Dissertations / Theses on the topic 'Discrete response models'

In a regression context, the aim is to analyse a response variable of interest conditional to a set of covariates. In many applications the response variable is discrete. Examples include the event of surviving a heart attack, the number of hospitalisation days, the number of times that individuals benefit of a health service, and so on. This thesis advances the methodology and the application of regression models with discrete response. First, we present a difference-in-differences approach to model a binary response in a health policy evaluation framework. In particular, generalized linear mixed methods are employed to model multiple dependent outcomes in order to quantify the effect of an adopted pay-for-performance program while accounting for the heterogeneity of the data at the multiple nested levels. The results show how the policy had a positive effect on the hospitals' quality in terms of those outcomes that can be more influenced by a managerial activity. Next, we focus on regression models for count response variables. In a parametric framework, Poisson regression is the simplest model for count data though it is often found not adequate in real applications, particularly in the presence of excessive zeros and in the case of dispersion, i.e. when the conditional mean is different to the conditional variance. Negative Binomial regression is the standard model for over-dispersed data, but it fails in the presence of under-dispersion. Poisson-Inverse Gaussian regression can be used in the case of over-dispersed data, Generalised-Poisson regression can be employed in the case of under-dispersed data, and Conway-Maxwell Poisson regression can be employed in both cases of over- or under-dispersed data, though the interpretability of these models is ot straightforward and they are often found computationally demanding. While Jittering is the default non-parametric approach for count data, inference has to be made for each individual quantile, separate quantiles may cross and the underlying uniform random sampling can generate instability in the estimation. These features motivate the development of a novel parametric regression model for counts via a Discrete Weibull distribution. This distribution is able to adapt to different types of dispersion relative to Poisson, and it also has the advantage of having a closed form expression for the quantiles. As well as the standard regression model, generalized linear mixed models and generalized additive models are presented via this distribution. Simulated and real data applications with different type of dispersion show a good performance of Discrete Weibull-based regression models compared with existing regression approaches for count data.

This dissertation examines the impact of stimulus and response similarity on response selection. Traditional models of response selection invoke a central processor that operates like a look-up table by matching the perceptually classified stimulus (e.g., green square) to the specified response (e.g., right button press). The look-up property of response selection affords the system the ability to map any stimulus onto any response, even if that stimulus-response has never been paired before. Under such an approach, the degree of perceptual similarity or dissimilarity that exists among stimuli in the environment should have little effect on central operations, the similarity or dissimilarity of the motor response executed in response to a stimulus should not influence response selection, and no interaction between stimulus and response features is permitted, given that stimulus features affect the encoding process, and response features affect the output process, but not response selection itself. Eight studies examine the influence of stimulus and response similarity during response selection. The first two experiments establish the interaction across different task demands between stimulus and response similarity. The interaction was not the result of perceptual difficulty (Experiment 3) and was extended to a new set of stimuli (Experiment 4). A consequence of the design in Experiments 1 - 4 was that response condition was confounded with response configuration. In one of the response conditions the target location had three competitors on one side of it compared to the other condition where the target had one competitor on one side and two others on the other side. Experiments 5 and 6 examined the separate roles that response configuration and response metrics had on the interaction between stimulus and response similarity. The mechanism that produced the interaction was the result of competition between partially activated stimulus-response alternatives. Experiments 7 and 8 further explored the role of competition during response selection by turning to traditional response selection methodologies that introduce competition through either the presentation of irrelevant stimulus information or through presenting the stimulus along an irrelevant spatial dimension. These data have broad implications for models of RS. To account for the ability to pair any stimulus modality with any response modality dominant accounts of RS assume that central operations are performed by a generic set of processes that operate over representations that are stripped of metric information (amodal representations). Response selection works as a look-up table that receives a categorized stimulus as an input and returns an abstract response code as output. This type of model cannot produce an interaction between stimulus and response similarity and thus, the present data provide a serious challenge to these types of models. Finally, the data provide evidence that the metric relationship between stimuli and response matter and influence response selection. The co-activation of stimulus-response alternatives are at a level of representation that includes both stimulus and response properties. A framework is presented that captures key aspects of the data.

