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Results for “Statistical Distributions”

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At least 1,171 records · Page 65Linked to original sources
PubMed2004

Statistical monitoring of weak spots for improvement of normalization and ratio estimates in microarrays.

BACKGROUND: Several aspects of microarray data analysis are dependent on identification of genes expressed at or near the limits of detection. For example, regression-based normalization methods rely on the premise that most genes in compared samples are expressed at similar levels and therefore require accurate identification of nonexpressed genes (additive noise) so that they can be excluded from the normalization procedure. Moreover, key regulatory genes can maintain stringent control of a given response at low expression levels. If arbitrary cutoffs are used for distinguishing expressed from nonexpressed genes, some of these key regulatory genes may be unnecessarily excluded from the analysis. Unfortunately, no accurate method for differentiating additive noise from genes expressed at low levels is currently available. RESULTS: We developed a multistep procedure for analysis of mRNA expression data that robustly identifies the additive noise in a microarray experiment. This analysis is predicated on the fact that additive noise signals can be accurately identified by both distribution and statistical analysis. CONCLUSIONS: Identification of additive noise in this manner allows exclusion of noncorrelated weak signals from regression-based normalization of compared profiles thus maximizing the accuracy of these methods. Moreover, genes expressed at very low levels can be clearly identified due to the fact that their expression distribution is stable and distinguishable from the random pattern of additive noise.

Animals↗
PubMed2003

A macroscopic approach to demography.

A canonical/lognormal model for human demography is established, specifying the net maternity function and the age distribution for mothers of new-borns using a single macroscopic parameter vector of dimension five. The age distribution of mothers is canonical, while the net maternity function normalizes to a lognormal density. Comparison of an actual population with the model serves to identify anomalies in the population which may be indicative of phase transitions or influences from levels outside the demographic. Tracking the time development of the parameter vector may be used to predict the future state of a population, or to interpolate for data missing from the record. In accordance with classical theoretical considerations of Backman, Prigogine, et al., it emerges that the logarithm of a mother's age is the most fundamental time variable for demographic purposes.

Adolescent↗
PubMed2006

Hemorrhage causes interalveolar perfusion maldistribution in the lungs of anesthetized rats.

BACKGROUND: Lung injury often occurs following hemorrhage and we hypothesized that this might be due to the effects of hemorrhage on perfusion distribution among alveoli. To test this, we measured interalveolar perfusion distribution in anesthetized, spontaneously breathing rats subjected to blood losses of 0%, 10%, 20%, or 30% of calculated blood volume. METHODS: We measured interalveolar perfusion distribution by analyzing trapping patterns of 4-mum diameter fluorescent latex particles infused into the pulmonary circulation. The particles (2 x 10) were infused 1 hour after each animal had been bled, and the lungs were then removed and air-dried. Using a confocal fluorescence microscope, we collected images of the particles in eight sections of each lung. Each image encompassed 3,360 x 3,360 x 100 microm (approximately 5,000 alveoli), and included 3-4,000 particles. Particle distributions in the images were measured using the method of dispersion index (DI) analysis. A DI value of zero corresponds to a statistically random distribution; the more DI exceeds zero, the more the distribution is clustered or inhomogenous. RESULTS: The largest DI values for the four groups were: 0%, 0.69 +/- 0.41; 10%, 0.57 +/- 0.58; 20%, 0.72 +/- 0.34; 30%, 1.38 +/- 0.41. The 30% blood loss group had a max DI value approximately twofold greater than those of the other three (p < 0.0001). CONCLUSIONS: Our results suggest that interalveolar perfusion distribution becomes markedly maldistributed at blood losses of 30%. This contributes to ventilation-perfusion mismatching, and may be a precipitating event for lung injury following hemorrhage.

Anesthesia↗
PubMed2004

[Analysis of the life span distribution mode in 128 successive generations of D. melanogaster].

A large body of experimental data consisting of 116 samples (sets) of individual life span (LS) values of D. melanogaster from the same laboratory strain Canton-S was analyzed. In total, 10180 Drosophila flies (5100 females and 5080 males) were studied. Each of 58 pairs of samples belonged to a definite generation in a continuous succession where every next generation was an offspring of the preceding one. Mathematical simulation made it possible to demonstrate that both the normal (Gaussian) and Gompertz distributions were equally good approximations of the experimental data. Both of them adequately described the LS distributions in laboratory populations of D. melanogaster. The confidence intervals for absolute deviations of the theoretical distributions from experimental ones were small (4-5%). In other words, the approximation error was no more than 5% in either case. The estimation of the dependence of approximation quality on the LS in the original (experimental) population showed that the normal distribution was preferable, because, in this case the absolute deviation from the experimental distribution was independent of the LS in the original population.

Age Distribution↗
PubMed2006

Building statistical models to analyze species distributions.

Models of the geographic distributions of species have wide application in ecology. But the nonspatial, single-level, regression models that ecologists have often employed do not deal with problems of irregular sampling intensity or spatial dependence, and do not adequately quantify uncertainty. We show here how to build statistical models that can handle these features of spatial prediction and provide richer, more powerful inference about species niche relations, distributions, and the effects of human disturbance. We begin with a familiar generalized linear model and build in additional features, including spatial random effects and hierarchical levels. Since these models are fully specified statistical models, we show that it is possible to add complexity without sacrificing interpretability. This step-by-step approach, together with attached code that implements a simple, spatially explicit, regression model, is structured to facilitate self-teaching. All models are developed in a Bayesian framework. We assess the performance of the models by using them to predict the distributions of two plant species (Proteaceae) from South Africa's Cape Floristic Region. We demonstrate that making distribution models spatially explicit can be essential for accurately characterizing the environmental response of species, predicting their probability of occurrence, and assessing uncertainty in the model results. Adding hierarchical levels to the models has further advantages in allowing human transformation of the landscape to be taken into account, as well as additional features of the sampling process.

Bayes Theorem↗
PubMed1999

A statistical method for correcting distortions of amplitude distribution histograms due to collisions of synaptic events.

The waveform of spontaneous synaptic potentials or currents comprising synaptic noise can be significantly distorted when these events are closely spaced due to a high frequency activity in the presynaptic inputs that generate them. It is essential to correct these alterations prior to measurements of overlapping miniature and/or postsynaptic potentials, in order to provide reliable information about their true amplitude distributions, and to avoid spurious peaks in the resulting histograms. In this paper we describe a statistical method for making these corrections, its range of application, and its theoretical background. Its use becomes necessary when the frequency of events is of the order of 8-50 Hz, depending upon their time to peak, which ranges from 6 to 1 ms in most synaptic potentials recorded in the central nervous system.

Action Potentials↗
PubMed1995

Radical prostatectomy versus expectant treatment for early carcinoma of the prostate. Twenty-three year follow-up of a prospective randomized study.

In a study by the Veterans Administration Cooperative Urological Research Group (VACURG), 142 patients with localized prostate cancer, VACURG stage I and II, were randomized between radical prostatectomy plus placebo versus placebo alone as initial treatment. 111 patients were evaluable for treatment comparison. Median follow-up for survival is 23 years. The prognostic value of Gleason histologic grading was confirmed. A difference in overall survival in favor of radical prostatectomy was observed in stage I patients. However, after adjustment for imbalance in age distribution, no statistically significant differences in survival could be demonstrated in either stage or in both stages combined. The results are discussed considering the small sample size and the limited statistical power of the study.

Adenocarcinoma↗