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At least 1,477 records · Page 82Linked to original sources
PubMed2006

Surveillance of the colorectal cancer disparities among demographic subgroups: a spatial analysis.

OBJECTIVE: The literature suggests that colorectal cancer mortality in Texas is distributed inhomogeneously among specific demographic subgroups and in certain geographic regions over an extended period. To understand the extent of the demographic and geographic disparities, the present study examined colorectal cancer mortality in 15 demographic groups in Texas counties between 1990 and 2001. METHODS: The Spatial Scan Statistic was used to assess the standardized mortality ratio, duration and age-adjusted rates of excess mortality, and their respective p-values for testing the null hypothesis of homogeneity of geographic and temporal distribution. RESULTS: The study confirmed the excess mortality in some Texas counties found in the literature, identified 13 additional excess mortality regions, and found 4 health regions with persistent excess mortality involving several population subgroups. CONCLUSION: Health disparities of colorectal cancer mortality continue to exist in Texas demographic subpopulations. Health education and intervention programs should be directed to the at-risk subpopulations in the identified regions.

Black People↗
PubMed2000

Stochastic epidemics in dynamic populations: quasi-stationarity and extinction.

Empirical evidence shows that childhood diseases persist in large communities whereas in smaller communities the epidemic goes extinct (and is later reintroduced by immigration). The present paper treats a stochastic model describing the spread of an infectious disease giving life-long immunity, in a community where individuals die and new individuals are born. The time to extinction of the disease starting in quasi-stationarity (conditional on non-extinction) is exponentially distributed. As the population size grows the epidemic process converges to a diffusion process. Properties of the limiting diffusion are used to obtain an approximate expression for tau, the mean-parameter in the exponential distribution of the time to extinction for the finite population. The expression is used to study how tau depends on the community size but also on certain properties of the disease/community: the basic reproduction number and the means and variances of the latency period, infectious period and life-length. Effects of introducing a vaccination program are also discussed as is the notion of the critical community size, defined as the size which distinguishes between the two qualitatively different behaviours.

Child↗
PubMedDate not supplied

Bayesian estimates of disease maps: how important are priors?

In the fully Bayesian (FB) approach to disease mapping the choice of the hyperprior distribution of the dispersion parameter is a key issue. In this context we investigated the sensitivity of the rate ratio estimates to the choice of the hyperprior via a simulation study. We also compared the performance of the FB approach to mapping disease risk to the conventional approach of mapping maximum likelihood (ML) estimates and p-values. The study was modelled on the incidence data of insulin dependent diabetes mellitus (IDDM) as observed in the communes of Sardinia.

Bayes Theorem↗
PubMed2005

Thermodynamics of DNA loops with long-range correlated structural disorder.

We study the influence of a structural disorder on the thermodynamical properties of 2D-elastic chains submitted to mechanical/topological constraint as loops. The disorder is introduced via a spontaneous curvature whose distribution along the chain presents either no correlation or long-range correlations (LRC). The equilibrium properties of the one-loop system are derived numerically and analytically for weak disorder. LRC are shown to favor the formation of small loop, larger the LRC, smaller the loop size. We use the mean first passage time formalism to show that the typical short time loop dynamics is superdiffusive in the presence of LRC. Potential biological implications on nucleosome positioning and dynamics in eukaryotic chromatin are discussed.

Chromatin↗
PubMed1998

Person-time analysis of paired community intervention trials when the number of communities is small.

Community intervention trials involve randomization of communities to either an intervention or control arm. The objective of this paper is to evaluate person-time methods of analysis of paired community intervention trials when the number of community pairs is small. We consider several test procedures and evaluate their performance by simulation. Naive methods that ignore intracluster correlation, such as standard Mantel-Haenszel type statistics, can be misleading. The performance of the paired t-test depends on the distribution of the random community effects. Permutation tests perform well for the ranges of situations considered. However, there can be considerable loss of power with permutation methods compared to standard Mantel-Haenszel methods if in fact there is no intracluster correlation when the number of pairs is small. We consider methods to account for individual level covariates. Motivation for this work came from recent randomized community intervention trials in Africa to prevent transmission of the human immunodeficiency virus (HIV).

Africa↗
PubMed2004

Statistics of defect trajectories in spatio-temporal chaos in inclined layer convection and the complex Ginzburg-Landau equation.

