On Models, Modelling and the Distinctive Nature of Model-Based Reasoning

M.H. Lee, On Models, Modelling and the Distinctive Nature of Model-Based Reasoning, AI Communications, 12 (3), pp127-137.1999.

A soft robust model for optimization under ambiguity

In this paper, we propose a framework for robust optimization that relaxes the standard notion of robustness by allowing the decision maker to vary the protection level in a smooth way across the uncertainty set. We apply our approach to the problem of maximizing the expected value of a payoff function when the underlying distribution is ambiguous and therefore robustness is relevant. Our primary objective is to develop this framework and relate it to the standard notion of robustness, which deals with only a single guarantee across one uncertainty set. First, we show that our approach connects closely to the theory of convex risk measures. We show that the complexity of this approach is equivalent to that of solving a small number of standard robust problems. We then investigate the conservatism benefits and downside probability guarantees implied by this approach and compare to the standard robust approach. Finally, we illustrate theme thodology on an asset allocation example consisting of historical market data over a 25-year investment horizon and find in every case we explore that relaxing standard robustness with soft robustness yields a seemingly favorable risk-return trade-off: each case results in a higher out-of-sample expected return for a relatively minor degradation of out-of-sample downside performance. © 2010 INFORMS.

Live Imaging of Host-Parasite Interactions in a Zebrafish Infection Model Reveals Cryptococcal Determinants of Virulence and Central Nervous System Invasion.

UNLABELLED: The human fungal pathogen Cryptococcus neoformans is capable of infecting a broad range of hosts, from invertebrates like amoebas and nematodes to standard vertebrate models such as mice and rabbits. Here we have taken advantage of a zebrafish model to investigate host-pathogen interactions of Cryptococcus with the zebrafish innate immune system, which shares a highly conserved framework with that of mammals. Through live-imaging observations and genetic knockdown, we establish that macrophages are the primary immune cells responsible for responding to and containing acute cryptococcal infections. By interrogating survival and cryptococcal burden following infection with a panel of Cryptococcus mutants, we find that virulence factors initially identified as important in causing disease in mice are also necessary for pathogenesis in zebrafish larvae. Live imaging of the cranial blood vessels of infected larvae reveals that C. neoformans is able to penetrate the zebrafish brain following intravenous infection. By studying a C. neoformans FNX1 gene mutant, we find that blood-brain barrier invasion is dependent on a known cryptococcal invasion-promoting pathway previously identified in a murine model of central nervous system invasion. The zebrafish-C. neoformans platform provides a visually and genetically accessible vertebrate model system for cryptococcal pathogenesis with many of the advantages of small invertebrates. This model is well suited for higher-throughput screening of mutants, mechanistic dissection of cryptococcal pathogenesis in live animals, and use in the evaluation of therapeutic agents. IMPORTANCE: Cryptococcus neoformans is an important opportunistic pathogen that is estimated to be responsible for more than 600,000 deaths worldwide annually. Existing mammalian models of cryptococcal pathogenesis are costly, and the analysis of important pathogenic processes such as meningitis is laborious and remains a challenge to visualize. Conversely, although invertebrate models of cryptococcal infection allow high-throughput assays, they fail to replicate the anatomical complexity found in vertebrates and, specifically, cryptococcal stages of disease. Here we have utilized larval zebrafish as a platform that overcomes many of these limitations. We demonstrate that the pathogenesis of C. neoformans infection in zebrafish involves factors identical to those in mammalian and invertebrate infections. We then utilize the live-imaging capacity of zebrafish larvae to follow the progression of cryptococcal infection in real time and establish a relevant model of the critical central nervous system infection phase of disease in a nonmammalian model.

The librarian is in: a consultation model for information literacy formative assessment

Formative assessment was introduced in Rehabilitation Therapy students’ information literacy programs in Fall Term 2006 in the course OT/PT 892: Evidence-Based Practice. It was subsequently employed in the Winter Term 2008 and again in the Spring Term 2008. Formative assessment during student/librarian face-to-face consultations was one of a variety of teaching techniques used in the program. Other techniques included: a required reading; an interactive hands-on searching session; and a summative assessment of the final revised search strategy assignment (these techniques varied somewhat over the 3 classes). With the 2008 entrance class, this course content moved to OT/PT 898: Critical Enquiry, largely in Module 3: Reviewing the Literature. One of the Critical Enquiry’s learning objectives is: “recognize and reflect on the complexity of gathering evidence to inform decision-making.”

