191 resultados para MLE
Resumo:
In collaboration with Merle. - cf. Quérard.
Resumo:
Mode of access: Internet.
Resumo:
Mode of access: Internet.
Resumo:
Mode of access: Internet.
Resumo:
Written by Merle in collaboration with Joseph Désessarts d'Ambreville and A.R.P. Alissan de Chazet.
Resumo:
Mode of access: Internet.
Resumo:
The correlated probit model is frequently used for multiple ordered data since it allows to incorporate seamlessly different correlation structures. The estimation of the probit model parameters based on direct maximization of the limited information maximum likelihood is a numerically intensive procedure. We propose an extension of the EM algorithm for obtaining maximum likelihood estimates for a correlated probit model for multiple ordinal outcomes. The algorithm is implemented in the free software environment for statistical computing and graphics R. We present two simulation studies to examine the performance of the developed algorithm. We apply the model to data on 121 women with cervical or endometrial cancer. Patients developed normal tissue reactions as a result of post-operative external beam pelvic radiotherapy. In this work we focused on modeling the effects of a genetic factor on early skin and early urogenital tissue reactions and on assessing the strength of association between the two types of reactions. We established that there was an association between skin reactions and polymorphism XRCC3 codon 241 (C>T) (rs861539) and that skin and urogenital reactions were positively correlated. ACM Computing Classification System (1998): G.3.
Resumo:
Thèse numérisée par la Direction des bibliothèques de l'Université de Montréal.
Resumo:
Thèse numérisée par la Direction des bibliothèques de l'Université de Montréal.
Resumo:
This dissertation is primarily an applied statistical modelling investigation, motivated by a case study comprising real data and real questions. Theoretical questions on modelling and computation of normalization constants arose from pursuit of these data analytic questions. The essence of the thesis can be described as follows. Consider binary data observed on a two-dimensional lattice. A common problem with such data is the ambiguity of zeroes recorded. These may represent zero response given some threshold (presence) or that the threshold has not been triggered (absence). Suppose that the researcher wishes to estimate the effects of covariates on the binary responses, whilst taking into account underlying spatial variation, which is itself of some interest. This situation arises in many contexts and the dingo, cypress and toad case studies described in the motivation chapter are examples of this. Two main approaches to modelling and inference are investigated in this thesis. The first is frequentist and based on generalized linear models, with spatial variation modelled by using a block structure or by smoothing the residuals spatially. The EM algorithm can be used to obtain point estimates, coupled with bootstrapping or asymptotic MLE estimates for standard errors. The second approach is Bayesian and based on a three- or four-tier hierarchical model, comprising a logistic regression with covariates for the data layer, a binary Markov Random field (MRF) for the underlying spatial process, and suitable priors for parameters in these main models. The three-parameter autologistic model is a particular MRF of interest. Markov chain Monte Carlo (MCMC) methods comprising hybrid Metropolis/Gibbs samplers is suitable for computation in this situation. Model performance can be gauged by MCMC diagnostics. Model choice can be assessed by incorporating another tier in the modelling hierarchy. This requires evaluation of a normalization constant, a notoriously difficult problem. Difficulty with estimating the normalization constant for the MRF can be overcome by using a path integral approach, although this is a highly computationally intensive method. Different methods of estimating ratios of normalization constants (N Cs) are investigated, including importance sampling Monte Carlo (ISMC), dependent Monte Carlo based on MCMC simulations (MCMC), and reverse logistic regression (RLR). I develop an idea present though not fully developed in the literature, and propose the Integrated mean canonical statistic (IMCS) method for estimating log NC ratios for binary MRFs. The IMCS method falls within the framework of the newly identified path sampling methods of Gelman & Meng (1998) and outperforms ISMC, MCMC and RLR. It also does not rely on simplifying assumptions, such as ignoring spatio-temporal dependence in the process. A thorough investigation is made of the application of IMCS to the three-parameter Autologistic model. This work introduces background computations required for the full implementation of the four-tier model in Chapter 7. Two different extensions of the three-tier model to a four-tier version are investigated. The first extension incorporates temporal dependence in the underlying spatio-temporal process. The second extensions allows the successes and failures in the data layer to depend on time. The MCMC computational method is extended to incorporate the extra layer. A major contribution of the thesis is the development of a fully Bayesian approach to inference for these hierarchical models for the first time. Note: The author of this thesis has agreed to make it open access but invites people downloading the thesis to send her an email via the 'Contact Author' function.