For spatio-temporal chaos observed in numerical simulations of the complex Ginzburg-Landau equation (CGL) and in experiments on inclined-layer convection (ILC) we report numerical and experimental data on the statistics of defects and of defect loops. These loops consist of defect trajectories in space-time that are connected to each other through the pairwise annihilation or creation of the associated defects. While most such loops are small and contain only a few defects, the loop distribution functions decay only slowly with the quantities associated with the loop size, consistent with power-law behavior. For the CGL, two of the three power-law exponents are found to agree, within our computational precision, with those from previous investigations of a simple lattice model. In certain parameter regimes of the CGL and ILC, our results for the single-defect statistics show significant deviations from the previously reported findings that the defect dynamics are consistent with those of random walkers that are created with fixed probability and annihilated through random collisions.

Models, Theoretical↗
PubMed1991

Incidence and prevalence of ulcerative colitis in Copenhagen county from 1962 to 1987.

The incidence of ulcerative colitis was estimated during the period 1962 to 1987 in the county of Copenhagen. Within this area of approximately 550,000 inhabitants, 1161 patients were diagnosed. The mean annual incidence for the period was 8.1 per 10(5) inhabitants. There were significant increases in incidence in the early 1970s and in the early 1980s, both of which were followed by significant decreases. A bimodal age distribution was found in men, with incidence peaks in young adult life and late in life. In women a peak incidence was found in the young adult age group. The interval from onset of symptoms to diagnosis in terms of years remained unchanged over the period, with a median of 1 year (range, 0-37 years). The extent of disease at diagnosis was total colon in 18% of the patients, a substantial part of the colon in 36%, and distal colon in 44%--with no changes during the study period. The distribution of different localizations did not differ among age groups except for a tendency towards more extensive disease in young patients, below 20 years of age. The disease activity in the 1st year was fulminant in 9.1% of the cases, moderate to high in 70.7%, and low in 20.2%, with a tendency towards higher activity in patients diagnosed late in the study period. The prevalence of ulcerative colitis increased steadily during the study period, reaching a value at the end of the study (31 December 1987) of 161 per 10(5) inhabitants.

Adult↗
PubMed1995

The number of segregating sites in expanding human populations, with implications for estimates of demographic parameters.

The frequency distribution of pairwise differences between sequences of mtDNA has recently been used to estimate the size of human populations before and after a hypothetical episode of rapid population growth and the time at which the population grew. To test the internal consistency of this method, we used three different sets of human mtDNA data and the corresponding demographic parameters estimated from the distribution of pairwise differences to determine by simulation the expected number of segregating sites, S, and its empirical distribution. The results indicate that the observed values of S are significantly lower than expected in two of three cases under the assumption of the infinite-sites model. Further simulations in which mutations were allowed to occur more than once at the same site and in which there was variation in mutation rate among sites show that the expected number of segregating sites can be much lower than under the infinite-site assumption. Nevertheless, the observed value of S is still significantly different from the value expected under the expansion hypothesis in two of three cases.

Animals↗
PubMed2001

Computational analysis of PKA-balanol interactions.

Protein kinases are important targets for designing therapeutic drugs. This paper illustrates a computational approach to extend the usefulness of a single protein-inhibitor structure in aiding the design of protein kinase inhibitors. Using the complex structure of the catalytic subunit of PKA (cPKA) and balanol as a guide, we have analyzed and compared the distribution of amino acid types near the protein-ligand interface for nearly 400 kinases. This analysis has identified a number of sites that are more variable in amino acid types among the kinases analyzed, and these are useful sites to consider in designing specific protein kinase inhibitors. On the other hand, we have found kinases whose protein-ligand interfaces are similar to that of the cPKA-balanol complex and balanol can be a useful lead compound for developing effective inhibitors for these kinases. Generally, this approach can help us discover new drug targets for an existing class of compounds that have already been well characterized pharmacologically. The relative significance of the charge/polarity of residues at the protein-ligand interface has been quantified by carrying out computational sensitivity analysis in which the charge/polarity of an atom or functional group was turned off/on, and the resulting effects on binding affinity have been examined. The binding affinity was estimated by using an implicit-solvent model in which the electrostatic contributions were obtained by solving the Poisson equation and the hydrophobic effects were accounted for by using surface-area dependent terms. The same sensitivity analysis approach was applied to the ligand balanol to develop a pharmacophoric model for searching new drug leads from small-molecule libraries. To help evaluate the binding affinity of designed inhibitors before they are made, we have developed a semiempirical approach to improve the predictive reliability of the implicit-solvent binding model.

Azepines↗
PubMed2005

Everybody else is: Networks, power laws and peer contagion in the aggressive recess behavior of elementary school boys.