Performance Management in the UK Public Sector: addressing multiple stakeholder complexity

Purpose The aim of this paper is to explore the issues involved in developing and applying performance management approaches within a large UK public sector department using a multiple stakeholder perspective and an accompanying theoretical framework. Design/methodology/approach An initial short questionnaire was used to determine perceptions about the implementation and effectiveness of the new performance management system across the organisation. In total, 700 questionnaires were distributed. Running concurrently with an ethnographic approach, and informed by the questionnaire responses, was a series of semi-structured interviews and focus groups. Findings Staff at all levels had an understanding of the new system and perceived it as being beneficial. However, there were concerns that the approach was not continuously managed throughout the year and was in danger of becoming an annual event, rather than an ongoing process. Furthermore, the change process seemed to have advanced without corresponding changes to appraisal and reward and recognition systems. Thus, the business objectives were not aligned with motivating factors within the organisation. Research limitations/implications Additional research to test the validity and usefulness of the theoretical model, as discussed in this paper, would be beneficial. Practical implications The strategic integration of the stakeholder performance measures and scorecards was found to be essential to producing an overall stakeholder-driven strategy within the case study organisation. Originality/value This paper discusses in detail the approach adopted and the progress made by one large UK public sector organisation, as it attempts to develop better relationships with all of its stakeholders and hence improve its performance. This paper provides a concerted attempt to link theory with practice.

A Fast Nonlinear Model Identification Method

The identification of nonlinear dynamic systems using linear-in-the-parameters models is studied. A fast recursive algorithm (FRA) is proposed to select both the model structure and to estimate the model parameters. Unlike orthogonal least squares (OLS) method, FRA solves the least-squares problem recursively over the model order without requiring matrix decomposition. The computational complexity of both algorithms is analyzed, along with their numerical stability. The new method is shown to require much less computational effort and is also numerically more stable than OLS.

An explanatory model for food-web structure and evolution

Food webs are networks describing who is eating whom in an ecological community. By now it is clear that many aspects of food-web structure are reproducible across diverse habitats, yet little is known about the driving force behind this structure. Evolutionary and population dynamical mechanisms have been considered. We propose a model for the evolutionary dynamics of food-web topology and show that it accurately reproduces observed food-web characteristics in the steady state. It is based on the observation that most consumers are larger than their resource species and the hypothesis that speciation and extinction rates decrease with increasing body mass. Results give strong support to the evolutionary hypothesis. (C) 2005 Elsevier B.V. All rights reserved.

High incidence of proviral integrations in the Hoxa locus in a new model of E2a-PBX1-induced B-cell leukemia

Relevant mouse models of E2a-PBX1-induced pre-B cell leukemia are still elusive. We now report the generation of a pre-B leukemia model using E2a-PBX1 transgenic mice, which lack mature and precursor T-cells as a result of engineered loss of CD3epsilon expression (CD3epsilon(-/-)). Using insertional mutagenesis and inverse-PCR, we show that B-cell leukemia development in the E2a-PBX1 x CD3epsilon(-/-) compound transgenic animals is significantly accelerated when compared to control littermates, and document several known and novel integrations in these tumors. Of all common integration sites, a small region of 19 kb in the Hoxa gene locus, mostly between Hoxa6 and Hoxa10, represented 18% of all integrations in the E2a-PBX1 B-cell leukemia and was targeted in 86% of these leukemias compared to 17% in control tumors. Q-PCR assessment of expression levels for most Hoxa cluster genes in these tumors revealed an unprecedented impact of the proviral integrations on Hoxa gene expression, with tumors having one to seven different Hoxa genes overexpressed at levels up to 6600-fold above control values. Together our studies set the stage for modeling E2a-PBX1-induced B-cell leukemia and shed new light on the complexity pertaining to Hox gene regulation. In addition, our results show that the Hoxa gene cluster is preferentially targeted in E2a-PBX1-induced tumors, thus suggesting functional collaboration between these oncogenes in pre-B-cell tumors.

Making Sense of Leaving Care: The contribution of Bridges Model of Transition to Understanding the Psycho-social process.

This paper is based on research into the transition of young people leaving public care in Romania. Using this specific country example, the paper aims to contribute to present understandings of the psycho-social transition of young people from care to independent living by introducing the use of Bridges (2002) to build on existing theories and literature. The research discussed involved mixed methods design and was implemented in three phases: semi-structured interviews with 34 care leavers, focus groups with 32 professionals, and a professional-service user working group. The overall findings confirmed that young people experience two different, but interconnected transitions - social and psychological - which take place at different paces. A number of theoretical perpectives are explored to make sense of this transition including attachment theory, focal theory and identity. In addition, a new model for understanding the complex process of transitions was adapted from Bridges’ (2002) to capture the clear complexity of transition which the findings demonstrated in terms of their psycho-social transition. The paper concludes with messages for leaving and after care services with an emphasis on managing the psycho-social transition from care to independent living.

A NEW CHEMICAL-MODEL OF THE CIRCUMSTELLAR ENVELOPE SURROUNDING IRC+10216

A new chemical model of the circumstellar envelope surrounding the carbon-rich star IRC+10216 has been developed. This model incorporates a variety of newly measured rapid neutral-neutral reactions between carbon atoms and hydrocarbons and between the radical CN and a variety of stable neutral molecules. In addition, other neutral-neutral reactions in the above two classes or involving atoms such as N or radicals such as C(2n)H have been included with large rate coefficients although they have not yet been studied in the laboratory. Unlike the interstellar case, where the inclusion of these neutral-neutral reactions destroys molecular complexity, our model results for IRC+10216 show that sufficient abundances of large hydrocarbon radicals and cyanpolyynes can be produced to explain observations. We also discuss the formation of H2CN and NH2CN, two potentially observable molecules in IRC+10216.