2010/2011
Quality is one important characteristic of products and services, but customers can find some difficulties in evaluating it. If information is not uniformly distributed (e.g. sellers have more information than buyers), trade can be difficult (Akerlof 1970). Certification, defined as the assurance that certain requirements are respected, is a possible mechanism to mitigate the asymmetric information. A typical example of certification is the university degree that assures of the fulfillment of academic requirements. This thesis focuses on the ISO 9000 certification, which assures that the quality management system respects the requirements of the ISO 9000 standards family. Having been adopted by more than 1.000.000 organizations in the world, ISO 9000 is a well-known family of standards based on Total Quality Management, a managerial approach aimed to improve quality and organization performance. Several studies investigated ISO 9000 focusing mainly on the manufacturing sector where ISO 9000 originated. However, over the last years, ISO 9000 has being increasingly adopted in service sector. The current study contributes to this research stream taking on both qualitative and quantitative methods to investigate the adoption and the impact of ISO 9000 certification in services. More precisely, the thesis focuses on two main research questions: Why do companies choose to be ISO 9000 certified? What is the relationship between ISO 9000 and innovation? The two research questions are investigated through the lens of the signaling model of education (Spence 1973, Weiss 1995). According to this framework, education can benefit the employee directly by increasing his expertise of an employee and by indirectly signaling his unobserved but relevant abilities (e.g. persistence). Similarly to education, ISO 9000 can improve organization performances and ISO 9000 certification can signal unobservable abilities of better companies. Some implications are drawn from this theoretical framework and they are tested using a dedicated dataset obtained by combining firm-level data from the Community Innovation Survey (CIS 2006) with the list of ISO 9000 certified companies from Mouvement Luxembourgeois pour la Qualité (MLQ). In the quantitative analysis, the decision of ISO 9000 certification is modeled within a discrete choice model. The quantitative analysis is integrated by a multiple-case study that considers both manufacturing and service companies rigorously selected with the Coarsened Exact Matching method. Addressing the first research question, the study focuses on the signaling effect of ISO 9000 certification. Results shows that companies seek for certification in order to signal to the market unobserved abilities only in specific contexts (e.g. when operating in international market). In addition, it appears that large companies are more likely to be certified than smaller companies. The quantitative analysis shows that the main motivation for certification is the requirements of business customers suggesting that certification is more effective in business to business market that than business to consumers. From a signaling point of view, it can be concluded that qualitative and quantitative findings are not in contrast but the hypothesis that ISO 9000 certification acts as signal is only partially supported. Addressing the second research question, the study suggests that management of quality and management of innovation are not conflicting. Even if distinguishing between organizational benefits and the signaling effect can be difficult, qualitative results shows that ISO 9000 certified companies are more likely to successfully introduce new products and services or new organization and marketing techniques. ISO 9000 certification is correlated with technological innovation (product and process) of manufacturing companies and with innovation of service sector companies when non-technological innovation is considered (organizational and marketing). The qualitative results suggest that ISO 9000 especially in recent versions is not hindering innovation. Reading together the results for both research questions, it appears that ISO 9000 could be a tool for policy-makers willing to improve innovation performance targeting specific groups of companies. Practitioners can better understand the features of firms for which the certification provides the best potential, also in terms of innovation. In this respect, management standards can be an effective tool to diffuse organizational skills among companies especially to companies that have less access to external managerial skills, like small companies. Additionally, the findings of the research can be interpreted as an example of the positive impact that standardization can have on innovation, in line with the policies of European Union that recognize standardization as a potential catalyst for innovation.
La qualità è una caratteristica importante di prodotti e servizi, ma gli acquirenti possono trovare alcune difficoltà nella sua valutazione. Nei caso che l’ informazione non sia uniformemente distribuita (ad esempio, chi vende ha più informazioni di chi compra), le transazioni possono essere difficili da realizzarsi (Akerlof 1970).La certificazione, definita come la garanzia che determinati requisiti sono rispettati, è un possibile meccanismo per mitigare l'asimmetria informativa. Ad esempio, la laurea é un tipico esempio di certificazione che assicura il rispetto dei requisiti accademici. Partendo da queste premesse, questa tesi studia la certificazione ISO 9000, che assicura che il sistema di gestione della qualità rispetti i requisiti della famiglia di standard ISO 9000. L’ ISO 9000, che trova origine nel Total Quality Management, un approccio manageriale volto a migliorare la qualità e le performances dell'organizzazione, è la piú nota famiglia di standards al mondo ed è adottata da più di 1.000.000 organizzazioni. Molte ricerche hanno studiato l’ ISO 9000, concentrandosi principalmente sul settore manifatturiero, ove l’ ISO 9000 ha avuto origine. Tuttavia, negli ultimi anni, ISO 9000 è sempre più adottato nel settore dei servizi. Il presente studio contribuisce a questo linea di ricerca adottando metodologie qualitative e quantitative per indagare l'adozione e l'impatto della certificazione ISO 9000 nel settore dei servizi. Più precisamente, la tesi si focalizza su i due seguenti quesiti: Perché le aziende scelgono di essere certificate ISO 9000? Qual è il rapporto tra ISO 9000 e l'innovazione? Le due domande sono studiate attraverso la lenti del modello di segnale sviluppato originariamente nella ambito degli studi sull’ istruzione (Spence 1973, Weiss 1995). In base a questo approccio teorico, l'istruzione può beneficiare il lavoratore sia aumentandone il capitale umano sia segnalando abilità inosservate, ma rilevanti (es. persistenza). Si ipotizza che in maniera simile all’ istruzione, l‘ ISO 9000 può migliorare le prestazioni aziendali di per sé e la certificazione ISO 9000 sia in grado di segnalare abilità inosservabili delle aziende. Alcune implicazioni tratte da questo quadro teorico sono state verificate su un set di dati ottenuto combinando i dati lussemburghesi a livello di impresa della Community Innovation Survey (CIS 2006) e l'elenco delle aziende certificate ISO 9000 del Mouvement Luxembourgeois pour la Qualité (MLQ). La decisione di certificazione è modellata quantitivamente in un modello a scelta discreta. L'analisi quantitativa è poi integrata dallo studio di casi indagando alcune imprese manifatturiere e di servizi opportunamente selezionate sulla base del Coarsened Exact Matching. Affrontando il primo quesito, la tesi si concentra sugli effetti di segnale della certificazione ISO 9000. I risultati quantitativi suggeriscono che le aziende si certificano al fine di segnalare al mercato capacità difficilmente osservabili in contesti specifici (ad esempio, quando opera sui mercati internazionali). Inoltre, lo studio quantitativo mostra che le imprese piú grandi hanno maggiore probabilità di essere certificate rispetto alle aziende più piccole. Lo studio di casi mostra che le aspettative della clientela sono il motivo principale per certificarsi e che le aziende sono più sensibili alla certificazione ISO 9000 rispetto a consumatori suggerendo che la certificazione è più efficace nel mercato Business to Business. In una ottica di segnale, si può concludere che i risultati quantitativi e qualititativi non sono in contrasto ma l'ipotesi che la certificazione ISO 9000 agisca da segnale è solo parzialmente supportata. Riguardo al seconda quesito, i risultati della tesi sembrano suggerire che la gestione della qualità e gestione dell'innovazione non sono in conflitto. Anche se distinguere tra effetto organizzativo e effetto di segnale non è agevole, i risultati qualitativi mostrano che le aziende certificate sono ,generalmente più propense a introdurre con successo nuovi prodotti e servizi o nuovi tipi di organizzazione e tecniche di marketing. Piú precisamente, la certificazione ISO 9000 è correlata con l'innovazione tecnologica (di prodotto e di processo) delle aziende manifatturiere e con l'innovazione delle imprese del settore dei servizi, quando l'innovazione non tecnologica è considerato (organizzative e di marketing). I risultati qualitativi suggeriscono che l’ ISO 9000, soprattutto nelle versioni più recenti, non è ostacola l'innovazione. Complessivamente, appare che ISO 9000 può essere un valido strumento per i policy makers che desiderano migliorare l' innovazione di specifici tipi di aziende. Inoltre, gli operatori del settore possono comprendere meglio le caratteristiche di imprese per le quali la certificazione fornisce il miglior potenziale, anche in termini di innovazione. In questo senso, standards di gestione possono essere uno strumento efficace per diffondere capacità organizzative tra le imprese, in particolare tra le aziende che hanno meno accesso a competenze manageriali esterne, come le piccole e imprese. Complessivamente, i risultati presentati nella tesi possono essere interpretati come un esempio di impatto positivo che la standardizazione può avere sull'innovazione, in linea con le politiche dell'Unione Europea che riconoscono la standardizzazione come potenziale catalizzatore per l'innovazione.
XXIII Ciclo
1981

