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At least 631 records · Page 35Linked to original sources
PubMed2004

Comparison of two cluster sampling methods for health surveys in developing countries.

BACKGROUND: The Expanded Program for Immunization (EPI) random walk method has been widely used by the World Health Organization and others for rapid cluster sample surveys where an up-to-date household sampling frame is not available. However, it is not a probability sample, does not allow for population movement since the last census, and does not ensure objectivity in household selection or permit call-backs for non-response. Compact segment sampling avoids these problems and has been proposed as a slower but cleaner alternative. METHODS: We conducted two surveys, one using the EPI scheme and one using compact segment sampling, to estimate vaccination coverage in Western Region of The Gambia within 3 months of each other in 2000-2001. RESULTS: Point estimates for vaccination coverage from the two surveys rarely differed by more than 2%. Any differences were more likely to be due to household selection than to population movement. A simple mathematical model showed that even in extreme situations, ignoring population movement since the last census is unlikely to have any appreciable effect. Rates of homogeneity did not differ systematically between the surveys. CONCLUSIONS: In situations where quality of fieldwork can be guaranteed, the EPI random walk method can give accurate and precise results. However, compact segment sampling is generally to be preferred as it ensures objectivity in household selection and permits the estimation of population totals (such as those unvaccinated), which are helpful for planning service provision.

BCG Vaccine↗
PubMed2001

Trapping and survival probability in two dimensions.

We investigate the survival probability Phi(n,c) of particles performing a random walk on a two-dimensional lattice that contains static traps, which are randomly distributed with a concentration c, as a function of the number of steps n. Phi(n,c) is analyzed in terms of a scaling ansatz, which allows us to locate quantitatively the crossover between the Rosenstock approximation (valid only at early times) and the asymptotic Donsker-Varadhan behavior (valid only at long times). While the existence of the crossover has been postulated before, its exact location has not been known. Our scaling hypothesis is based on the mean value of the quantity S(n), the number of sites visited in an n-step walk. We make use of the idea of self-interacting random walks, and a "slithering" snake algorithm, available in the literature, and we are thus able to obtain accurate survival probability data indirectly by Monte Carlo simulation techniques. The crossover can now be determined by our method, and it is found to depend on a combination of c and n. It occurs at small Phi(n,c) values, which is typically the case for large values of n.

Journal Article↗
PubMed2001

Micron-scale positioning of features influences the rate of polymorphonuclear leukocyte migration.

Microfabrication technology was used to create regular arrays of micron-size holes (2 microm x 2 microm x 210 nm) on fused quartz and photosensitive polyimide surfaces. The patterned surfaces, which possessed a basic structural element of a three-dimensional (3-D) network (i.e., spatially separated mechanical edges), were used as a model system for studying the effect of substrate microgeometry on neutrophil migration. The edge-to-edge spacing between features was systematically varied from 6 microm to 14 microm with an increment of 2 microm. In addition, collagen was used to coat the patterned quartz surfaces in an attempt to change the adhesive properties of the surfaces. A radial flow detachment assay revealed that cell adhesion was the strongest on the quartz surface (approximately 50% cell attached), whereas it was relatively weaker on polyimide and collagen-coated quartz (approximately 25% cell attached). Cell adhesion to each substrate was not affected either by the presence of holes or by the spacing between holes. A direct visualization assay showed that neutrophil migration on each patterned surface could be characterized as a persistent random walk; the dependence of the random motility coefficient (mu) as a function of spacing was biphasic with the optimal spacing at approximately 10 microm on each substrate. The presence of evenly distributed holes at the optimal spacing of 10 microm enhanced mu by a factor of 2 on polyimide, a factor of 2.5 on collagen-coated quartz, and a factor of 10 on uncoated quartz. The biphasic dependence on the mechanical edges of neutrophil migration on 2-D patterned substrate was strikingly similar to that previously observed during neutrophil migration within 3-D networks, suggesting that microfabricated materials provide relevant models of 3-D structures with precisely defined physical characteristics. In addition, our results demonstrate that the microgeometry of a substrate, when considered separately from adhesion, can play a significant role in cell migration.

Cell Adhesion↗
PubMed1999

Dissociated cells of the calcareous sponge clathrina: a model for investigating cell adhesion and cell motility in vitro.