A New Model for Automated Examination Timetabling

Automated examination timetabling has been addressed by a wide variety of methodologies and techniques over the last ten years or so. Many of the methods in this broad range of approaches have been evaluated on a collection of benchmark instances provided at the University of Toronto in 1996. Whilst the existence of these datasets has provided an invaluable resource for research into examination timetabling, the instances have significant limitations in terms of their relevance to real-world examination timetabling in modern universities. This paper presents a detailed model which draws upon experiences of implementing examination timetabling systems in universities in Europe, Australasia and America. This model represents the problem that was presented in the 2nd International Timetabling Competition (ITC2007). In presenting this detailed new model, this paper describes the examination timetabling track introduced as part of the competition. In addition to the model, the datasets used in the competition are also based on current real-world instances introduced by EventMAP Limited. It is hoped that the interest generated as part of the competition will lead to the development, investigation and application of a host of novel and exciting techniques to address this important real-world search domain. Moreover, the motivating goal of this paper is to close the currently existing gap between theory and practice in examination timetabling by presenting the research community with a rigorous model which represents the complexity of the real-world situation. In this paper we describe the model and its motivations, followed by a full formal definition.

‘What Adult Worker Model? A Critical Look at Recent Social Policy Reform in Europe from a Gender and Family Perspective

Analyses regularly feature claims that European welfare states are in the process of creating an adult worker model. The theoretical and empirical basis of this argument is examined here by looking first at the conceptual foundations of the adult worker model formulation and then at the extent to which social policy reform in western Europe fits with the argument. It is suggested that the adult worker formulation is under-specified. A framework incorporating four dimensions—the treatment of individuals vis-à-vis their family role and status for the purposes of social rights, the treatment of care, the treatment of the family as a social institution, and the extent to which gender inequality is problematized—is developed and then applied. The empirical analysis reveals a strong move towards individualization as social policy promotes and valorizes individual agency and self-sufficiency and shifts some childcare from the family. Yet evidence is also found of continued (albeit changed) familism. Rather than an unequivocal move to an individualized worker model then, a dual earner, gender-specialized, family arrangement is being promoted. The latter is the middle way between the old dependencies and the new “independence.” This makes for complexity and even ambiguity in policy, a manifestation of which is that reform within countries involves concurrent moves in several directions.

Multi-Modal CTL:Completeness, Complexity, and an Application

We define a multi-modal version of Computation Tree Logic (ctl) by extending the language with path quantifiers E and A where d denotes one of finitely many dimensions, interpreted over Kripke structures with one total relation for each dimension. As expected, the logic is axiomatised by taking a copy of a ctl axiomatisation for each dimension. Completeness is proved by employing the completeness result for ctl to obtain a model along each dimension in turn. We also show that the logic is decidable and that its satisfiability problem is no harder than the corresponding problem for ctl. We then demonstrate how Normative Systems can be conceived as a natural interpretation of such a multi-dimensional ctl logic. © 2009 Springer Science+Business Media B.V.

An efficient and flexible synthesis of model lignin oligomers

The use of model compounds in the development of selective lignin depolymerisation processes has been limited by the lack of complexity of these models compared with lignin itself. In this paper we report a convergent and efficient synthetic method for the flexible, multi-gram preparation of model lignin hexamers and octamers containing three of the most common connectivity motifs found within native lignin, namely ß-O-4', 5-5' and ß-5', which will be used to further the mechanistic understanding of lignin depolymerisation processes.

A multi-output two-stage locally regularized model construction method using the extreme learning machine

This paper investigates the construction of linear-in-the-parameters (LITP) models for multi-output regression problems. Most existing stepwise forward algorithms choose the regressor terms one by one, each time maximizing the model error reduction ratio. The drawback is that such procedures cannot guarantee a sparse model, especially under highly noisy learning conditions. The main objective of this paper is to improve the sparsity and generalization capability of a model for multi-output regression problems, while reducing the computational complexity. This is achieved by proposing a novel multi-output two-stage locally regularized model construction (MTLRMC) method using the extreme learning machine (ELM). In this new algorithm, the nonlinear parameters in each term, such as the width of the Gaussian function and the power of a polynomial term, are firstly determined by the ELM. An initial multi-output LITP model is then generated according to the termination criteria in the first stage. The significance of each selected regressor is checked and the insignificant ones are replaced at the second stage. The proposed method can produce an optimized compact model by using the regularized parameters. Further, to reduce the computational complexity, a proper regression context is used to allow fast implementation of the proposed method. Simulation results confirm the effectiveness of the proposed technique. © 2013 Elsevier B.V.