This paper develops a simple random network model of peer contagion in aggressive behavior among inner-city elementary school boys during recess periods. The model predicts a distribution of aggressive behaviors per recess period with a power law tail beginning at two aggressive behaviors and having a slope of approximately -1.5. Comparison of these values with values derived from observations of aggressive behaviors during recess at an inner-city elementary school provides empirical support for the model. These results suggest that fluctuations in aggressive behaviors during recess arise from the interactions between students, rather than from variations in the behavior of individual students. The results therefore support those interventions that aim to change the pattern of interaction between students.

Child↗
PubMed2001

Distribution of patients' paroxysmal atrial tachyarrhythmia episodes: implications for detection of treatment efficacy.

INTRODUCTION: Clinical trials of treatments for paroxysmal atrial tachyarrhythmia (pAT) often compare different treatment groups using the time to first episode recurrence. This approach assumes that the time to the first recurrence is representative of all times between successive episodes in a given patient. We subjected this assumption to an empiric test. METHODS AND RESULTS: Records of pAT onsets from a chronologic series of 134 patients with dual chamber implantable defibrillators were analyzed; 14 had experienced >10 pAT episodes, which is sufficient for meaningful statistical modeling of the time intervals between episodes. Episodes were independent and randomly distributed in 9 of 14 patients, but a fit of the data to an exponential distribution, required by the stated assumption, was rejected in 13 of 14. In contrast, a Weibull distribution yielded an adequate goodness of fit in 5 of the 9 cases with independent and randomly distributed data. Monte Carlo methods were used to determine the impact of violations of the exponential distribution assumption on clinical trials using time from cardioversion to first episode recurrence as the dependent measure. In a parallel groups design, substantial loss of power occurs with sample sizes <500 patients per group. In a cross-over design, there is insufficient power to detect a 30% reduction in episode frequency even with 300 patients. CONCLUSION: Clinical trials that rely on time to first episode recurrence may be considerably less able to detect efficacious treatments than may have been supposed. Analysis of multiple episode onsets recorded over time should be used to avoid this pitfall.

Aged↗
PubMed1985

Magnetic reversals and mass extinctions.

Previous analyses of the time distribution of reversals of the Earth's magnetic field have yielded mixed results. Some authors have claimed significant periodicities of order 10(7) yr whereas others have reported failure to reject null hypothesis of random spacing at that scale. Because of repeated suggestions that field reversal is linked to biological extinction, further analysis of the magnetic times series is appropriate. I present here the results of a study of the reversal record of the past 165 Myr. A stationary periodicity of 30 Myr emerges (superimposed on the non-stationarities already established by others), which predicts pulses of increased reversal activity centered at 10, 40, 70, ... Myr BP.

Biological Evolution↗
PubMed1998

Mechanical wall stress in abdominal aortic aneurysm: influence of diameter and asymmetry.

PURPOSE: Risk for rupture of an abdominal aortic aneurysm is widely believed to be related to its maximum diameter. From a biomechanical standpoint, however, risk is probably more precisely related to mechanical wall stress. Many abdominal aortic aneurysms are asymmetric (for example because of anterior bulging with posterior expansion limited by the vertebral column). The purpose of this work was to investigate the effect of maximum diameter and asymmetric bulge on wall stress. METHODS: Three-dimensional computer models of abdominal aortic aneurysms were generated. In one protocol, maximum diameter was held constant while bulge shape factor was varied. The shape factor took into account the asymmetric shape of the bulge. In a second protocol, the shape of the aneurysmal wall was held constant while maximum diameter was varied. Wall stress was computed in each instance with a commercial software package and assumption of physiologic intraluminal pressure. RESULTS: Both maximum diameter and the shape factor were found to have substantial influence on the distribution of wall stress within the aneurysm. In some instances the maximum stress occurred at the midsection, and in others it occurred elsewhere. The magnitude of peak stress acting on the aneurysm increased nonlinearly with increasing maximum diameter or increasing asymmetry. CONCLUSIONS: Our computer models showed that the stress within the wall of an abdominal aortic aneurysm and possibly the potential for rupture are as dependent on aneurysm shape as they are on maximum diameter. This information may be important in determining severity of individual abdominal aortic aneurysms and in improving understanding of the natural history of the disease.

Aorta, Abdominal↗
PubMed2005

Determination of the packing mode of the coiled-coil domain of cGMP-dependent protein kinase Ialpha in solution using charge-predicted dipolar couplings.

Coiled-coil motifs are ubiquitous in biology and play essential roles in protein assembly and molecular recognition. Here, we show that the relative orientation and stoichiometry of coiled-coil proteins in solution can be determined by comparison of residual dipolar couplings (RDCs) measured in charged liquid-crystalline medium with values predicted from the three-dimensional charge distribution of the protein. Comparison of charge-predicted RDCs with a small set of one-bond 1DNH dipolar couplings, measured in the negatively charged liquid-crystalline Pf1 bacteriophage medium, identified the coiled-coil region of the cGMP-dependent protein kinase I as a parallel homodimer in solution and ruled out an antiparallel dimeric or monomeric state. The method is very rapid, applicable to a wide variety of liquid crystals used in biological NMR to date, and can be applied to coiled-coil structures and other proteins with higher order assembly.

