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Unrealistic Assumptions (unrealistic + assumption)
Selected AbstractsWhen density dependence is not instantaneous: theoretical developments and management implicationsECOLOGY LETTERS, Issue 2 2008Irja I. Ratikainen Abstract Most organisms live in changing environments or do not use the same resources at different stages of their lives or in different seasons. As a result, density dependence will affect populations differently at different times. Such sequential density dependence generates markedly different population responses compared to the unrealistic assumption that all events occur simultaneously. Various field studies have also shown that the conditions that individuals experience during one period can influence success and per capita vital rates during the following period. These carry-over effects further complicate any general principles and increase the diversity of possible population dynamics. In this review, we describe how studies of sequential density dependence have diverged in directions that are both taxon-specific and have non-overlapping terminology, despite very similar underlying problems. By exploring and highlighting these similarities, we aim to improve communication between fields, clarify common misunderstandings, and provide a framework for improving conservation and management practices, including sustainable harvesting theory. [source] Beyond Mule Kicks: The Poisson Distribution in Geographical AnalysisGEOGRAPHICAL ANALYSIS, Issue 2 2006Daniel A. Griffith The Poisson model, discovered nearly two centuries ago, is the basis for analyses of rare events. Its first applications included descriptions of deaths from mule kicks. More than half a century ago the Poisson model began being used in geographical analysis. Its initial descriptions of geographic distributions of points, disease maps, and spatial flows were accompanied by an assumption of independence. Today this unrealistic assumption is replaced by one allowing for the presence of spatial autocorrelation in georeferenced counts. Contemporary statistical theory has led to the creation of powerful Poisson-based modeling tools for geographically distributed count data. [source] Patterns of woody plant species richness in the Iberian Peninsula: environmental range and spatial scaleJOURNAL OF BIOGEOGRAPHY, Issue 10 2008Ole R. Vetaas Abstract Aim, Climate-based models often explain most of the variation in species richness along broad-scale geographical gradients. We aim to: (1) test predictions of woody plant species richness on a regional spatial extent deduced from macro-scale models based on water,energy dynamics; (2) test if the length of the climate gradients will determine whether the relationship with woody species richness is monotonic or unimodal; and (3) evaluate the explanatory power of a previously proposed ,water,energy' model and regional models at two grain sizes. Location, The Iberian Peninsula. Methods, We estimated woody plant species richness on grid maps with c. 2500 and 22,500 km2 cell size, using geocoded data for the individual species. Generalized additive models were used to explore the relationships between richness and climatic, topographical and substrate variables. Ordinary least squares regression was used to compare regional and more general water,energy models in relation to grain size. Variation partitioning by partial regression was applied to find how much of the variation in richness was related to spatial variables, explanatory variables and the overlap between these two. Results, Water,energy dynamics generate important underlying gradients that determine the woody species richness even over a short spatial extent. The relationships between richness and the energy variables were linear to curvilinear, whereas those with precipitation were nonlinear and non-monotonic. Only a small fraction of the spatially structured variation in woody species richness cannot be accounted for by the fitted variables related to climate, substrate and topography. The regional models accounted for higher variation in species richness than the water,energy models, although the water,energy model including topography performed well at the larger grain size. Elevation range was the most important predictor at all scales, probably because it corrects for ,climatic error' due to the unrealistic assumption that mean climate values are evenly distributed in the large grid cells. Minimum monthly potential evapotranspiration was the best climatic predictor at the larger grain size, but actual evapotranspiration was best at the smaller grain size. Energy variables were more important than precipitation individually. Precipitation was not a significant variable at the larger grain size when examined on its own, but was highly significant when an interaction term between itself and substrate was included in the model. Main conclusions, The significance of range in elevation is probably because it corresponds to several aspects that may influence species