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At least 469 records · Page 26Linked to original sources

PubMed2005

A fuzzy reinforcement learning approach to power control in wireless transmitters.

We address the issue of power-controlled shared channel access in wireless networks supporting packetized data traffic. We formulate this problem using the dynamic programming framework and present a new distributed fuzzy reinforcement learning algorithm (ACFRL-2) capable of adequately solving a class of problems to which the power control problem belongs. Our experimental results show that the algorithm converges almost deterministically to a neighborhood of optimal parameter values, as opposed to a very noisy stochastic convergence of earlier algorithms. The main tradeoff facing a transmitter is to balance its current power level with future backlog in the presence of stochastically changing interference. Simulation experiments demonstrate that the ACFRL-2 algorithm achieves significant performance gains over the standard power control approach used in CDMA2000. Such a large improvement is explained by the fact that ACFRL-2 allows transmitters to learn implicit coordination policies, which back off under stressful channel conditions as opposed to engaging in escalating "power wars."

Algorithms↗

PubMed2005

From fuzzy logic toward plurimonism: the science of active and empathic observation.

Plurimonism is a new philosophy and method of science. It holds that the revolution in computer science and artificial intelligence has reached the point that all the sciences in general can now account for the complex relations of an irreducible plurality of unique observers engaged in describing the same event. Plurimonism seeks to describe the conscious and unconscious relations of the scientific observer during the act of observation of a given event while preserving the historical uniqueness and indivisible identity of each such observer. Using the framework of plurimonism, we mathematically formulate the problem of empathy. This self-reflective mathematical model entails four components of the empathic process involving two observers. They are: 1) the self; 2) the self's-other; 3) the other; and 4) the other's-self. It measures the degree of accuracy of the therapist-observer's empathy, as well as conscious and unconscious processes involved in the patient-observer's idealization and the therapist-observer's confidence in clinical psychotherapy. Ratings are obtained from both patient and therapist from four different points of view. The plural views of the patient's global assessment of functioning (GAF) are from: 1) the therapist's view (TGAF); 2) the patient's view (PGAF); 3) the therapist empathic view (TEGAF), which represents the therapist's estimate of PGAF; and 4) the patient's empathic estimate of the TGAF. The GAF scale is the standard dimensional 100-point-scale measure used in psychiatry for recording a patient's functioning. The patient's estimate of the therapist's degree of accuracy as well as the therapist's confidence in his or her empathic accuracy is also represented. Three formulae are presented that describe the degree of the therapist's empathic accuracy, the patient's over-idealization/under-idealization, and the therapist's over-confidence/under-confidence. The concept of empathy is here restricted to mean the degree to which one observer can take the point of view of another observer when both are observing the same thing.

Cybernetics↗

PubMed2006

Planar-shape prototype generation using a tree-based random greedy algorithm.

A prototype is representative of a set of similar objects. This paper proposes an approach that formulates the problem of prototype generation as finding the mean from a given set of objects, where the prototype solution must satisfy certain constraints. These constraints describe the important perceptual features of the sample shapes that the proposed prototype must retain. The contour prototype generated from a set of planar objects was used as an example of the approach, and the corners were used as the perceptual features to be preserved in the proposed prototype shape. However, finding a prototype solution for more than two contours is computationally intractable. A tree-based approach is therefore proposed in which an efficient greedy random algorithm is used to obtain a good approximation of the proposed prototype and analyze the expected complexity of the algorithm. The proposed prototype-generation process for hand-drawn patterns is described and discussed in this paper.

Algorithms↗

PubMed2003

Sensitivity analysis for the assessment of causal vaccine effects on viral load in HIV vaccine trials.

Vaccines with limited ability to prevent HIV infection may positively impact the HIV/AIDS pandemic by preventing secondary transmission and disease in vaccine recipients who become infected. To evaluate the impact of vaccination on secondary transmission and disease, efficacy trials assess vaccine effects on HIV viral load and other surrogate endpoints measured after infection. A standard test that compares the distribution of viral load between the infected subgroups of vaccine and placebo recipients does not assess a causal effect of vaccine, because the comparison groups are selected after randomization. To address this problem, we formulate clinically relevant causal estimands using the principal stratification framework developed by Frangakis and Rubin (2002, Biometrics 58, 21-29), and propose a class of logistic selection bias models whose members identify the estimands. Given a selection model in the class, procedures are developed for testing and estimation of the causal effect of vaccination on viral load in the principal stratum of subjects who would be infected regardless of randomization assignment. We show how the procedures can be used for a sensitivity analysis that quantifies how the causal effect of vaccination varies with the presumed magnitude of selection bias.