The Internet has been more and more intensively used in recent years. Although network infrastructure has been regularly upgraded, and the ability to manage heavy traffic greatly increased, especially on the core networks, congestion never ceases to appear, as the amount of traffic that flow on the Internet seems to be increasing at an even faster rate. Thus, congestion control mechanisms play a vital role in the functioning of the Internet. Active Queue Management (AQM) is a popular type of congestion control mechanism that is implemented on gateways (most notably routers), which can predict and avoid the congestion before it happens. When properly configured, AQMs can effectively reduce the congestion, and alleviate some of the problems such as global synchronisation and unfairness to bursty traffic. However, there are still many problems regarding AQMs. Most of the AQM schemes are quite sensitive to their parameters setting, and these parameters may be heavily dependent on the network traffic profile, which the administrator may not have intensive knowledge of, and is likely to change over time. When poorly configured, many AQMs perform no better than the basic drop-tail queue. There is currently no effective method to compare the performance of these AQM algorithms, caused by the parameter configuration problem. In this research, the aim is to propose a new analytical model, which mainly uses discrete-time queueing theory. A novel transient modification to the conventional equilibrium-based method is proposed, and it is utilised to further develop a dynamic interactive model of responsive traffic and bottleneck queues. Using step-by-step analysis, it represents the bursty traffic and oscillating queue length behaviour in practical network more accurately. It also provides an effective way of predicting the behaviour of a TCP-AQM system, allowing easier parameter optimisation for AQM schemes. Numerical solution using MATLAB and software simulation using NS-2 are used to extensively validate the proposed models, theories and conclusions.

