Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Statistical Distributions”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

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

At least 1,387 records · Page 77Linked to original sources
PubMed2001

A general family of distributions for longitudinal dependence with special reference to event histories.

Event histories play an increasingly important role in medical studies. Examples include times between recurrences of tumours, as with bladder cancer, and between repeated infections, as with chronic granulotomous disease. A general method for generating new distributions is proposed by introducing an intensity function into a density. This procedure yields, as special cases, several distributions already proposed in the literature. The families of distributions based on the Pareto distribution are of particular interest for event history analysis because of their relationship to the Laplace transform of a gamma distribution. They can yield multivariate distributions, with longitudinal (serial) dependence by a procedure similar to updating in the Kalman filter and with uniform dependence in a similar way to copulas. For longitudinal dependence, several such updating procedures are proposed.

Antineoplastic Agents, Alkylating↗
PubMed2005

Characterization of degree frequency distribution in protein interaction networks.

In this work, we analyze the degree frequency distribution in the yeast protein interaction network by studying a previously proposed duplication network model. This model correctly predicts the observed degree distribution (a power law for large degree values and a departure from this behavior for small degree). We numerically and analytically characterize this distribution as a mixture of random and power-law behavior, and make a comparative study of the robustness of the network model against realistic perturbations. We conclude that the particular distribution observed in both the model and the experimental data has many advantages in terms of dynamical and topological robustness and could have emerged in the evolutionary history as a sort of compromise between purely deterministic and random underlying mechanisms of network growth.

Computer Simulation↗
PubMed2006

Some remarks on the indoor radon distribution in a country.

From the point of view of indoor radon activity concentrations, a country is characterized correctly by two parameters (GM and GSD) of a lognormal distribution if and only if hypothesis tests do not reject the distribution of the local radon activity concentrations as a lognormal one. The authors introduce a way to obtain an empirical distribution using a stratum system covering almost the whole country and in each stratum the lognormal model is acceptable according to hypothesis tests. The sample for the analysis is provided by the measurement of radon level in 15,619 Hungarian homes.

Air Pollutants, Radioactive↗
PubMed2005

Species lifetime distribution for simple models of ecologies.

Interpretation of empirical results based on a taxa's lifetime distribution shows apparently conflicting results. Species' lifetime is reported to be exponentially distributed, whereas higher-order taxa, such as families or genera, follow a broader distribution, compatible with power-law decay. We show that both forms of evidence are consistent with a simple evolutionary model that does not require specific assumptions on species interaction. The model provides a zero-order description of the dynamics of ecological communities, and its species lifetime distribution can be computed exactly. Different behaviors are found as follows: an initial t(-3/2) power law, emerging from a random walk type of dynamics, which crosses over to a steeper t(-2) branching process-like regime and finally is cut off by an exponential decay that becomes weaker and weaker as the total population increases. Sampling effects also can be taken into account and shown to be relevant. If species in the fossil record were sampled according to the Fisher log-series distribution, lifetime should be distributed according to a t(-1) power law. Such variability of behaviors in a simple model, combined with the scarcity of data available, casts serious doubt on the possibility of validating theories of evolution on the basis of species lifetime data.

Animals↗
PubMed1999

Testing the robustness of the likelihood-ratio test in a variance-component quantitative-trait loci-mapping procedure.

Detection of linkage to genes for quantitative traits remains a challenging task. Recently, variance components (VC) techniques have emerged as among the more powerful of available methods. As often implemented, such techniques require assumptions about the phenotypic distribution. Usually, multivariate normality is assumed. However, several factors may lead to markedly nonnormal phenotypic data, including (a) the presence of a major gene (not necessarily linked to the markers under study), (b) some types of gene x environment interaction, (c) use of a dichotomous phenotype (i.e., affected vs. unaffected), (d) nonnormality of the population within-genotype (residual) distribution, and (e) selective (extreme) sampling. Using simulation, we have investigated, for sib-pair studies, the robustness of the likelihood-ratio test for a VC quantitative-trait locus-detection procedure to violations of normality that are due to these factors. Results showed (a) that some types of nonnormality, such as leptokurtosis, produced type I error rates in excess of the nominal, or alpha, levels whereas others did not; and (b) that the degree of type I error-rate inflation appears to be directly related to the residual sibling correlation. Potential solutions to this problem are discussed. Investigators contemplating use of this VC procedure are encouraged to provide evidence that their trait data are normally distributed, to employ a procedure that allows for nonnormal data, or to consider implementation of permutation tests.