diversity, such as climatic variability within grid cells, enhanced surface area, and location for refugia. The relative explanatory power of energy and water variables was high, and was influenced by the length of the climate gradient, substrate and grain size of the analysis. Energy appeared to have more influence than precipitation, but water availability is also determined by energy, substrate and topographic relief. [source] An explanatory model of medical practice variation: a physician resource demand perspectiveJOURNAL OF EVALUATION IN CLINICAL PRACTICE, Issue 2 2002Michael J. Long MA PhD Abstract Practice style variation, or variation in the manner in which physicians treat patients with a similar disease condition, has been the focus of attention for many years. The research agenda is further intensified by the unrealistic assumption that by reducing variation, quality will be improved, costs will be reduced, or both. There is a wealth of literature that identifies differences in health care use of many kinds, in apparently similar communities. Attempts have been made by many scholars to identify the determinants of variation in terms of differences in the population characteristics (e.g. age, sex, insurance, etc.) and geographical characteristics (e.g. distance to provider, number of physicians, number of hospital beds, etc.). When significant differences in use rates prevail after controlling for differences in population characteristics, it is often attributed to ,uncertainty', or the fact that there is no consensus on what constitutes the optimum treatment process. It is suggested by this literature that the greatest variation can be found in the circumstances where there is the most ,uncertainty'. In this work, a physician resource demand model is proposed in which it is suggested that, during the diagnosis and treatment process, physicians demand resources consistent with the clinical needs of the patients, modified by the intervening forces under which they practice. These intervening forces, or constraints, are categorized as patient agency constraints, organizational constraints and environmental constraints, which are characterized as ,induced variation'. It is suggested that when all of the variables that constitute these constraints are identified, the remaining variance represents ,innate variance', or practice style differences. It is further suggested that the more completely this model is specified, the more likely area differences will be attenuated and the smaller will be the residual variance. [source] Does food allergy cause atopic dermatitis?DERMATOLOGIC THERAPY, Issue 2 2006Food challenge testing to dissociate eczematous from immediate reactions ABSTRACT:, The objective is to evaluate and diagnose, in a controlled setting, suspected food allergy causation in patients hospitalized for management of severe, unremitting atopic dermatitis (AD). Nineteen children were hospitalized at Oregon Health and Science University with atopic dermatitis from 1986 to 2003 for food restriction, then challenge, following standard recommendations. Challenges were prioritized by categories of (a) critical foods (e.g., milk, wheat, egg, soy); (b) important foods; and (c) other suspected foods. Patients were closely observed for evidence of pruritus, eczematous responses, or IgE-mediated reactions. If results were inconsistent, double-blind, placebo-controlled food challenge was performed. A total of 17 children with atopic dermatitis were assessed. Two could not be fully evaluated, thus were excluded from data tabulations. Only one positive eczematous food response was observed of 58 challenges. Three children had well-documented histories of food-induced IgE-mediated anaphylactoid or urticaria reactions to seafood and/or nuts and were not challenged with those foods. Atopic dermatitis, even in the highest-risk patients, is rarely induced by foods. Undocumented assumptions of food causation detract from proper anti-inflammatory management and should be discouraged. Immediate IgE-mediated food reactions are common in atopic dermatitis patients; such reactions are rapid onset, typically detected outside the clinic, and must be distinguished from eczematous reactions. Diagnosis of food-induced eczema cannot be made without food challenge testing. Such tests can be practical and useful for dispelling unrealistic assumptions about food allergy causation of atopic dermatitis. [source] The ,New Minimalist Approach' to Private-Sector Development: A Critical AssessmentDEVELOPMENT POLICY REVIEW, Issue 4 2006Tilman Altenburg Recent literature on private-sector development emphasises the need to establish a ,level playing field' and tends to disregard selective supportive interventions. The most commonly highlighted elements are administrative simplification and effective property rights policies, with business services largely left to private providers - what we call the ,new minimalist approach' (NMA). However, the NMA is based on certain unrealistic assumptions and is barely backed by empirical