Overfitting Bayesian Mixture Models with an Unknown Number of Components

This paper proposes solutions to three issues pertaining to the estimation of finite mixture models with an unknown number of components: the non-identifiability induced by overfitting the number of components, the mixing limitations of standard Markov Chain Monte Carlo (MCMC) sampling techniques, and the related label switching problem. An overfitting approach is used to estimate the number of components in a finite mixture model via a Zmix algorithm. Zmix provides a bridge between multidimensional samplers and test based estimation methods, whereby priors are chosen to encourage extra groups to have weights approaching zero. MCMC sampling is made possible by the implementation of prior parallel tempering, an extension of parallel tempering. Zmix can accurately estimate the number of components, posterior parameter estimates and allocation probabilities given a sufficiently large sample size. The results will reflect uncertainty in the final model and will report the range of possible candidate models and their respective estimated probabilities from a single run. Label switching is resolved with a computationally light-weight method, Zswitch, developed for overfitted mixtures by exploiting the intuitiveness of allocation-based relabelling algorithms and the precision of label-invariant loss functions. Four simulation studies are included to illustrate Zmix and Zswitch, as well as three case studies from the literature. All methods are available as part of the R package Zmix, which can currently be applied to univariate Gaussian mixture model

Challenging aspects of critical thinking : A mixed-methods study of students’ test results, students’ reasoning, and teaching strategies

There is a growing focus on critical thinking throughout the education system. Overall, the efforts that have been made to improve students’ critical thinking have not yielded the desired results. This could indicate a need for more research concerning which specific aspects of critical thinking that are challenging for students and what barriers students face in this regard. Furthermore, this could also indicate a need for more research on effective strategies for teaching critical thinking, and, more specifically, the details that make these strategies effective in some cases and not in others. The aim of this dissertation is to provide pragmatic (i.e., useful) knowledge of students’ struggles with critical thinking problems. This could hopefully lead to further integration of the insights from critical thinking research into teaching practice. Furthermore, the aim of this dissertation is to investigate whether supposedly effective yet general strategies for teaching critical thinking are effective in the context of lower secondary classrooms. In this research project, we have used a well-known test of critical thinking skills, together with a modified version of this test, to identify students’ challenges concerning their use of critical thinking skills. The modified version of the test includes written justifications to selected multiple-choice items from the test. This could give insights into students’ reasoning when facing these items and indicate certain skills and knowledge that should be of particular focus in instruction. Moreover, we measured the effect of the ARGUMENT project—which included supposedly effective strategies for teaching critical thinking—on students’ performance on the test. In Phase 1 of the study in Article I, we quantitatively explored lower secondary students’ test results on the unmodified critical thinking test, and tentatively identified challenging items. The items were then qualitatively analyzed and divided into five categories based on their proposed solution strategies (i.e., from the test manual). According to the analyses, three categories of items were particularly challenging for these students. First, the items that required students to discern between observations and inferences seemed to be the most challenging. Second, many students struggled with the items requiring that test takers recognize a conflict of interest and take that into account when evaluating the credibility of sources and statements. Third, some students also struggled with the items requiring that test takers recognize that certain methods of observation are better than others. In Phase 2 of the study, we administered the modified test which asked for written justifications to selected multiple-choice items from these tentatively identified difficult categories of items. We have not seen any previously published studies that have used this method. The results from the modified test support the hypothesis that these items are challenging, and, importantly, that the challenges relate to the required critical thinking skills. In Article II, we conducted a thematic analysis of the written justifications from the dataset we collected in Phase 2 of Article I. We identified six overarching general themes of reasoning encompassing 21 sub-themes. In sum, more than a quarter of the responses expressed strong inductive logic yet contained incorrect reasons because the premises used were either based on alternative evidence or were made up by students who introduced elements not originally included in the context of the items. Only a few responses did not express strong inductive logic. Most of these were responses from students who seemed to believe that an inference is just as, or more, believable than an observation. We discuss potential barriers to critical thinking that students seemed to face when working with the test items, and how these barriers relate to skills, dispositions, knowledge, and motivation. In Article III, we conducted a quasi-experimental study comparing the gain in critical thinking test scores of the lower-secondary students in the ARGUMENT project with a control group. Teachers in the schools within the ARGUMENT project worked with researchers to develop and implement inquiry-based teaching methods with a focus on scientific argumentation and critical thinking in the context of socioscientific issues. The project aspired to implement general strategies for teaching critical thinking which have been found effective in previous research. Students in the three treatment schools and the three non-treatment schools improved their critical thinking scores significantly from pretest to posttest. However, we did not find a difference in the gain in scores between the two groups. The article discusses potential reasons for this, including the theoretical rationale used in the research and the degree to which the implementation of the project aligns with this rationale. Importantly, we also suggest that the strategies for teaching critical thinking found in the literature could be too general. The article proposes potential avenues that should be explored in future research. In particular, the discussion of the results indicates that there is a need for more detailed insights into the characteristics of the types of authentic inquiry, dialogue, explication of critical thinking principles, and teacher training that are effective in improving critical thinking. The findings and tentative conclusions from the first two articles could contribute to the literature on critical thinking instruction by providing preliminary insights into which aspects of critical thinking that might be particularly difficult for secondary students, as well as how these students reason when faced with critical thinking problems representing these aspects. With time, especially if future research is able to further validate these conclusions, these insights could indicate which aspects of critical thinking that should be the focus of instruction. Moreover, the insights from Article III might aid instruction and design of other projects with similarities to the ARGUMENT project. Further research on how to explicate critical thinking principles, for example through dialogue and scaffolds for inquiry into authentic issues, could then make use of the insights (from the first two articles) into which aspects of critical thinking that should be the focus of such explication.Doktorgradsavhandlin