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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)
In this thesis, a wide general class of models for mixed responses is proposed in which joint distributions are constructed by the conditional approach (probability density functions, (pdf), as the product of a marginal pdf and a conditional pdf). It is assumed that the distribution of the discrete response and the conditional distribution of the continuous response given the discrete variable belong to one- or two-parameter exponential family of distributions. Furthermore, the marginal means are related to the covariates by link functions using linear and/or nonlinear and/or non-parametric predictors and a dependency structure between the responses is introduced into the model via the conditional mean. Estimation methods, diagnostic analysis and in uence techniques are presented as well as a simulation study considering the Bernoulli-exponential and Poisson-normal semiparametric models, two particular cases of the proposed class. Finally, one of the proposed models is used in a real data set involving the total cost of care for each patient during hospitalization, the use or not of the intensive treatment units and the age of the patient.
Nesta tese é proposta uma classe ampla e geral de modelos bivariados para respostas mistas em que as distribuições conjuntas são construídas pelo método da fatoração (função densidade de probabilidade, (fdp), como o produto de uma fdp marginal e uma fdp condicional). É assumido que a distribuição da variável resposta discreta e a distribuição condicional da variável resposta contínua dada a variável discreta pertencem à família exponencial de distribuições uniparamétrica ou biparamétrica. Além disso, as médias marginais são relacionadas a covariáveis através de funções de ligação usando preditores linear e/ou não linear e/ou não paramétrico e uma estrutura de dependência entre as respostas é inserida no modelo via a média condicional. Métodos de estimação, análises de diagnóstico e técnicas de in uência são apresentadas assim como um estudo de simulação considerando os modelos Bernoulli-exponencial e Poisson-normal semiparamétrico, dois casos particulares da classe proposta. Finalmente, um dos modelos propostos é usado em um conjunto de dados reais envolvendo gastos totais com cuidados para cada paciente durante a hospitalização, o uso ou não da unidade de tratamento intensivo e a idade do paciente.