The study of cell-cell and cell-substratum adhesion in vitro is useful for understanding cell behavior in a three-dimensional pattern. We have used dissociated cells (choanocytes represent the main fraction) from the calcareous sponge Clathrina, namely C. cerebrum and C. clathrus, to illustrate our present understanding on three main aspects of cell-cell and cell-substratum adhesion in vitro: (1) cytoskeletal protrusions; (2) cell behaviours on organic substrata; and (3) paths of locomotory sponge cell. Cell locomotion occurs by the extensions of scleropodial and lamellipodial protrusions, by way of actin polymerization. The extent to which cells produce these cytoplasmic processes varies according to the substratum (e.g., collagen, fibronectin, laminin, polylysine). It was found that more cell extensions were produced on collagen substrata, and this led to greater cell movement. Advancing choanocytes are not polarized. Their paths are particularly complicated, showing linear segments, which produce a more efficent cellular translocation, and winding tracts with frequent turns or loops. Small amoeboid cells describe more linear paths with a wide range of speed variation than larger cells. The presence of cell-derived substratum reduces the progressive dispersion of cells and allows cells to encounter one another in such a way that the initial random walking later turns into non-random displacement. Even though cAMP-treated cells exhibit different aggregative tactics, cAMP 10(-8) M remarkably enhances cell encounters and supports the existing information that this cyclic nucleotide represents a signal that affects cell morphology and locomotion. The bulk of data on sponge cell-cell and cell-substratum adhesion has been evaluated by mentioning the significant advances and references concerning studies of other cell systems.

Animals↗
PubMed2002

Aggregation kinetics of well and poorly differentiated human prostate cancer cells.

Aggregation of attachment-dependent animal cells represents a series of motility, collision, and adhesion events applicable to such diverse fields as tissue engineering, bioseparations, and drug testing. Aggregation of human prostate cancer cells in liquid-overlay culture was modeled using Smoluchowski's collision theory. Using well (LNCaP) and poorly differentiated (DU 145 and PC 3) cell lines, the biological relevance of the model was assessed by comparing aggregation rates with diffusive and adhesive properties. Diffusion coefficients ranged from 5 to 90 microm(2)/min for single LNCaP and PC 3 cells, respectively. Similar diffusivities were predicted by the persistent random walk model and Einstein relation, indicating random motion. LNCaP cells were the most adhesive in our study with reduced cell shedding, 100% adhesion probability, and enhanced expression of E-cadherin. There was an increase in DU 145 cells staining positive for E-cadherin from nearly 20% of single cells to uniform staining across the surface of all aggregates; under 30% of PC 3 aggregates stained positive. Aggregation rates were more consistent with adhesive properties than with motilities, suggesting that aggregation in our study was reaction-controlled. Relative to other assays employed here, aggregation rates were more sensitive to phenotypic differences in cell lines and described size-dependent changes in aggregation at a finer resolution. In particular, model results suggest similar aggregation rates for two-dimensional DU 145 and PC 3 aggregates and upwards of 4-fold higher rates for larger three-dimensional DU 145 spheroids, consistent with expression of E-cadherin. The kinetic model has application to spheroid production, to cell flocculation and as an adhesion assay.

Cell Adhesion↗
PubMed2004

Reappraisal of four different approaches for finding the mean reaction time in the multi-trap variant of the Adam-Delbrück problem.

Adam and Delbrück argued that the dimensionality of the diffusion space determines the average lifetime of a diffusing particle confined to a region with a central trap. Doubts have often been aired as to whether their calculation is relevant to real biological systems, where the number of traps is usually much larger than unity; or whether the rate enhancement is merely a manifestation of an increase in the concentration of the traps; or whether the diverse multi-trap versions of their expression for the mean lifetime in two dimensions are trustworthy. These issues are addressed, and the long-standing problem of finding the low-density limit of trapping time in two dimensions solved, by examining previous treatments of the problem, and by carrying out simulations of two-dimensional systems in which the particles undergo a Pearsonian random walk, and the traps are distributed randomly or on a square lattice. The mean lifetimes are found to be different in the two situations, and it is concluded that the neglect of this aspect lies at the root of the conflict between some of the existing expressions for the mean lifetime. Relations expressing the mean lifetime as a function of the concentration of the traps are presented together with a discussion of their applicability.