Binding Sites↗
PubMed1999

Response features determining spike times.

Interpreting messages encoded in single neuronal responses requires knowing which features of the responses carry information. That the number of spikes is an important part of the code has long been obvious. In recent years, it has been shown that modulation of the firing rate with about 25 ms precision carries information that is not available from the total number of spikes across the whole response. It has been proposed that patterns of exactly timed (1 ms precision) spikes, such as repeating triplets or quadruplets, might carry information that is not available from knowing about spike count and rate modulation. A model using the spike count distribution, the low-pass filtered PSTH (bandwidth below 30 Hz), and, to a small degree, the interspike interval distribution predicts the numbers and types of exactly-timed triplets and quadruplets that are indistinguishable from those found in the data. From this it can be concluded that the coarse (< 30 Hz) sequential correlation structure over time gives rise to the exactly timed patterns present in the recorded spike trains. Because the coarse temporal structure predicts the fine temporal structure, the information carried by the fine temporal structure must be completely redundant with that carried by the coarse structure. Thus, the existence of precisely timed spike patterns carrying stimulus-related information does not imply control of spike timing at precise time scales.

Animals↗
PubMed2006

Single subject image analysis using the complex general linear model--an application to functional magnetic resonance imaging with multiple inputs.

A linear time invariant model is applied to functional fMRI blood flow data. Based on traditional time series analysis, this model assumes that the fMRI stochastic output sequence can be determined by a constant plus a linear filter (hemodynamic response function) of several fixed deterministic inputs and an error term assumed stationary with zero mean. The input function consists of multiple exponential distributed (time delay between images) visual stimuli consisting of negative and erotic images. No a priori assumptions are made about the hemodynamic response function that, in essence, is calculated at each spatial position from the data. The sampling rate for the experiment is 400 ms in order to allow for filtering out higher frequencies associated with the cardiac rate. Since the statistical analysis is carried out in the Fourier domain, temporal correlation problems associated with inference in the time domain are avoided. This formal model easily lends itself to further development based on previously developed statistical techniques.

Adult↗
PubMed1997

Extension of a numerical algorithm to proton dose calculations. I. Comparisons with Monte Carlo simulations.

A numerical algorithm originally developed for electron dose calculations [Med. Phys. 21, 1591 (1994)] has been modified for use with proton beams. The algorithm recursively propagates the proton distribution in energy, angle, and space from one level in an absorbing medium to another at slightly greater depth until all protons stop. Vavilov's theory is used to predict, at any point in the absorber, the broadening of the primary proton energy-spectrum. Moliere's theory is applied to describe the angular distribution, and it is shown that the Gaussian first term of Moliere's series expansion is of sufficient accuracy for dose calculations. These multiple scattering and energy loss distributions are sampled using equal probability spacing to optimize computational speed while maintaining calculational accuracy. Inelastic nuclear collisions along the proton trajectories are modeled by a simple exponential extinction. Predictions of the algorithm for absolute dose deposition by a 160 MeV initially monoenergetic proton beam are compared with the results of Monte Carlo simulations performed with the PTRAN code. The excellent level of agreement between the results of these two methods of dose calculation (< 5% dose and < 3 mm spatial deviations) demonstrate that dose deposition from proton beams may be computed to high accuracy using this algorithm without the need for extensive empirical measurement as input.

Algorithms↗
PubMed1994

The effect of impurities on the stress distribution at the tooth/denture base resin interface.

The most common cause of teeth detaching from the denture base is the presence of impurities along the tooth denture-base interface. However detachment after a period of wear of the denture probably occurs due to the presence of stress concentrations along the interface from which cracks may initiate. The presence of the impurity will inevitably lower the stress threshold for cracks to propagate thereby increasing the risk of tooth detachment. Although this is obvious, there is no evidence to show the effect of impurities on the stress distribution at this interface. This pilot study utilised the finite element method to examine stresses at the tooth-denture base interface in the presence of simulated impurities interspersed along different positions on the interface. Within the limitations of the two-dimensional model used, it was concluded that the presence of an impurity at the edge of the palatal aspect of the interface has a significant effect on the stress magnitude. However, the presence of impurities elsewhere along the interface had no significant effect on the stress magnitude. This would have important implications during the processing of dentures.

Acrylic Resins↗