carbonized pitch interface in carbon anodes by etching

A novel, simple, quick, and economic method has been developed to etch samples for characterizing the structural aspects of carbonized pitch alone and in baked anodes. Hot air is used to etch the polished carbonized pitch surface for creating its topography; followed by the characterization of the structure using scanning electron microscope. Hot air preferentially etches the carbonized pitch, which make the differentiation of carbonized pitch from the calcined coke particles possible in baked anode. After etching, lamellar parallel cracks are created and fine granular mosaics are observed on the surfaces of carbonized pitch. The structural composition in baked anode differs visibly from the pure carbonized pitch baked under the same conditions. This may be due to the effect of fine coke particles in anode on the formation of structure during baking. The etching technique permits the determination of the internal structure of carbonized pitch and its interface with coke in anode