The direct simple shear (DSS) device is one of the most commonly used laboratory testing tools to characterize the shear behavior of soils. In the Norwegian Geotechnical Institute (NGI) version of the DSS test, where a cylindrical soil specimen is confined by a wire-reinforced membrane, only normal and shear stresses on the horizontal planes are measured. The knowledge of these stresses alone does not provide adequate information to calculate friction angles for use in geotechnical design. Further, the absence of complementary shear stresses at the soil-membrane interface causes stress non-uniformities within DSS specimens, which makes the task of interpreting DSS testing results even more difficult. With the recent advances in computers, it is now possible to model soil in a realistic manner as a collection of particles using the discrete element method (DEM). With this background, a DEM model of a cylindrical DSS specimen was developed to provide insight on the state of stress and strain in DSS specimens. A laboratory DSS testing program was undertaken on glass beads as part of this study. The results of the glass beads tests were used for comparison with the DEM model results. Further, free-form sensors (paper-thin flexible pressure sensors mounted on the reinforced part of the DSS membrane) were used to measure lateral stresses acting on reconstituted Fraser River silt specimens. It was shown that: i) the adopted DEM modeling approach is effective in capturing the salient characteristics of the DSS behavior of the tested glass beads; ii) during the shearing phase, the distribution of shear strains across the specimen is more uniform at lower shear strain levels; iii) significant stress non-uniformities during shearing are limited to a narrow zone of about two particles diameter near the lateral boundaries, while stresses at central specimen locations are relatively more uniform (i.e. most representative of “ideal” simple shear conditions); and iv) at large shear strains, the horizontal plane becomes the plane of maximum obliquity, and the friction angle calculated using the stress state on the horizontal plane is a good approximation to the mobilized friction angle at such strain levels.
Applied Science, Faculty of
Civil Engineering, Department of
Graduate

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One of the global security challenges the United States faces is disaster coupled with political instability. The U.S. Military‘s ability to rapidly respond to disasters enhances regional and global security and stability. Foreign Humanitarian Assistance and Disaster Relief (FHA/DR), increasingly a mission that relies on a significant military component, focuses on the provision of goods and services such as health care, supplies necessary for survival, and infrastructure repair, with the goal of reducing the immediate human suffering. The disaster in this project‘s scenario is catastrophic flooding that occurs in one of Africa‘s most populated and wealthiest countries that threatens the stability and development of West Africa. This project, employing a systems engineering methodology, focuses on the 60 days after the disaster and the requirements to provide this assistance in the form of goods and services. Many system-of-systems architectures were developed to investigate the effectiveness of utilizing a Seabase for the primary delivery of aid. Two simulation tools, SimKit, and STELLA, were used to model and examine these architectures with the former addressing the delivery and throughput concerns while the latter focused on the satisfaction of the population and the limitation of criminal activity. Based on the results of modeling, the team provided recommendations relative to the most effective architectures in influencing the population of this coastal area as well as accomplishing the FHA/DR mission.

For modeling of particulate materials, discrete element method (DEM) is commonly used. It perceives every particle like a single body. A railway ballast loading by trains is a typical example of a particulate discrete material. By a passing train, static and dynamic forces act on a track bed. Cycling loading results in pernament changes in the railway ballast. Cavity creation, agglomeration and ballast cracking lead to damages in rail traffic. Usage of the discrete element method may reveal the real issues of the railway ballast and it may leads to a reduction of costs associated with a design and repairs. This thesis is focused on the ballast modeling and identification of the discrete model parameters. Obtained results are compared with real experiments from Nottingham University.