Journal Article↗
PubMed2002

Persistence of a continuous stochastic process with discrete-time sampling: non-Markov processes.

We consider the problem of "discrete-time persistence," which deals with the zero crossings of a continuous stochastic process X(T) measured at discrete times T=nDeltaT. For a Gaussian stationary process the persistence (no crossing) probability decays as exp(-theta(D)T)=[rho(a)](n) for large n, where a=exp(-DeltaT/2) and the discrete persistence exponent theta(D) is given by theta(D)=(ln rho)/(2 ln a). Using the "independent interval approximation," we show how theta(D) varies with DeltaT for small DeltaT and conclude that experimental measurements of persistence for smooth processes, such as diffusion, are less sensitive to the effects of discrete sampling than measurements of a randomly accelerated particle or random walker. We extend the matrix method developed by us previously [Phys. Rev. E 64, 015101(R) (2001)] to determine rho(a) for a two-dimensional random walk and the one-dimensional random-acceleration problem. We also consider "alternating persistence," which corresponds to a<0, and calculate rho(a) for this case.

Journal Article↗
PubMed2001

Peculiar scaling of self-avoiding walk contacts.

The nearest neighbor contacts between the two halves of an N-site lattice self-avoiding walk offer an unusual example of scaling random geometry: for N-->infinity they are strictly finite in number but their radius of gyration R(c) is power law distributed proportional to R(-tau)(c), where tau>1 is a novel exponent characterizing universal behavior. A continuum of diverging length scales is associated with the R(c) distribution. A possibly superuniversal tau = 2 is also expected for the contacts of a self-avoiding or random walk with a confining wall.

Journal Article↗
PubMed2025

DyNDG: Identifying Leukemia-related Genes Based on Time-series Dynamic Network by Integrating Differential Genes.

Leukemia is a malignant disease characterized by progressive accumulation with high morbidity and mortality rates, and investigating its disease genes is crucial for understanding its etiology and pathogenesis. Network propagation methods have emerged and been widely employed in disease gene prediction, but most of them focus on static biological networks, which hinders their applicability and effectiveness in the study of progressive diseases. Moreover, there is currently a lack of special algorithms for the identification of leukemia disease genes. Here, we proposed a novel Dynamic Network-based model integrating Differentially expressed Genes (DyNDG) to identify leukemia-related genes. Initially, we constructed a time-series dynamic network to model the development trajectory of leukemia. Then, we built a background-temporal multilayer network by integrating both the dynamic network and the static background network, which was initialized with differentially expressed genes at each stage. To quantify the associations between genes and leukemia, we extended a random walk process to the background-temporal multilayer network. The results demonstrate that DyNDG achieves superior accuracy compared to several state-of-the-art methods. Moreover, after excluding housekeeping genes, DyNDG yields a set of promising candidate genes associated with leukemia progression or potential biomarkers, indicating the value of dynamic network information in identifying leukemia-related genes. The implementation of DyNDG is available at both https://ngdc.cncb.ac.cn/biocode/tool/BT7617 and https://github.com/CSUBioGroup/DyNDG.

Leukemia↗
PubMed2001

Partial and random covering times in one dimension.

We study the recently introduced random walk problems of partial covering time (PCT) and random covering time (RCT). We generalize the concept of first-passage time to a given set of m sites by considering the probability of visiting all m sites for the first time on the tth step. For the one-dimensional case we derive an explicit result for the mean time needed to visit m sites for the first time. Using this result we are able to solve the PCT and RCT problems exactly in one dimension.

Journal Article↗
PubMed2003

Bilinear diffusion quantum Monte Carlo methods.