Resumo:
Most unsignalised intersection capacity calculation procedures are based on gap acceptance models. Accuracy of critical gap estimation affects accuracy of capacity and delay estimation. Several methods have been published to estimate drivers’ sample mean critical gap, the Maximum Likelihood Estimation (MLE) technique regarded as the most accurate. This study assesses three novel methods; Average Central Gap (ACG) method, Strength Weighted Central Gap method (SWCG), and Mode Central Gap method (MCG), against MLE for their fidelity in rendering true sample mean critical gaps. A Monte Carlo event based simulation model was used to draw the maximum rejected gap and accepted gap for each of a sample of 300 drivers across 32 simulation runs. Simulation mean critical gap is varied between 3s and 8s, while offered gap rate is varied between 0.05veh/s and 0.55veh/s. This study affirms that MLE provides a close to perfect fit to simulation mean critical gaps across a broad range of conditions. The MCG method also provides an almost perfect fit and has superior computational simplicity and efficiency to the MLE. The SWCG method performs robustly under high flows; however, poorly under low to moderate flows. Further research is recommended using field traffic data, under a variety of minor stream and major stream flow conditions for a variety of minor stream movement types, to compare critical gap estimates using MLE against MCG. Should the MCG method prove as robust as MLE, serious consideration should be given to its adoption to estimate critical gap parameters in guidelines.
Resumo:
My doctoral dissertation is on Johan Jakob Tikkanen (1857 1930), the first professor of art history in Finland, and his significance and methods in the context of late 19th and early 20th-century European art history. Tikkanen was one of the pioneering scholars in the field of medieval art research, and, along with Anton Springer, Heinrich Wölfflin, Aloïs Riegl, Adolfo Venturi, Franz Wickhoff, Julius von Schlosser, Aby Warburg, Emile Mâle and others, one of the scholars who defined art history as an independent academic discipline. Tikkanen s scholarly interests and his methods resemble those of many formalistically oriented German and Austrian art historians of his time. He became well known throughout Europe, mainly for his studies on illustrated medieval manuscripts. Tikkanen s dissertation, Der Malerische Styl Giotto s Versuch zu einer Characteristik Desselben, from 1884 was regarded in its day as the best form-analytical study on the painter. It has a central position in the present thesis, as it already included nearly all the methods that Tikkanen used and elaborated upon throughout his career. Giotto also gives a good perspective for comparing Tikkanen s ideas with a long art-historical tradition. Tikkanen was profoundly interested in artistic creativity. In his own words, he wanted to study das künstlerische Können , artistic ability, instead of das künstlerische Wollen or artistic will, which was an important theoretical issue in art history in the late 19th century. This starting point led him to the history of style and iconographical research. Along with the Danish art historian, Julius Lange, he was one of the first scholars who began to study the meaning of gestures and postures in art. In my dissertation I have emphasized the importance of Tikkanen s personal art education. I regard it as having influenced both his scholarly argumentation and his working methods. I have also written a short overview of the situation of art history in Finland and in Northern Countries before Tikkanen s time in order to give an idea of his scientific background. My thesis is a critical and historiographical study on J. J. Tikkanen s role in the development of art history and its methodology.
Resumo:
We propose a molecular mechanism for the intra-cellular measurement of the ratio of the number of X chromosomes to the number of sets of autosomes, a process central to both sex determination and dosage compensation in Drosophila melanogaster. In addition to the two loci, da and Sxl, which have been shown by Cline (Genetics, 90, 683, 1978)and others to be involved in these processes, we postulate two other loci, one autosomal (ω) and the other, X-linked (π). The product of the autosomal locus da stimulates ω and initiates synthesis of a limited quantity of repressor. Sxl and π ,both of which are X-linked, compete for this repressor as well as for RNA polymerase. It is assumed that Sxl has lower affinity than π for repressor as well as polymerase and that the binding of polymerase to one of these sites modulates the binding affinity of the other site for the enzyme. It can be shown that as a result of these postulated interactions transcription from the Sxl site is proportional to the X/A ratio such that the levels of Sxl+ product are low in males, high in females and intermediate in the intersexes. If, as proposed by Cline, the Sxl- product is an inhibitor of X chromosome activity, this would result in dosage compensation. The model leads to the conclusion that high levels of Sxl+ product promote a female phenotype and low levels, a male phenotype. One interesting consequence of the assumptions on which the model is based is that the level of Sxl+ product in the cell, when examined as a function of increasing repressor concentration, first goes up and then decreases, yielding a bell-shaped curve. This feature of the model provides an explanation for some of the remarkable interactions among mutants at the Sxl, da and mle loci and leads to several predictions. The proposed mechanism may also have relevance to certain other problems, such as size regulation during development, which seem to involve measurement of ratios at the cellular level.
Resumo:
This publication captures the outputs of a JISC infoNet conference held in February 2004 in the Royal York Hotel, York. The event was entitled ‘When Worlds Collide’ because it aimed to explore the cultural differences between the different communities involved in developing and supporting learning environments hence the event was promoted in collaboration with ALT and UCISA. We defined learning environment in the broadest sense to encompass all of the structures, systems and processes that impact on the learning experience. For many this is summed up in the term Managed Learning Environment (MLE) although others favour alternative terminologies. The event was thus unusual in that it brought together people, from learning and teaching, MIS, library, administrative and senior management backgrounds, that don’t often get the opportunity to network with one another.