Analysis of Variance↗
PubMed2006

To permute or not to permute.

UNLABELLED: Permutation test is a popular technique for testing a hypothesis of no effect, when the distribution of the test statistic is unknown. To test the equality of two means, a permutation test might use a test statistic which is the difference of the two sample means in the univariate case. In the multivariate case, it might use a test statistic which is the maximum of the univariate test statistics. A permutation test then estimates the null distribution of the test statistic by permuting the observations between the two samples. We will show that, for such tests, if the two distributions are not identical (as for example when they have unequal variances, correlations or skewness), then a permutation test for equality of means based on difference of sample means can have an inflated Type I error rate even when the means are equal. Our results illustrate permutation testing should be confined to testing for non-identical distributions. CONTACT: calian@raunvis.hi.is.

Computer Simulation↗
PubMed2003

Kinetic model for the inactivation of Lactobacillus plantarum by pulsed electric fields.

The kinetics of Lactobacillus plantarum inactivation by pulsed electric fields (PEF) was studied in two different growth stages (exponential and stationary), but in the same reference medium (0.6% peptone water). Electric field intensity and treatment time varied from 20 to 28 kV/cm and 30 to 240 micros, respectively. The experimental data showed that cells in the exponential growth stage were more sensitive to PEF treatment than those in the stationary stage. The inactivation data were adjusted to the Bigelow and Hülsheger models and the Weibull frequency distribution function, and constants were calculated for both growth stages in each model. The models were tested and their accuracy was assessed by using the Accuracy Factor. According to this parameter, the Weibull frequency distribution function gave better fittings for the inactivation by PEF than Bigelow or Hülsheger models. On the other hand, the Bigelow model gave a good accuracy factor and is simpler.

Electric Stimulation↗
PubMed1995

Prevalence of dental fluorosis in Sudanese children from two villages with 0.25 and 2.56 ppm fluoride in the drinking water.

Dental fluorosis is endemic in Eastern Africa and a high prevalence has been found even in low-fluoride (approximately 0.5 ppm) areas. Substantial seasonal changes in the fluoride content of water have also been reported. The aim of the present study was to ascertain, through one year, the fluoride concentration in water from two underground reservoirs in Sudan, in Treit el Biga (TeB) and Abu Groon (AG), and to assess dental fluorosis in children aged 7-16 years who had been lifelong residents. The water in one of the reservoirs (TeB) was shown to have a low, very stable fluoride concentration (0.25 ppm SD 0.04) whereas the other (AG) had a tenfold higher, and slightly varying, fluoride concentration (2.56 ppm SD 0.26). Dental fluorosis was recorded on maxillary central incisors according to Dean's index. In TeB 91% of the children showed signs of dental fluorosis whereas in AG all children had fluorotic teeth. The Community Fluorosis Index in TeB and AG was 1.40 and 2.44, respectively. There was a significantly higher degree of fluorosis in boys than in girls in the low-fluoride area. In TeB, older boys tended to have more fluorosis than younger boys; the difference, however, was not statistically significant. No significant sex or age differences in fluorosis were found in AG. In both villages great inter-individual variations in dental fluorosis were recorded. The prevalence and severity of dental fluorosis in TeB was higher than that previously reported in areas with similar fluoride concentrations in the drinking water.

Adolescent↗
PubMed2004

Bone mineral density in patients on maintenance hemodialysis and effect of chronic hepatitis C virus infection.