AIDS Vaccines↗

PubMed1999

Maximum likelihood analysis for heteroscedastic one-way random effects ANOVA in interlaboratory studies.

This article presents results for the maximum likelihood analysis of several groups of measurements made on the same quantity. Following Cochran (1937, Journal of the Royal Statistical Society 4(Supple), 102-118; 1954, Biometrics 10, 101-129; 1980, in Proceedings of the 25th Conference on the Design of Experiments in Army Research, Development and Testing, 21-33) and others, this problem is formulated as a one-way unbalanced random-effects ANOVA with unequal within-group variances. A reparametrization of the likelihood leads to simplified computations, easier identification and interpretation of multimodality of the likelihood, and (through a non-informative-prior Bayesian approach) approximate confidence regions for the mean and between-group variance.

Analysis of Variance↗

PubMed1999

Multiple comparison of entropies with application to dinosaur biodiversity.

Did the biodiversity of dinosaurs decline, or did it remain more or less constant before their mass extinction 65 million years ago? Sheehan et al. (1991, Science, 835-839) reported that the biodiversity of families of dinosaur species remained more or less constant preceding their extinction, suggesting extinction due to a cataclysmic event such as an asteroid strike. But that claim was based on the incorrect interpretation that a large p value associated with a test of null hypothesis of equality supports that null hypothesis. To assess whether there is a basis for such a claim, we formulate the problem as one of practical equivalence, in analalogy to bioequivalence. We then develop reliable practical equivalence confidence intervals for differences of entropies by applying the bootstrap-t technique to a nearly pivotal quantity. Confidence intervals for changes in the biodiversity of dinosaurs are then computed, allowing the reader to assess whether there is evidence of near constancy of dinosaur biodiversity before extinction.

Animals↗

PubMed2001

Addressing an idiosyncrasy in estimating survival curves using double sampling in the presence of self-selected right censoring.

We investigate the use of follow-up samples of individuals to estimate survival curves from studies that are subject to right censoring from two sources: (i) early termination of the study, namely, administrative censoring, or (ii) censoring due to lost data prior to administrative censoring, so-called dropout. We assume that, for the full cohort of individuals, administrative censoring times are independent of the subjects' inherent characteristics, including survival time. To address the loss to censoring due to dropout, which we allow to be possibly selective, we consider an intensive second phase of the study where a representative sample of the originally lost subjects is subsequently followed and their data recorded. As with double-sampling designs in survey methodology, the objective is to provide data on a representative subset of the dropouts. Despite assumed full response from the follow-up sample, we show that, in general in our setting, administrative censoring times are not independent of survival times within the two subgroups, nondropouts and sampled dropouts. As a result, the stratified Kaplan-Meier estimator is not appropriate for the cohort survival curve. Moreover, using the concept of potential outcomes, as opposed to observed outcomes, and thereby explicitly formulating the problem as a missing data problem, reveals and addresses these complications. We present an estimation method based on the likelihood of an easily observed subset of the data and study its properties analytically for large samples. We evaluate our method in a realistic situation by simulating data that match published margins on survival and dropout from an actual hip-replacement study. Limitations and extensions of our design and analytic method are discussed.

Arthroplasty, Replacement, Hip↗

PubMed1977

Brief psychotherapy: a case report illustrating its potential effectiveness.

The advance of industrialization in Mexico and elsewhere has brought about psychosocial problems of aging similar to those encountered in the United States. A case study is presented of a 64-year-old Mexican widow who had suicidal thoughts and feeling of uselessness. External constraints on the therapist dictated that psychotherapy be completed within 15 days. A strategy of brief psychotherapy was adopted. The focus was not on establishing transference, probing the unconscious or reconstructing the patient's personality, but on identifying problems and formulating practical solutions promptly. Four hour-long sessions were sufficient to gather information and to counsel the patient and her family. The diagnosis was depression. Treatment centered around increasing the quality of the patient's activity. The initial success of these measure made for a good prognosis. Both patient and family were advised of the availability of antidepressant drugs, and advised against relocating the patient.

Adjustment Disorders↗

PubMed1986

Nitrate tolerance in angina pectoris.

