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Modelling Assumptions (modelling + assumption)

Distribution by Scientific Domains

Mathematics and Statistics	33%
Business, Economics, Finance and Accounting	33%

Selected Abstracts

On the Pervasiveness of Home Market Effects

ECONOMICA, Issue 275 2002
Keith Head
Paul Krugman's model of trade predicts that the country with the relatively large number of consumers is the net exporter and hosts a disproportionate share of firms in the increasing returns sector. He terms these results ,home market effects'. This paper analyses three additional models featuring increasing returns, firm mobility, and trade costs to assess the robustness of home market effects to alternative modelling assumptions. We find strikingly similar results for two of the models that relax assumptions about the nature of demand, competition and trade costs. However, a model that links varieties to nations rather than firms can generate opposite results. [source]

A measure of disclosure risk for microdata

JOURNAL OF THE ROYAL STATISTICAL SOCIETY: SERIES B (STATISTICAL METHODOLOGY), Issue 4 2002
C. J. Skinner
Summary. Protection against disclosure is important for statistical agencies releasing microdata files from sample surveys. Simple measures of disclosure risk can provide useful evidence to support decisions about release. We propose a new measure of disclosure risk: the probability that a unique match between a microdata record and a population unit is correct. We argue that this measure has at least two advantages. First, we suggest that it may be a more realistic measure of risk than two measures that are currently used with census data. Second, we show that consistent inference (in a specified sense) may be made about this measure from sample data without strong modelling assumptions. This is a surprising finding, in its contrast with the properties of the two ,similar' established measures. As a result, this measure has potentially useful applications to sample surveys. In addition to obtaining a simple consistent predictor of the measure, we propose a simple variance estimator and show that it is consistent. We also consider the extension of inference to allow for certain complex sampling schemes. We present a numerical study based on 1991 census data for about 450 000 enumerated individuals in one area of Great Britain. We show that the theoretical results on the properties of the point predictor of the measure of risk and its variance estimator hold to a good approximation for these data. [source]

Sensitivity analysis for incomplete contingency tables: the Slovenian plebiscite case

JOURNAL OF THE ROYAL STATISTICAL SOCIETY: SERIES C (APPLIED STATISTICS), Issue 1 2001
Geert Molenberghs
Classical inferential procedures induce conclusions from a set of data to a population of interest, accounting for the imprecision resulting from the stochastic component of the model. Less attention is devoted to the uncertainty arising from (unplanned) incompleteness in the data. Through the choice of an identifiable model for non-ignorable non-response, one narrows the possible data-generating mechanisms to the point where inference only suffers from imprecision. Some proposals have been made for assessing the sensitivity to these modelling assumptions; many are based on fitting several plausible but competing models. For example, we could assume that the missing data are missing at random in one model, and then fit an additional model where non-random missingness is assumed. On the basis of data from a Slovenian plebiscite, conducted in 1991, to prepare for independence, it is shown that such an ad hoc procedure may be misleading. We propose an approach which identifies and incorporates both sources of uncertainty in inference: imprecision due to finite sampling and ignorance due to incompleteness. A simple sensitivity analysis considers a finite set of plausible models. We take this idea one step further by considering more degrees of freedom than the data support. This produces sets of estimates (regions of ignorance) and sets of confidence regions (combined into regions of uncertainty). [source]

On the Specification and Estimation of the Production Function for Cognitive Achievement*

THE ECONOMIC JOURNAL, Issue 485 2003
Petra E. Todd
This paper considers methods for modelling the production function for cognitive achievement in a way that captures theoretical notions that child development is a cumulative process depending on the history of family and school inputs and on innate ability. It develops a general modelling framework that accommodates many of the estimating equations used in the literatures. It considers different ways of addressing data limitations, and it makes precise the identifying assumptions needed to justify alternative approaches. Commonly used specifications are shown to place restrictive assumptions on the production technology. Ways of testing modelling assumptions and of relaxing them are discussed. [source]