The standard method of quantum Monte Carlo for the solution of the Schrödinger equation in configuration space can be described quite generally as devising a random walk that generates-at least asymptotically-populations of random walkers whose probability density is proportional to the wave function of the system being studied. While, in principle, the energy eigenvalue of the Hamiltonian can be calculated with high accuracy, estimators of operators that do not commute the Hamiltonian cannot. Bilinear quantum Monte Carlo (BQMC) is an alternative in which the square of the wave function is sampled in a somewhat indirect way. More specifically, one uses a pair of walkers at positions x and y and introduces stochastic dynamics to sample phi(i)(x)t(x,y)phi(j)(y), where phi(i)(x) and phi(j)(y) are eigenfunctions of (possibly different) Hamiltonians, and t(x,y) is a kernel that correlates positions x and y. Using different Hamiltonians permits the accurate computation of small energy differences. We review the conceptual basis of BQMC, discuss qualitatively and analytically the problem of the fluctuations in the branching, and present partial solutions to that problem. Finally we exhibit numerical results for some model systems including harmonic oscillators and the hydrogen and helium atoms. Further research will be necessary to make this a practical and generally applicable scheme.

Journal Article↗
PubMed2004

Effect of hydration status on thirst, drinking, and related hormonal responses during low-intensity exercise in the heat.

During exercise-heat stress, ad libitum drinking frequently fails to match sweat output, resulting in deleterious changes in hormonal, circulatory, thermoregulatory, and psychological status. This condition, known as voluntary dehydration, is largely based on perceived thirst. To examine the role of preexercise dehydration on thirst and drinking during exercise-heat stress, 10 healthy men (21 +/- 1 yr, 57 +/- 1 ml x kg(-1) x min(-1) maximal aerobic power) performed four randomized walking trials (90 min, 5.6 km/h, 5% grade) in the heat (33 degrees C, 56% relative humidity). Trials differed in preexercise hydration status [euhydrated (Eu) or hypohydrated to -3.8 +/- 0.2% baseline body weight (Hy)] and water intake during exercise [no water (NW) or water ad libitum (W)]. Blood samples taken preexercise and immediately postexercise were analyzed for hematocrit, hemoglobin, serum aldosterone, plasma osmolality (P(osm)), plasma vasopressin (P(AVP)), and plasma renin activity (PRA). Thirst was evaluated at similar times using a subjective nine-point scale. Subjects were thirstier before (6.65 +/- 0.65) and drank more during Hy+W (1.65 +/- 0.18 liters) than Eu+W (1.59 +/- 0.41 and 0.31 +/- 0.11 liters, respectively). Postexercise measures of P(osm) and P(AVP) were significantly greater during Hy+NW and plasma volume lower [Hy+NW = -5.5 +/- 1.4% vs. Hy+W = +1.0 +/- 2.5% (P = 0.059), Eu+NW = -0.7 +/- 0.6% (P < 0.05), Eu+W = +0.5 +/- 1.6% (P < 0.05)] than all other trials. Except for thirst and drinking, however, no Hy+W values differed from Eu+NW or Eu+W values. In conclusion, dehydration preceding low-intensity exercise in the heat magnifies thirst-driven drinking during exercise-heat stress. Such changes result in similar fluid regulatory hormonal responses and comparable modifications in plasma volume regardless of preexercise hydration state.

Adult↗
PubMed2001

"Generalized des Cloizeaux" exponent for self-avoiding walks on the incipient percolation cluster.

We study the asymptotic shape of self-avoiding random walks (SAW) on the backbone of the incipient percolation cluster in d-dimensional lattices analytically. It is generally accepted that the configurational averaged probability distribution function for the end-to-end distance r of an N step SAW behaves as a power law for r-->0. In this work, we determine the corresponding exponent using scaling arguments, and show that our suggested "generalized des Cloizeaux" expression for the exponent is in excellent agreement with exact enumeration results in two and three dimensions.

Journal Article↗
PubMed2003

Escaping from cycles through a glass transition.

A random walk is performed over a disordered media composed of N sites random and uniformly distributed inside a d-dimensional hypercube. The walker cannot remain in the same site and hops to one of its n neighboring sites with a transition probability that depends on the distance D between sites according to a cost function E(D). The stochasticity level is parametrized by a formal temperature T. In the case T=0, the walk is deterministic and ergodicity is broken: the phase space is divided in a O(N) number of attractor basins of two-cycles that trap the walker. For d=1, analytic results indicate the existence of a glass transition at T(1)=1/2 as N--> infinity. Below T1, the average trapping time in two-cycles diverges and an out-of-equilibrium behavior appears. Similar glass transitions occur in higher dimensions when the right cost function is chosen. We also present some results for the statistics of distances for Poisson spatial point processes.

Journal Article↗