OBJECTIVE: To determine the prevalence of osteopenia and osteoporosis in HD patients at our center; to investigate whether HCV infection affects BMD in hemodialysis patients; to test for correlations between bone mineral density (BMD) and clinical and laboratory parameters in this population. SUBJECTS AND METHODS: The study involved 76 end-stage renal disease patients. Forty-three (56.6%) patients were tested negative for anti-HCV antibodies and HCV-RNA. Thirty-three (43.4%) of them had positivity of anti-HCV antibodies and permanent or intermittent HCV-RNA positivity at least for two years. Mean HD duration was 86.4 months. Patients completed a standard questionnaire that listed age, sex, occupation, education level; cause of renal failure, smoking history, dialysis duration, and sports activities engaged in during life, and pathologic bone fractures. The women answered additional items about age at menarche, number of pregnancies and menopausal status. Each subject underwent a baseline physical examination, including measurement of body weight and height for calculation of body mass index. The results of laboratory tests that had been done at monthly visits in the previous year were retrospectively evaluated, and mean levels for the year were used for correlation testing. Bone mineral density was measured in the spine, femoral neck and forearm. Relationships between BMD values and chronic HCV infection, laboratory results and clinical parameters were analyzed. RESULTS: In the 43 patients who were negative for anti-HCV antibodies and HCV-RNA, spine BMD testing showed osteopenia in 16 (37.2%) cases and osteoporosis in 7 (16.3%) cases. The corresponding values for the neck of the femur were 14 (32.6%) and 6 (14.0%), and for the forearm were 19 (44.2%) and 15 (34.9%). In the 33 anti-HCV antibodies and HCV-RNA positive patients; spine BMD testing showed osteopenia in 10 (30.3%) cases and osteoporosis in 7 (21.2%) cases. The corresponding values for the neck of the femur were 17 (51.5%) and 4 (12.1%), and for the forearm were 4 (12.1%) and 25 (75.8%). Bone mineral density decreased as dialysis duration increased (p<0.05). There was no statistical difference between BMD measurements of chronic HCV infection positive and negative group. CONCLUSION: However the mean BMD values for all three sites in the 76 HD patients were low HCV infection may not be a risk factor for low BMD in this population.

Absorptiometry, Photon↗
PubMed2003

Hyperlactatemia in human immunodeficiency virus-infected children receiving antiretroviral treatment.

BACKGROUND: Hyperlactatemia and lactic acidosis occur in HIV-infected adults receiving antiretroviral treatment. Our objective was to determine the incidence, course and risk factors for hyperlactatemia in our HIV-infected pediatric patients. DESIGN: A prospective observational study of venous lactate concentrations during a 28-month period in 80 HIV-infected children, most of whom were receiving antiretrovirals. METHODS: Venous blood lactate concentrations were measured every 6 months under optimal sample-obtaining conditions. Alanine values from the same blood sample were performed when lactate concentrations were elevated. Hyperalaninemia is observed only when mitochondrial oxidative phosphorylation is chronically disturbed. RESULTS: Twenty-three patients (29%) were identified with hyperlactatemia, in 9 of the cases with normal alaninemia, probably caused by difficult venous punctures. The other 14 children (17%) had pathologic alanine concentrations with a mean lactate peak of 2.67 mmol/l (range, 2.05 to 4.9 mmol/l); none of them showed metabolic acidosis, and they were all symptom-free. Treatment was continued in all cases, and lactate has progressed spontaneously to normal values in 5 patients. CONCLUSIONS: Symptom-free hyperlactatemia was observed in HIV-infected children receiving nucleoside analog reverse transcriptase inhibitors. In our study, only a younger age at the beginning of antiretroviral treatment was a statistically significant risk factor for hyperlactatemia. Random measurements of blood lactate concentrations should be included in the clinical follow-up of those HIV-infected children <3 years of age who are treated with nucleoside analog reverse transcriptase inhibitors, symptomatic or not.