Barocke Dachwerke: Konstruktion und Analyse des Tragverhaltens

BAUTECHNIK, Issue 1 2009
Stefan M. Holzer Univ.-Prof.
Konstruktion und Tragverhalten historischer Bauwerke haben schon öfter im Zentrum von Forschungsprojekten gestanden, zuletzt ganz besonders intensiv im Karlsruher DFG-Sonderforschungsbereich 315 (Erhalten historisch bedeutsamer Bauwerke). Der hier vorliegende Beitrag ist einem Thema gewidmet, das bei allen diesen Forschungen bisher am Rande gestanden hat, nämlich den barocken Holzkonstruktionen. Die Autoren haben in den letzten Jahren eine große Anzahl barocker Dachwerke in Bayern untersucht, vorwiegend weitgespannte Kirchendächer. Der Bestand solcher Dächer gerade im Gebiet südlich der Donau ist nahezu unüberschaubar groß, und die statische Beurteilung und Ertüchtigung solcher Dächer stellt für den Bauingenieur eine sowohl praktisch und ökonomisch relevante als auch in der Modellbildung anspruchsvolle Aufgabe dar. Bei der Untersuchung der bayerischen Barockdächer hat es sich herausgestellt, dass die dort anzutreffenden Tragkonstruktionen in erheblichem Maße von den typischen mittelalterlichen Dachwerken oder auch von barocken Dachwerken anderer Regionen abweichen: Die barocken Dachtragwerke Süddeutschlands haben , in markantem Unterschied zum mittelalterlichen Dach mit seiner Reihung identischer, voneinander nahezu völlig unabhängiger Gespärre , ausgeprägte firstparallele Tragwirkung. Noch wichtiger , gerade auch im Hinblick auf notwendige Sanierungen , ist aber das äußerst häufige Fehlen eines Zugbandes in Form einer durchgehenden Anker- oder Zerrbalkenlage auf Höhe der Dachtraufe. Die oft weit in den Dachraum ragenden, meist aus Ziegeln gemauerten Gewölbe und Kuppeln machen komplizierte, vielfach statisch unbestimmte Konstruktionen zur Aufnahme der Schubkräfte der Gespärre notwendig. In diesem Beitrag werden typische Konstruktionsvarianten zur Lösung dieser barocken Aufgabe vorgestellt und deren Tragverhalten beleuchtet. Bei allen wichtigen Konstruktionsvarianten stellt sich heraus, dass diese Tragwerke bei der statischen Berechnung im Gegensatz zum klassischen Sparrendachwerk mit Zugband auf Traufenhöhe sehr sensibel auf Modellierungsannahmen reagieren, vor allem auf die angesetzten Steifigkeiten der Verbindungen und Auflager. Nichtlineare Modelle für Verbindungen (z. B. Berücksichtigung der herstellungs- und schwindbedingten Klaffungen) könnten einen wesentlichen Beitrag zum Verständnis des Lastabtrags und zum Nachweis der verbleibenden Standsicherheit solcher Tragwerke liefern (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) Construction and structural analysis of baroque roofs. Several research projects, notably within the priority research programme 315, Karlsruhe, have already been devoted to historic constructions, in particular timber constructions. However, it turns out that the results obtained from these previous studies do not carry over to a large , economically as well as historically significant , group of German heritage constructions, namely, the baroque roofs of southern Germany. While numerous constructions of this type are preserved, foundations for their realistic structural assessment are still lacking: These roofs have a much more markedly three-dimensional behaviour than the typical mediaeval roof, due to various longitudinal members such as purlins and St-Andrew's crosses; furthermore, the typical southern German wide-span baroque church roof lacks a tie-beam at the base of the roof, thus requiring complicated, statically indeterminate systems to carry the thrust. The authors of the present paper have conducted an extensive survey of this kind of roof; on the basis of the data collected, characteristic types of such constructions are described, as well as analysed from a structural point of view. It turns out that computational results on such constructions react far more sensitively to changes in the underlying modelling assumptions than other types of roofs where a tie-beam is present. [source]

Principles of pharmacoeconomics and their impact on strategic imperatives of pharmaceutical research and development

BRITISH JOURNAL OF PHARMACOLOGY, Issue 7 2010
József Bodrogi
The importance of evidence-based health policy is widely acknowledged among health care professionals, patients and politicians. Health care resources available for medical procedures, including pharmaceuticals, are limited all over the world. Economic evaluations help to alleviate the burden of scarce resources by improving the allocative efficiency of health care financing. Reimbursement of new medicines is subject to their cost-effectiveness and affordability in more and more countries. There are three major approaches to calculate the cost-effectiveness of new pharmaceuticals. Economic analyses alongside pivotal clinical trials are often inconclusive due to the suboptimal collection of economic data and protocol-driven costs. The major limitation of observational naturalistic economic evaluations is the selection bias and that they can be conducted only after registration and reimbursement. Economic modelling is routinely used to predict the cost-effectiveness of new pharmaceuticals for reimbursement purposes. Accuracy of cost-effectiveness estimates depends on the quality of input variables; validity of surrogate end points; and appropriateness of modelling assumptions, including model structure, time horizon and sophistication of the model to differentiate clinically and economically meaningful outcomes. These economic evaluation methods are not mutually exclusive; in practice, economic analyses often combine data collection alongside clinical trials or observational studies with modelling. The need for pharmacoeconomic evidence has fundamentally changed the strategic imperatives of research and development (R&D). Therefore, professionals in pharmaceutical R&D have to be familiar with the principles of pharmacoeconomics, including the selection of health policy-relevant comparators, analytical techniques, measurement of health gain by quality-adjusted life-years and strategic pricing of pharmaceuticals. [source]