SEARCH · PubMed Health

Results for “Problem Formulation”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 415 records · Page 23Linked to original sources

PubMed2000

A genetic algorithm approach to multi-disorder diagnosis.

One of the common limitations of expert systems for medical diagnosis is that they make an implicit assumption that multiple disorders do not co-occur in a single patient. The need for this simplifying assumption stems from the fact that finding minimal sets of disorders that cover all symptoms for a given patient is generally computationally intractable (NP-hard). In this paper, we explain the need for performing multi-disorder diagnosis, review previous approaches, formulate the problem using set theory notation, and propose the use of a search method based on a genetic algorithm. We test the algorithm and compare it to another approach using a simple example. The genetic algorithm performs well independently of the order of symptoms, and has the potential to perform multi-disorder diagnosis using existing or newly developed knowledge bases.

Algorithms↗

PubMed1990

Decision analysis in obstetrics and gynaecology.

Decision analysis has its weaknesses: it may oversimplify problems, and probability and utility estimates may be biased. However, the alternative--decision-making by intuition--is likely to oversimplify problems even more and to be subject to even greater bias. Clinicians and patients often ignore major items of data, they handle probabilistic information badly, and their decisions are subject to a number of well-documented biases (Tversky and Kahnemann, 1974). Concern over these problems is not a reason to avoid decision analysis. Probably the greatest virtue of the decision tree and the other components of decision analysis is that the method forces decision-makers to make the bases for their decisions explicit. Generating the information often clarifies problems, and the source of difference of opinion can then be elucidated. Most of the benefits of decision analysis come to the decision-maker in the early part of the analysis as the problem is formulated and probabilities and values are estimated. Only occasionally is it necessary to perform new research, formally measure experts' opinions by the Delphi method, or do elaborate sensitivity analysis.

Bias↗

PubMed2002

Fundamental issues, recent advances, and future directions in myodynamics.

A state-of-the-art report is presented on recent progress in selected areas of myodynamics, but also on problems that severely hamper the further development of the discipline. Significant advances have been made in elucidating the force-producing interaction between actin and the myosin-S1-subunit, including the localization of the most probable molecular site of power stroke initiation. Concerning the architecture of the myostructures, strong experimental evidence has accumulated for numerous intra-, inter-, and extramuscular pathways for lateral force transmission in addition to the serial sarcomere-to-sarcomere myotendinous path. It is shown that contemporary muscle models are inadequate in most respects and lag far behind the requirements an appropriate myodynamic model should fulfil. A similar comment applies to the current approaches designed to solve the myoskeletal indeterminacy problem. These formulations neglect myodynamic properties and do not allow for the implementation of biologically realistic objective functions. The solutions currently obtained are highly unsatisfactory. New research directions to rectify these situations are suggested, also with regard to the identification of subject-specific myodynamic parameters.

Adenosine Triphosphate↗

PubMed2000

A finite difference model of load-induced fluid displacements within bone under mechanical loading.

Load-induced fluid flow in the lacunocanalicular network, induced by the mechanical loading of bone, is believed to play an important role in bone modelling, remodelling and adaptation processes. There are strong indications that this fluid flow is responsible for the mechanotransduction from external mechanical loads to the cells responsible for bone apposition or removal. Since direct flow measurements (especially in compact bone, in vivo and in situ) are not yet possible, theoretical modelling offers an alternative approach to determine the fluid flow velocities, displacements and effects of interstitial fluid flow. In this model, the fluid displacements in a middiaphyseal slab of a rat tibia under a cyclic four-point-bending load were calculated by applying Biot's theory of poroelasticity. The resulting differential equations were solved numerically for the fluid displacement vectors using the finite difference method. Thereby, the cross section located in the middle between the two inner points of force application was chosen for examination, such that the problem, although formulated in three dimensions, reduced itself to an essentially planar form. The maximal fluid displacements for the vector components in the cross sectional plane were found in the proximity of the neutral axis of bending. The direction of the displacement vectors was from the lateral aspect, which was in compression in the examined loading situation, towards the medial aspect in tension. In a parameter study it was found that the fluid displacement pattern and the distribution of fluid displacements remained constant for all the examined parameters, while the magnitude was influenced by the model parameters Young's modulus, Poisson's ratio and porosity. This study represents a further step in the examination of load-induced fluid displacements in loaded bone using theoretical models, aiming to understand the relationship between mechanical loading and bone modelling, remodelling and functional adaptation.

