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At least 577 records · Page 32Linked to original sources
PubMed2000

Statistical models for carbon-nitrogen film growth

We studied models of deposition and erosion, with two species of particles, that represent quantitatively many features of amorphous carbon-nitrogen film grown under plasma enhanced chemical vapor deposition. In the original model, the columns of the deposit are independent, and particles C and N are released with probabilities p and 1-p, respectively. An incident C particle always aggregates upon contact with the surface. An N particle annihilates with a top C particle with probability q and aggregates with probability 1-q. An N particle always annihilates with a top N. A critical line separates the regimes of growth (p>q/2) and erosion (p<q/2). For fixed q, when p decreases towards the critical value p(c)=q/2, the bulk concentration of N (x(N)) increases, and the growth rate r decreases. The rxx(N) curve for q=0.25 agrees with data from films grown in acetylene-nitrogen atmospheres. In order to represent the blocking of surface bonds by hydrogen atoms, we considered a second model in which any aggregation process is accepted with probability alpha, otherwise it is rejected. For q=0.25 and alpha=0.3, the rxx(N) curve agrees with data from films grown in methane-nitrogen and methane-ammonia atmospheres. The fitting values of q and alpha were inferred from related experiments. In order to test the influence of lattice structure and spatial correlations, we also studied those models in simple cubic lattices, considering that the aggregation must satisfy the restricted solid-on-solid model conditions for the difference of heights in neighboring columns, while the erosion is random. We obtained similar results for rxx(N) curves, confirming the validity of those models to represent the kinetics of amorphous films growth. It was also observed that the surface roughness increases with x(N), which agrees qualitatively with several experiments on carbon-nitrogen films growth with ion bombardment.

Journal Article↗
PubMed2005

Statistical model for intermittent plasma edge turbulence.

The probability distribution function of plasma density fluctuations at the edge of fusion devices is known to be skewed and strongly non-Gaussian. The causes of this peculiar behavior are, up to now, largely unexplored. On the other hand, understanding the origin and the properties of edge turbulence is a key issue in magnetic fusion research. In this paper we show that a stochastic fragmentation model, already successfully applied to fluid turbulence, is able to predict an asymmetric distribution that closely matches experimental data. The asymmetry is found to be a direct consequence of intermittency. A discussion of our results in terms of recently suggested Bramwell-Holdsworth-Pinton universal curve [S. T. Bramwell, P. C. W. Holdsworth, and J.-F. Pinton, Nature (London) 396, 552 (1998)], that should hold for strongly correlated and critical systems, is also proposed.

Journal Article↗
PubMed2001

Worm algorithms for classical statistical models.

We show that high-temperature expansions provide a basis for the novel approach to efficient Monte Carlo simulations. "Worm" algorithms utilize the idea of updating closed-path configurations (produced by high-temperature expansions) through the motion of end points of a disconnected path. An amazing result is that local, Metropolis-type schemes using this approach appear to have dynamical critical exponents close to zero (i.e., their efficiency is comparable to the best cluster methods) as proved by finite-size scaling of the autocorrelation time for various universality classes.

Journal Article↗
PubMed2006

Statistical modelling of the seasonality of preterm delivery and intrauterine growth restriction in rural Gambia.

We have developed a methodology for comparing the seasonal influences on two outcomes, when those influences may act cumulatively or instantaneously. We have used this to compare the seasonal pattern of intrauterine growth retardation (IUGR, as reflected by weight-for-gestational-age) and preterm delivery (as assessed by Dubowitz scoring) among 1718 infants born in rural Gambia. Both outcomes were analysed as binary variables: small-for-gestational-age (SGA, <10th centile of reference standard) and preterm (<37 weeks) respectively. Percentages of preterm and SGA babies show divergent seasonal patterns that might indicate separate aetiologies. However, seasonal effects influencing intrauterine growth are likely to be cumulative over the last few months of pregnancy. By modelling seasonality with truncated Fourier series we were able to deconvolve the underlying seasonal influences on fetal growth from the pattern for SGA. This enabled us to use seemingly unrelated biprobit regression to compare the underlying seasonal pattern of intrauterine growth with that governing the incidence of preterm delivery. We conclude that, if the seasonal factors affecting intrauterine growth operate over more than the last 2 months of pregnancy, then the seasonal patterns of the factors causing IUGR and preterm delivery are indistinguishable if the factors are assumed to trigger preterm delivery immediately, but differ if preterm delivery is assumed to be programmed by factors acting at conception.

Epidemiologic Methods↗
PubMed1986

Statistical modeling of animal bioassay data with variable dosing regimens: example--vinyl chloride.

We consider animal bioassay experiments with variable dosing regimens in which groups of animals are dosed beginning at different ages and for varying durations. Two response models are discussed and then applied to data from an experiment on vinyl chloride exposure of F-344 rats, B6C3F1 and Swiss CD-1 mice, and Syrian Golden hamsters. The multistage model of Armitage and Doll, as extended by Whittemore, Day and Brown, and Crump and Howe, is used to estimate the dose effect on the ordered stages of tumor development. The data for all endpoints and species/strains examined consistently indicate a predominant effect on the first stage, suggesting that vinyl chloride is primarily a tumor initiator. This is consistent with evidence from two-stage experiments on this chemical. The second response model, new to this article, adjusts for survival nonparametrically. It is used to test for an age difference in susceptibility, to evaluate alternative exposure durations, and to compare the effectiveness of alternative dosing regimens for detecting carcinogenicity.

Animals↗
PubMedDate not supplied

Abdominal versus vaginal hysterectomy: a statistical model for determining physician decision making and patient outcome.

To identify the effects of preoperative assessment and physician practice style on the outcomes of hysterectomy, the authors conducted a small-area analysis of 640 women under-going abdominal or vaginal hysterectomy in a St. Louis, Missouri, hospital. Of these patients, excluding outliers, 115 met the conditions for inclusion in the study. Hysterectomies were performed by the abdominal route in 55 (47.8%) and by the vaginal route in 60 (52.2%) of the 115 patients. A total of 29 physicians performed the hysterectomies. Of these 29, 15 (51.7%) were predisposed toward the abdominal approach, 13 (44.8%) had no appreciable predisposition, and one (3.5%) was predisposed toward the vaginal procedure. Path analysis revealed that physician decision making about the type of hysterectomy procedure performed was primarily influenced by practice style (predisposition) and variables related to physician preoperative assessments (uterine size and uterine mobility), some of which are prone to inaccuracy. Factors that traditionally determine operative approach (such as obesity) did not always act in the expected direction. Furthermore, the decision to perform hysterectomy vaginally had positive outcomes for both cost and length of hospital stay. Shorter hospital stays were associated with physician factors that included selection of the vaginal route, training site, predisposition toward the vaginal procedure, and preoperative assessment of uterine size. Length of hospital stay and duration of surgery were the strongest predictors of cost. Other factors being equal, the mean cost of a vaginal procedure is $224 less than that of an abdominal hysterectomy. Establishing the vaginal approach as the recommended procedure for this specific population should result in cost reductions and shorter hospital stays without negatively impacting quality of care.

Costs and Cost Analysis↗