Acidosis, Lactic↗
PubMed2003

Distribution of plasma membrane Ca2+ pump activity in normal human red blood cells.

The plasma membrane calcium pump (PMCA) is the only active Ca2+ transporter in human red blood cells (RBCs). Previous measurements of maximal Ca2+ extrusion rates (Vmax) reported only mean values in the RBC population. Despite early evidence for differences in Ca2+ extrusion capacity among RBCs, the precise Vmax distribution remained unknown. It was important to characterize this distribution to assess the range and modality (uni- or multimodal) of PMCA Vmax variation and the likelihood of RBCs with elevated [Ca2+]i in the circulation participating in physiologic and pathologic processes. We report here the application of a new method to investigate the detailed distribution of PMCA Vmax activity in RBCs. The migrating profile of osmotic lysis curves was used to identify and quantify the fraction of cells that extrude a uniform Ca2+ load at different rates. The results revealed that RBCs from single donors have large variations in PMCA activity that follow a unimodal, broad distribution pattern consistently skewed toward higher Vmax values, suggesting an excess of cells with Vmax higher than the mean value. The method applied may provide a way of evaluating whether the observed variation in PMCA Vmax is related to cell age.

Calcium-Transporting ATPases↗
PubMed2006

International intercomparison of retrospective luminescence dosimetry method: sampling and distribution of the brick samples from Dolon' village, Kazakhstan.

There are still many differences in dose estimates depending on the applied methods of retrospective dosimetry in the areas, which were affected by nuclear tests at the Semipalatinsk nuclear test site (SNTS). In order to provide more correct estimation of radiation doses to population in proximity to the SNTS an International Intercomparison of Retrospective Luminescence Dosimetry (RLD) method had been proposed. It was suggested there be a comparison of the dose estimates for the brick samples from the buildings in the settlement, suffered following nuclear tests at the SNTS. With this purpose, during the September-October 2002 field mission, the team of specialists from Kazakhstan and Japan had collected four whole bricks for RLD International Intercomparison. Three buildings were selected as sampling locations in Dolon' village (Kazakhstan). The slices from these bricks were distributed between six laboratories in Finland, Germany, Japan, Russia, UK and USA for independent estimations by the RLD method of the accumulated dose of external irradiation. The descriptions of sampling, locations, data on geographical coordinates, dates of building construction, mode of cutting of samples for distribution, labeling, condition of transportation, order of the distribution of samples and information concerning 137Cs and 239+240Pu soil contamination density in the village and near sampling locations are presented in the paper.

Air Pollution, Indoor↗
PubMed2004

Partitioning of nongaussian-distributed biochemical reference data into subgroups.

BACKGROUND: The aim of this study was to develop new methods for partitioning biochemical reference data, covering in particular nongaussian distributions. METHODS: We recently proposed partitioning criteria for gaussian distributions. These criteria relate to proportions of the subgroups outside each of the reference limits of the combined distribution (proportion criteria) and to distances between the subgroup distributions as correlates of these proportions (distance criteria). However, distance criteria do not seem to be ideal for nongaussian distributions because a generally valid relationship between proportions and distances cannot be established for these. RESULTS: Proportion criteria appear preferable to distance criteria for two additional reasons: (a) The prevalences of the subgroup populations may have a considerable effect on stratification, but these are hard to account for by using distance criteria. Two methods to handle prevalences are described, the root method and the multiplication method. (b) Tied reference values, another complication of the partitioning problem, could also be hard to take care of using distance criteria. Some solutions to the problems caused by tied reference values are suggested. CONCLUSIONS: Partitioning of biochemical reference data should preferably be based on proportion criteria; this is particularly true for nongaussian distributions. Both of the described complications of the partitioning problem, the prevalences of the subgroups and tied reference values, are hard to deal with using distance criteria, but the proposed methods make it possible to account for them when proportion criteria are applied.

Clinical Laboratory Techniques↗