evidence. A range of complementary public policies is needed to create competitive sectors and overcome internal constraints, especially in small-scale economies. [source] The maximum entropy formalism and the idiosyncratic theory of biodiversityECOLOGY LETTERS, Issue 11 2007Salvador Pueyo Abstract Why does the neutral theory, which is based on unrealistic assumptions, predict diversity patterns so accurately? Answering questions like this requires a radical change in the way we tackle them. The large number of degrees of freedom of ecosystems pose a fundamental obstacle to mechanistic modelling. However, there are tools of statistical physics, such as the maximum entropy formalism (MaxEnt), that allow transcending particular models to simultaneously work with immense families of models with different rules and parameters, sharing only well-established features. We applied MaxEnt allowing species to be ecologically idiosyncratic, instead of constraining them to be equivalent as the neutral theory does. The answer we found is that neutral models are just a subset of the majority of plausible models that lead to the same patterns. Small variations in these patterns naturally lead to the main classical species abundance distributions, which are thus unified in a single framework. [source] Enforcement of environmental charges: some economic aspects and evidence from the German Waste Water ChargeENVIRONMENTAL POLICY AND GOVERNANCE, Issue 5 2001Professor Dr Erik Gawel Enforcement problems are usually analysed with respect to command-and-control measures of environmental regulation. The recognition that any environmental policy instrument entails an enforcement problem in principle is basic to a comparative analysis of enforcement effects. This paper deals with the comparative enforcement effects of charges: How does enforcement of a charge function? Which problems occur particularly in the enforcement of charges? Could enforcement be facilitated by a specific construction of charge laws? Are economic concepts of charge enforceable at all, and if so, under what conditions? Are charges more readily enforceable than other instruments? Therefore, some economic theory assessments of enforcement processes are presented first. In a third part, the paper sheds light on the practical experience made with the enforcement of the German Waste Water Charge. It is argued that the well worn thesis of an enforcement-friendly ,self-control' of market instruments is based on unrealistic assumptions. Whether against this background enforcement of environmental policy can be facilitated by an increased application use of charges must be viewed sceptically in an overall assessment of the problematic. Moreover, the transition from allocative control tasks to fiscal environmental charges may well be a symptom of rather than a contribution to the solution of the political and administrative crisis of enforcement. Especially for charges, the crucial question seems to be the political implementation rather than concrete enforcement by local authorities. Copyright © 2001 John Wiley & Sons, Ltd. and ERP Environment [source] A class of nonseparable and nonstationary spatial temporal covariance functionsENVIRONMETRICS, Issue 5 2008Montserrat Fuentes Abstract Spectral methods are powerful tools to study and model the dependency structure of spatial temporal processes. However, standard spectral approaches as well as geostatistical methods assume separability and stationarity of the covariance function; these can be very unrealistic assumptions in many settings. In this work, we introduce a general and flexible parametric class of spatial temporal covariance models, that allows for lack of stationarity and separability by using a spectral representation of the process. This new class of covariance models has a unique parameter that indicates the strength of the interaction between the spatial and temporal components; it has the separable covariance model as a particular case. We introduce an application with ambient ozone air pollution data provided by the U.S. Environmental Protection Agency (U.S. EPA). Copyright © 2007 John Wiley & Sons, Ltd. [source] Hybrid approach with an expert system and a genetic algorithm to production management in the supply netINTELLIGENT SYSTEMS IN ACCOUNTING, FINANCE & MANAGEMENT, Issue 1-2 2006awrynowicz A new approach to solving production management problems in the supply net is proposed. An expert system designed to help companies in medium-term and short-term production planning is discussed. The proposed expert system considers alternative process plans for a job and outsourcing, when a bottleneck exists in the machine. The proposed hybrid system uses the output of the expert system as the input of the genetic algorithm. The output of the genetic algorithm is a near optimal schedule. The proposed method does not require any unrealistic assumptions. It can be used to solve highly complicated and non-linear functions of a realistic problem. Copyright © 2007 John Wiley & Sons, Ltd. [source] Representing