Adaptation, Physiological↗

PubMed2006

Automatic deconvolution of isotope-resolved mass spectra using variable selection and quantized peptide mass distribution.

We present an algorithm for the deconvolution of isotope-resolved mass spectra of complex peptide mixtures where peaks and isotope series often overlap. The algorithm formulates the problem of mass spectrum deconvolution as a classical statistical problem of variable selection, which aims to interpret the spectrum with the least number of peptides. The LASSO method is used to perform automatic variable selection. The algorithm also makes use of the quantized distribution of peptide masses in the NCBInr database after in silico trypsin digestion as filters to aid the deconvolution process. Errors in the expected isotope pattern are accounted for to avoid spurious isotope series. The effectiveness of the algorithm is demonstrated with annotated ESI spectrum of known peptides for which the peaks and isotope series are highly overlapping. The algorithm successfully finds all correct masses in the experimental spectrum, except for one spectrum where an additional refinement procedure is required to obtain the correct results. Our results compare favorably to those from a widely used commercial program.

Animals↗

PubMed2005

Finite time thermodynamics: limiting performance of diffusion engines and membrane systems.

In this paper, the limiting performance of membrane systems with inhomogeneous composition is studied within the class of fixed rate processes. The problem of maintaining a nonequilibrium state in such a system using minimal power (separation problem) and the problem of extracting maximal power from such a system (diffusion engine problem) are formulated and solved. Results are obtained for diffusion engines with constant and periodic contact between the working body and the reservoirs.

Journal Article↗

PubMed2006

Distributions of ionic concentrations and electric field around the three-phase contact at high rates of Langmuir-Blodgett deposition.

A mathematical problem is formulated and numerically solved for addressing the electric field and ionic concentration distributions developing around the three-phase contact line during the Langmuir-Blodgett deposition of charged monolayers. Compared to a previous paper dealing with the same effect (J. Phys. Chem. B 2004, 108, 13449), the present analysis is not restricted to the case of low deposition rates and small concentration changes. The obtained results show that, for sufficiently high deposition rates, the subphase composition substantially changes in the immediate vicinity of the three-phase contact line. It is shown that the predicted changes in the subphase composition can drastically affect the adhesion work and the dynamic contact angle. On this basis, the influence of the concentration polarization effect on meniscus behavior is discussed.

Computer Simulation↗

PubMed2004

Inferring the most likely geographical origin of mtDNA sequence profiles.

In a number of practical cases it is important to determine the likely geographical origin of an individual or a biological sample. A dead body, old bones or a sample of semen may be available. Information on where the sample might come from can assist investigation or research. The first part of this paper is independent of specific data structure. We formulate the problem as a classification problem. Bayes' theorem allows different sources of information or data to be reconciled conveniently. The main part of the paper involves high dimensional data for which simple, standard methods are not likely to work properly. Mitochondrial DNA (mtDNA) data is a typical example of such data. We propose a procedure involving essentially two steps. First, principal component analysis is used to reduce the dimension of the data. Next, quadratic discriminant analysis performs the actual classification. A cross validation procedure is implemented to select the optimal number of principal components. The importance of using separate data sets for model fitting and testing is emphasized. This method distinguishes well between individuals with a self reported European (Icelandic or German) origin and SE Africans. In this case the error rate is 2.0%.

Africa↗

PubMed1987

[Obstetric-perinatologic data collection using a personal computer].

A concept covering the collecting and processing of obstetrical and perinatological data is described. Collecting of patient data is effected on the basis of a case history that has been drawn up in an EDP-adequate manner, which, however, can also be used in the conventional way. This procedure was chosen because for some time to come one cannot do without a document for handwritten notes to avoid duplication of work by "double tracking" and to eliminate transmission errors, and also to continue the present procedure of dealing with the patient. A commercial data base system was chosen for data collection and storage (dBase III by Ashton Tate). This relational data base has its own programming language with very powerful macro calls. Data input is effected by means of programme masks which the user can solicit via menu monitoring. The requisite hardware configuration consists of a computer with a main storage comprising 640 KB, system MS-DOS, and a hard disk of at least 20 MB. This data collection system operates in an obstetric hospital with annually more than 1,600 births and more than 1,200 entries during early pregnancy, to the satisfaction of the users. Besides compiling the usual statistical analyses and formulating research problems, the system automatically prepares the discharge reports. This rationalisation procedure compels the user to collect the data with care and also completely. On the whole, such a data collection system offers to hospitals of any size quick and easy access to data at any time, as well as optimised patient care, without additional effort and at a reasonable cost level.