A review of the current literature concerning development of nitrate tolerance in the treatment of angina pectoris suggests that: development of nitrate tolerance is a clinical reality development of tolerance do not seem to differ for nitroglycerin, isosorbide-dinitrate or isosorbide-5-mononitrate a high nitrate dose seems to be more likely to induce tolerance a nitrate free interval during some part of the day or the night seems to restore vascular responsiveness by optimal use of different nitrate formulations the problem with development of tolerance may be reduced.

Angina Pectoris↗

PubMed1981

A semi-empirical model of aerosol deposition in the human respiratory tract for mouth inhalation.

A mathematical model for regional deposition of aerosols following inhalations via mouth has been developed. The model is in the form of algebraic equations which make it particularly efficient for computation of deposition of polydisperse aerosols. The parameters of the model were derived from average experimental data for "head' and tracheobronchial deposition which were supplemented by results of previous theoretical calculations and mass balance considerations. An example is presented to illustrate an application of the model to a problem in formulation of inhalation aerosols. To make the calculations more reliable for particular patho-physiological groups of patients, some modifications of the parameters used in the model are necessary. The model may be suitable, e.g. for testing the changes in regional deposition which would be likely to result from modification of particle size and related formulation properties of inhalation aerosols.

Aerosols↗

PubMed2003

A model of fluid flow in solid tumors.

Solid tumors consist of a porous interstitium and a neoplastic vasculature composed of a network of capillaries with highly permeable walls. Blood flows across the vasculature from the arterial entrance point to the venous exit point, and enters the tumor by convective and diffusive extravasation through the permeable capillary walls. In this paper, an integrated theoretical model of the flow through the tumor is developed. The flow through the interstitium is described by Darcy's law for an isotropic porous medium, the flow along the capillaries is described by Poiseuille's law, and the extravasation flux is described by Starling's law involving the pressure on either side of the capillaries. Given the arterial, the venous, and the ambient pressure, the problem is formulated in terms of a coupled system of integral and differential equations for the vascular and interstitial pressures. The overall hydrodynamics is described in terms of hydraulic conductivity coefficients for the arterial and venous flow rates whose functional form provides an explanation for the singular behavior of the vascular resistance observed in experiments. Numerical solutions are computed for an idealized case where the vasculature is modeled as a single tube, and charts of the hydraulic conductivities are presented for a broad range of tissue and capillary wall conductivities. The results in the physiological range of conditions are found to be in good agreement with laboratory observations. It is shown that the assumption of uniform interstitial pressure is not generally appropriate, and predictions of the extravasation rate based on it may carry a significant amount of error.

Arteries↗

PubMed2000

A hybrid, inverse approach to the design of magnetic resonance imaging magnets.

This paper describes a hybrid numerical method of an inverse approach to the design of compact magnetic resonance imaging magnets. The problem is formulated as a field synthesis and the desired current density on the surface of a cylinder is first calculated by solving a Fredholm equation of the first kind. Nonlinear optimization methods are then invoked to fit practical magnet coils to the desired current density. The field calculations are performed using a semi-analytical method. The emphasis of this work is on the optimal design of short MRI magnets. Details of the hybrid numerical model are presented, and the model is used to investigate compact, symmetric MRI magnets as well as asymmetric magnets. The results highlight that the method can be used to obtain a compact MRI magnet structure and a very homogeneous magnetic field over the central imaging volume in clinical systems of approximately 1 m in length, significantly shorter than current designs. Viable asymmetric magnet designs, in which the edge of the homogeneous region is very close to one end of the magnet system are also presented. Unshielded designs are the focus of this work. This method is flexible and may be applied to magnets of other geometries.

Algorithms↗

PubMed2003

Scattering of elastic waves in damaged media.

The scattering of elastic waves in a medium with damage from microcracking is discussed. The influence of damage from penny-shaped microcracks within a homogeneous medium is considered. The microcracks are assumed to be randomly oriented and uniformly distributed. Explicit expressions are derived for the attenuation of longitudinal and shear elastic waves in terms of the damage parameter and the effective elastic moduli of the medium. A generalized tensor-based approach is used such that the results are coordinate free. The derivation is based upon diagrammatic methods. The problem is formulated in terms of the Dyson equation, which is solved for the mean field response within the limits of the first-order smoothing approximation. The longitudinal and shear attenuations are discussed in terms of their frequency dependence and damage dependence. In particular, the attenuations are shown to scale linearly with the damage parameter.