species in reserves from patterns of assemblage diversityJOURNAL OF BIOGEOGRAPHY, Issue 7 2004M. B. Araújo Abstract Aim, A positive relationship between assemblage diversity (AD) , equivalent to the biotic version of the environment diversity, ED, method , and species diversity has been reported. This being true, reserve networks with many different assemblages would be expected to represent more species than reserve networks including fewer and less different assemblages. This idea was tested using European species atlas distributions of terrestrial vertebrates and plants. It is asked whether: (1) maximizing AD within one group would represent species diversity of this group better than expected by chance; and (2) maximizing AD within one group would represent species diversity of other groups better than expected by chance. Location, Europe. Methods, Three ordination techniques (non-metric multidimensional scaling, detrended correspondence analysis and correspondence analysis) are used to summarize patterns of compositional turnover within assemblages. p -Median location-allocation models are then calculated from ordination space to measure the degree of expected species representation within the group being sampled as well as the expected representation within other groups. Results are compared with near-optimal solutions obtained with complementarity-based algorithms and to a null model obtained by simulating selection of areas at random. Matrix correlation analysis was also performed to investigate broad patterns of covariation in compositional turnover of assemblages of species belonging to different taxonomic groups and these values were compared with correlation in species richness scores between groups. Results, The AD model did not always represent more species of the group being sampled than expected by chance (P < 0.05). Results were independent of the method and taxonomic group considered. Effectiveness of AD within one group to represent species of other groups varied, but in most cases it was worse than using complementarity-based algorithms as a surrogate strategy. Even when correlations indicated high coincidence between assemblages, taxonomic-based surrogates did not always recover more species than expected by chance (P < 0.05). Main conclusions, Results are discussed in the light of two possible explanations: (1) the AD model is based on unrealistic assumptions, namely that species have equal probability of having the centre of their distributions anywhere in ordination space and that species display unimodal, symmetrical, bell-shaped response curves to gradients; (2) particular implementation of methods may be inadequate to summarize useful complementarity among assemblages, especially for restricted-range species. We conclude that both arguments are likely to play a role in explaining results, but that opportunities exist to improve performance of existing surrogate strategies. [source] The problem of phase mixed shear Alfvén waves in the solar corona revisitedASTRONOMISCHE NACHRICHTEN, Issue 8 2008G. Mocanu Abstract The problem of phase mixing of shear Alfvén waves is revisited taking into account dissipative phenomena specific for the solar corona. In regions of space plasmas where the dynamics is controlled by the magnetic field, transport coefficients become anisotropic with transport mechanism having different behavior and magnitude depending on the orientation with respect to the ambient magnetic field. Taking into account realistic values for dissipative coefficients we obtain that the previous results derived in context of torsional Alfvén wave phase mixing are actually heavily underestimated so phase mixing cannot be used to explain the damping of torsional Alfvén waves and heating of open coronal structures. The presented results indicate that in order for phase mixing to still be a viable mechanism to explain heating or wave damping unrealistic assumptions have to be made. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Hovenkamp's ostracized vicariance analysis: testing new methods of historical biogeographyCLADISTICS, Issue 4 2008Article first published online: 7 DEC 200, Simone Fattorini All methods currently employed in cladistic biogeography usually give contrasting results and are theoretically disputed. In two overlooked papers, Hovenkamp (1997, 2001) strongly criticized methods currently used by biogeographers and proposed two other methods. However, his criticisms have remained unanswered and his methods rarely applied. I used three different data sets to show the superiority of Hovenkamp's methods. Both methods proposed by Hovenkamp do not suffer from the unrealistic assumptions that underlie other methods commonly used in cladistic biogeography. The method proposed in 2001 is more powerful than the previous method published in 1997, because it does not use a priori assumptions about the areas involved. However, the method proposed in 1997 may be a valid alternative for large data sets. © The Willi Hennig Society 2007. [source] | ||||||||||||||||