Preserving monumental historic buildings has not been an easy task due to their high vulnerability to seismic events. Throughout the years, several studies have tried to predict their behavior with the use of different numerical models, but the response is such complex that it remains a challenge. One of the trending tools to simulate masonry is the Discrete Element Model (DEM), but unfortunately few researches have implemented the physical simulation to validate the numerical results, and that is the main motivation of this study, which aims to contribute to the better understanding of masonry structures using a DEM and a physical model of large dimensions. This investigation is part of the “SEBESMOVA3D” project (SEeismic BEhavior of Scaled MOdels of groin VAults made by 3D printers) granted by the Seismology and Earthquake Engineering Research Infrastructure Alliance for Europe SERA. This investigation starts from the design of a 2m x 2m groin vault, which represents a very common typology of ceiling systems in historical masonry monuments. During the first project campaign, the springings are fixed to the base. Each block is formed by a plastic skin (hollow section) and the inner core is filled with mortar to acquire the corresponding mass for dynamic tests. The blocks are easy and fast to assemble, so a significant number of tests can be executed. Prior to the experimental phase, a series of numerical simulations are carried out to predict both static and dynamic behavior based on a defined material characterization, allowing to establish a frequency range to test the physical model. Experimental tests are performed on a 3m x 3m shaking table, and the data obtained from a motion capture system is processed to evaluate displacements and cumulative damage. DEM simulations are run to calibrate and validate the numerical model. The results will be relevant and considered for the next project campaign.

Maximum likelihood is an ubiquitous method used in the estimation of generalized linear mixed model (GLMM). However, the method entails computational difficulties and relies on the normality assumption for random effects. We propose a second-order least squares (SLS) estimator based on the first two marginal moments of the response variables. The proposed estimator is computationally feasible and requires less distributional assumptions than the maximum likelihood estimator. To overcome the numerical difficulties of minimizing an objective function that involves multiple integrals, a simulation-based SLS estimator is proposed. We show that the SLS estimators are consistent and asymptotically normally distributed under fairly general conditions in the framework of GLMM. Missing data is almost inevitable in longitudinal studies. Problems arise if the missing data mechanism is related to the response process. This thesis develops the proposed estimators to deal with response data missing at random by either adapting the inverse probability weight method or applying the multiple imputation approach. In practice, some of the covariates are not directly observed but are measured with error. It is well-known that simply substituting a proxy variable for the unobserved covariate in the model will generally lead to biased and inconsistent estimates. We propose the instrumental variable method for the consistent estimation of GLMM with covariate measurement error. The proposed approach does not need any parametric assumption on the distribution of the unknown covariates. This makes the method less restrictive than other methods that rely on either a parametric distribution of the covariates, or to estimate the distribution using some extra information. In the presence of data outliers, it is a concern that the SLS estimators may be vulnerable due to the second-order moments. We investigated the robustness property of the SLS estimators using their influence functions. We showed that the proposed estimators have a bounded influence function and a redescending property so they are robust to outliers. The finite sample performance and property of the SLS estimators are studied and compared with other popular estimators in the literature through simulation studies and real world data examples.

The aim of this work is to present a model for the superelastic response of Shape Memory Alloys (SMAs) by developing a Preisach Model with thermodynamics basis. The special features of SMA superelastic response is useful in a variety of applications (eg. seismic dampers and arterial stents). For example, under seismic loads the SMA dampers undergo rapid loading{unloading cycles, thus going through a number of internal hysteresis loops, which are responsible for dissipating the vibration energy. Therefore the design for such applications requires the ability to predict the response, particularly internal loops. It is thus intended to develop a model for the superelastic response which is simple, computationally fast and can predict internal loops. The key idea here is to separate the elastic response of SMAs from the dissipative response and apply a Preisach Model to the dissipative response as opposed to the popular notion of applying the Preisach Model to the stress{strain response directly. Such a separation allows for the better prediction of internal hysteresis, avoids issues due to at/negative slopes in the stress{strain plot, and shows good match with experimental data, even when minimal input is given to the model. The model is developed from a Gibbs Potential, which allows us to compute a driving force for the underlying phase transformation in the superelastic response. The hysteresis between the driving force for transformation and the extent of transformation (volume fraction of martensite) is then used with a Preisach model. The Preisach model parameters are identi ed using a least squares approach. ASTM Standards for the testing of NiTi wires (F2516-07^sigma 2), are used for the identi cation of the parameters in the Gibbs Potential. The simulations are run using MATLAB R . Results under di erent input conditions are discussed. It is shown that the predicted response shows good agreement with the experimental data. A couple of attempts at extending the model to bending and more complex response of SMAs is also discussed.