Journal Article↗

PubMed1989

Genetic instability and aging: theories, facts, and future perspectives.

The fundamental mechanisms involved in the physiological deterioration observed with age in mammalian organisms have not yet been elucidated. It appears that random alterations in informational biomolecules and in their synthesis could be the basis of such physiological changes. There is, however, a lack of knowledge with respect to the frequency and characteristics of changes introduced in the cellular molecular machinery. Moreover, the driving force initiating the generation of such alterations and the order of events in which they occur are unknown at present. In this article, data concerning the hypothesis that the aging process is associated with widespread genetic instability are reviewed in the context of the complex interactions between the three major informational biomolecules, DNA, RNA, and protein. We conclude that the results obtained to date do not rule out the possibility that genetic instability in a wide sense is a major causal factor in a number of age-related phenomena. However, it appears that new strategies based on a new technology are ultimately necessary to elucidate the alterations in the intricately interwoven patterns of molecular control that could underlie the various aspects of the aging process. A first attempt is made to formulate the problems in this field and to provide some solutions.

Aging↗

PubMed2004

Kernel-based data fusion and its application to protein function prediction in yeast.

Kernel methods provide a principled framework in which to represent many types of data, including vectors, strings, trees and graphs. As such, these methods are useful for drawing inferences about biological phenomena. We describe a method for combining multiple kernel representations in an optimal fashion, by formulating the problem as a convex optimization problem that can be solved using semidefinite programming techniques. The method is applied to the problem of predicting yeast protein functional classifications using a support vector machine (SVM) trained on five types of data. For this problem, the new method performs better than a previously-described Markov random field method, and better than the SVM trained on any single type of data.

Algorithms↗

PubMed2002

Vertebrate phylogenomics: reconciled trees and gene duplications.

Ancient gene duplication events have left many traces in vertebrate genomes. Reconciled trees represent the differences between gene family trees and the species phylogeny those genes are sampled from, allowing us to both infer gene duplication events and estimate a species phylogeny from a sample of gene families. We show that analysis of 118 gene families yields a phylogeny of vertebrates largely in agreement with other data. We formulate the problem of locating episodes of gene duplication as a set cover problem: given a species tree in which each node has a set of gene duplications associated with it, the smallest set of species nodes whose union includes all gene duplications specifies the locations of gene duplication episodes. By generating a unique mapping from this cover set we can determine the minimal number of such episodes at each location. When applied to our data, this method reveals a complex history of gene duplications in vertebrate evolution that does not conform to the "2R" hypothesis.

Animals↗

PubMed2005

Robust reinforcement learning.

This letter proposes a new reinforcement learning (RL) paradigm that explicitly takes into account input disturbance as well as modeling errors. The use of environmental models in RL is quite popular for both offline learning using simulations and for online action planning. However, the difference between the model and the real environment can lead to unpredictable, and often unwanted, results. Based on the theory of H(infinity) control, we consider a differential game in which a "disturbing" agent tries to make the worst possible disturbance while a "control" agent tries to make the best control input. The problem is formulated as finding a min-max solution of a value function that takes into account the amount of the reward and the norm of the disturbance. We derive online learning algorithms for estimating the value function and for calculating the worst disturbance and the best control in reference to the value function. We tested the paradigm, which we call robust reinforcement learning (RRL), on the control task of an inverted pendulum. In the linear domain, the policy and the value function learned by online algorithms coincided with those derived analytically by the linear H(infinity) control theory. For a fully nonlinear swing-up task, RRL achieved robust performance with changes in the pendulum weight and friction, while a standard reinforcement learning algorithm could not deal with these changes. We also applied RRL to the cart-pole swing-up task, and a robust swing-up policy was acquired.

Learning↗

PubMedDate not supplied

Filtering strategies in mass population screening for colorectal cancer: an economic evaluation.

On the basis of clinical trial data, 13 strategies for initial screening (filtering) for colorectal cancer are modeled for the purpose of economic evaluation. A wide range of detection cost estimates are generated, although ranking options by detection cost ignores the important consideration of undetected cancers. Formulating the problem as one of cost-effectiveness, however, allows the authors to demonstrate that members of a subset of strategies uniquely outperform all others and that the optimum strategy can be identified by the ex ante specification of the valuation of cancers missed on screening.

Colorectal Neoplasms↗