Computers↗

PubMed2005

Water waves as a spatial dynamical system; infinite depth case.

We review the mathematical results on traveling waves in one or several superposed layers of potential flow, subject to gravity, with or without surface and interfacial tension, where the bottom layer is infinitely deep. The problem is formulated as a "spatial dynamical system," and it is shown that the linearized operator of the resulting reversible system has an essential spectrum filling the real line. We consider three cases where bifurcation occurs. (i) The first case is when, in moving a parameter, two pairs of imaginary eigenvalues merge into one pair of double eigenvalues, and then split into four symmetric complex conjugate eigenvalues. (ii) The second case is when one pair of imaginary eigenvalues meet in 0, and disappear; (iii) the third case is when the phenomenon described in (ii) is superposed to the presence of another pair of imaginary eigenvalues sitting at finite distance from 0. We give a physical example for each case and more specially study the solitary waves and generalized solitary waves, emphasizing the differences, in the methods and in the results, between these cases and the finite depth case.

Algorithms↗

PubMed1985

Modification of the age transition model with implications for regional migration rates and their influence on the reproduction structure of a population.

"Models are developed for the dynamics of the age structure of the population of a region into which there are migration flows from other regions with different fertility coefficients. The model is explored in detail for a three-age-group population. A number of optimisation problems are formulated which can be used for various planning purposes."

Age Distribution↗

PubMed2001

Self-similar intermediate asymptotics for nonlinear degenerate parabolic free-boundary problems that occur in image processing.

In the boundary layers around the edges of images, basic nonlinear parabolic equations for image intensity used in image processing assume a special degenerate asymptotic form. An asymptotic self-similar solution to this degenerate equation is obtained in an explicit form. The solution reveals a substantially nonlinear effect-the formation of sharp steps at the edges of the images, leading to edge enhancement. Positions of the steps and the time shift parameter cannot be determined by direct construction of a self-similar solution; they depend on the initial condition of the pre-self-similar solution. The free-boundary problem is formulated describing the image intensity evolution in the boundary layer.

Journal Article↗

PubMed1992

How reporting delay, duration of follow-up and number of cases affect the estimates of the incubation time of transfusion-associated AIDS cases.

The authors discuss the impact of reporting delay, duration of follow-up, and number of cases in a sample on estimates of the incubation time of transfusion-associated AIDS cases. "This article comes to the conclusion that the accuracy of the incubation time estimate would depend on the sample size rather than on the duration of follow-up." (SUMMARY IN FRE)

Acquired Immunodeficiency Syndrome↗

PubMed1999

Teaching fractional factorial experiments via course delegate designed experiments.

Industrial experiments are fundamental in enhancing the understanding and knowledge of a process and product behavior. Designed industrial experiments assist people in understanding, investigating, and improving their processes. The purpose of a designed experiment is to understand which factors might influence the process output and then to determine those factor settings that optimize the process output. Teaching "design of experiments" using textbook examples does not fully shed light on how to identify and formulate the problem, identify factors, and determine the performance of the physical experiment. Presented here is an example of how to teach fractional factorial experiments in a course on designed experiments. Also presented is a practical, hands-on experiment that has been found to be extremely successful in instilling confidence and motivation in course delegates. The experiment provides a great stimulus to the delegates for the application of experimental design in their own work environment.

Analysis of Variance↗

PubMed2000

Osmotically controlled oral drug delivery.