The Dynamics of Brazilian Rock Art Landscape: An Agent-Based Modelling Approach to Theories

The first attempts at synthetizing the diversity of Brazilian rock art sites and the spread of graphic similarities envolved a certain amount of environmental determinism and traditionalism. We propose an agent-based model able to verify the possible effects of theoretical perspectives on the landscape. Our model uses a number of hunters moving randomly and a set of shelters where they can make new paintings according to simples rules. Three different mechanisms can be modified: exogenous (by nature, some shelters are fit for painting and not others), endogenous (by culture, some shelters are preferred by each hunter, and not others) and cumulative (shelters with paintings are more attractive). Compared to the archaeological context, only exogenous and cumulative constraints seem able to result in a landscape where a few shelters are concentrating most of the paintings. Endogenous constraints alone seem unable to produce the same results without another mechanism for transmission

Feedback: Still the Simplest and Best Solution

Most engineers are (indirectly) trained to be "feedforward thinkers" and they immediately think of "model inversion" when it comes to doing control. Thus, they prefer to rely on models instead of data, although feedback solutions in most cases are much simpler and more robust

Visual Summarization of Scholarly Videos using Word Embeddings and Keyphrase Extraction

Effective learning with audiovisual content depends on many factors. Besides the quality of the learning resource's content, it is essential to discover the most relevant and suitable video in order to support the learning process most effectively. Video summarization techniques facilitate this goal by providing a quick overview over the content. It is especially useful for longer recordings such as conference presentations or lectures. In this paper, we present an approach that generates a visual summary of video content based on semantic word embeddings and keyphrase extraction. For this purpose, we exploit video annotations that are automatically generated by speech recognition and video OCR (optical character recognition).Comment: 12 pages, 5 figure

Seasonal Hypomagnesemia in reindeer on Kautokeino winter pasture in Finnmark County, Norway

Hypomagnesemia was diagnosed in reindeer on Kautokeino winter pasture in Finnmark County, Norway. The affected animals were paretic or ataxic. Mean serum magnesium levels were 0.19 +/- 0.20 mm/L (n = 6), compared to a serum Mg level of 0.82 +/- 0.17 mm/L for the reference group

Modelling and control of growing slugs in horizontal multiphase pipe flows

In this paper, the use of active control to restrict the length of growing slugs in horizontal pipelines is investigated. Specifically, the paper attempts to determine if such control can be attained with realistic measurements and actuators. Simulations in OLGA2000 show that a feedback controller can use measurements or estimates of slug length to control the growth of a slug in a horizontal pipeline by partially closing inlet or outlet chokes. A control-volume approach is used to develop a low-order model of inlet choke-slug growth dynamics based on mass- and impulse balances. The resulting model is a system of nonlinear differential-algebraic equations, which is suitable for observer-design. The tuned model is found to be in good agreement with experiments and OLGA2000-simulations. Linearizations of the model are found to be observable around realistic trajectories when rates and pressures at the inlet and outlet are measured. An extended Luenberger-observer is shown to give good estimates of slug length and -position in simulations even under model uncertainty