It is advantageous to deliver some drugs with short half-life, and which are to be given frequently for chronic ailments, in the form of controlled-release (CR) formulations. The orally administered drugs, in the form of conventional matrix or reservoir type formulations, pose problems of bioavailability fluctuations due to gastric pH variations. Moreover, the release of drug(s) from these systems is affected by the hydrodynamic conditions of the body. Osmotically controlled drug delivery systems utilize the principles of osmotic pressure for the controlled delivery of active agent(s). The release rate of drug(s) from these systems is independent of the physiological factors of the gastrointestinal (GI) tract to a large extent. In the present review, theory underlying the delivery of drugs from osmotic systems is presented. Different types of oral osmotic systems, their advantages over conventional matrix and reservoir types of systems, and their applications are also discussed. Finally, some of the limitations, adverse effects, and patent and market status of these systems are reviewed. These systems form a major segment of drug delivery products. Because of their advantages and strong market potential, it appears that the future of osmotic systems in rate-controlled oral drug delivery is promising.

Administration, Oral↗

PubMed2003

The global impact of drug resistance.

Measuring the impact of drug resistance is an important step in understanding the scope of the problem and formulating policies to limit the emergence and spread of resistant organisms. Studies have focused on measuring the increased costs, morbidity, and mortality in patients with infections due to resistant versus susceptible organisms. These have generally found that resistance worsens outcomes. By focusing only on infected patients, however, they may understate the impact of resistance. It is important to recognize that resistance also affects the treatment of individuals with nonresistant organisms. In areas with high rates of resistance, physicians and governments have changed empiric therapy for malaria, tuberculosis, acute respiratory infections, and other diseases, increasing overall treatment costs. In some instances, these costs may exceed those attributable to treatment failure.

Communicable Diseases↗

PubMed2003

Haplotype block partition with limited resources and applications to human chromosome 21 haplotype data.

Recent studies have shown that the human genome has a haplotype block structure such that it can be decomposed into large blocks with high linkage disequilibrium (LD) and relatively limited haplotype diversity, separated by short regions of low LD. One of the practical implications of this observation is that only a small fraction of all the single-nucleotide polymorphisms (SNPs) (referred as "tag SNPs") can be chosen for mapping genes responsible for human complex diseases, which can significantly reduce genotyping effort, without much loss of power. Algorithms have been developed to partition haplotypes into blocks with the minimum number of tag SNPs for an entire chromosome. In practice, investigators may have limited resources, and only a certain number of SNPs can be genotyped. In the present article, we first formulate this problem as finding a block partition with a fixed number of tag SNPs that can cover the maximal percentage of the whole genome, and we then develop two dynamic programming algorithms to solve this problem. The algorithms are sufficiently flexible to permit knowledge of functional polymorphisms to be considered. We apply the algorithms to a data set of SNPs on human chromosome 21, combining the information of coding and noncoding regions. We study the density of SNPs in intergenic regions, introns, and exons, and we find that the SNP density in intergenic regions is similar to that in introns and is higher than that in exons, results that are consistent with previous studies. We also calculate the distribution of block break points in intergenic regions, genes, exons, and coding regions and do not find any significant differences.

Chromosomes, Human, Pair 21↗

PubMed1999

On the assessment of statistical significance in disease-gene discovery.

One of the major challenges facing genome-scan studies to discover disease genes is the assessment of the genomewide significance. The assessment becomes particularly challenging if the scan involves a large number of markers collected from a relatively small number of meioses. Typically, this assessment has two objectives: to assess genomewide significance under the null hypothesis of no linkage and to evaluate true-positive and false-positive prediction error rates under alternative hypotheses. The distinction between these goals allows one to formulate the problem in the well-established paradigm of statistical hypothesis testing. Within this paradigm, we evaluate the traditional criterion of LOD score 3.0 and a recent suggestion of LOD score 3.6, using the Monte Carlo simulation method. The Monte Carlo experiments show that the type I error varies with the chromosome length, with the number of markers, and also with sample sizes. For a typical setup with 50 informative meioses on 50 markers uniformly distributed on a chromosome of average length (i.e., 150 cM), the use of LOD score 3.0 entails an estimated chromosomewide type I error rate of.00574, leading to a genomewide significance level >.05. In contrast, the corresponding type I error for LOD score 3.6 is.00191, giving a genomewide significance level of slightly <.05. However, with a larger sample size and a shorter chromosome, a LOD score between 3.0 and 3.6 may be preferred, on the basis of proximity to the targeted type I error. In terms of reliability, these two LOD-score criteria appear not to have appreciable differences. These simulation experiments also identified factors that influence power and reliability, shedding light on the design of genome-scan studies.

Chromosome Mapping↗