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At least 1,081 records · Page 60Linked to original sources
PubMed2000

Comparing time to onset of response in antidepressant clinical trials using the cure model and the Cramer-von Mises test.

Currently available antidepressants have a delayed onset of action and there is considerable interest in developing new products which exhibit a shorter time to response. Previous authors have suggested using a cure model to characterize the time to response for the combination of the patients who respond and the patients who do not respond. In this paper, we use the cure model and propose a Cramer-von Mises statistic to compare the time to response distributions for patients who respond to therapy. The asymptotic null distribution of the statistic is derived. A bootstap procedure is also proposed for sample sizes typical in antidepressant clinical trials. Results of the simulation study suggest that for common designs and sample sizes used in such trials, the statistic has reasonable size and power properties as long as censoring is moderate.

Adrenergic beta-Antagonists↗
PubMed1999

Hydration effects on the electrostatic potential around tuftsin.

The electrostatic potential and component dielectric constants from molecular dynamics (MD) trajectories of tuftsin, a tetrapeptide with the amino acid sequence Thr-Lys-Pro-Arg in water and in saline solution are presented. The results obtained from the analysis of the MD trajectories for the total electrostatic potential at points on a grid using the Ewald technique are compared with the solution to the Poisson-Boltzmann (PB) equation. The latter was solved using several sets of dielectric constant parameters. The effects of structural averaging on the PB results were also considered. Solute conformational mobility in simulations gives rise to an electrostatic potential map around the solute dominated by the solute monopole (or lowest order multipole). The detailed spatial variation of the electrostatic potential on the molecular surface brought about by the compounded effects of the distribution of water and ions close to the peptide, solvent mobility, and solute conformational mobility are not qualitatively reproducible from a reparametrization of the input solute and solvent dielectric constants to the PB equation for a single structure or for structurally averaged PB calculations. Nevertheless, by fitting the PB to the MD electrostatic potential surfaces with the dielectric constants as fitting parameters, we found that the values that give the best fit are the values calculated from the MD trajectories. Implications of using such field calculations on the design of tuftsin peptide analogues are discussed.

Computer Simulation↗
PubMed1989

Probability and sampling.

The concept of probability is introduced, and the role of probability distributions in statistical theory is discussed, with particular reference to the Normal distribution and its characteristics. Sampling and sampling variation are described, together with sampling error, the standard error of the mean, and confidence intervals for determining the likely magnitude of a population mean.

Child↗
PubMed2006

Correlated transition between two activity states of neurons.

In order to understand the dynamical properties of a neural network, it is important to characterize the relation between spike trains of two neurons in the network. In this study, we show that in some neuron pairs in inferior temporal cortices of macaque monkeys, spike trains of a pair are described by a two-dimensional Poisson process whose means are modulated by a common two-state Markov process. The common two-state Markov process describes a correlated state transition between firing and nonfiring states of the constituent neurons of the pair.

Action Potentials↗
PubMed1995

Issues in biomedical statistics: comparing means under normal distribution theory.

The test used most commonly in biomedical research to compare means when measurements have been made on a continuous scale is Student's t-test, followed closely by various forms of analysis of variance. These tests require that defined populations have been randomly sampled, but there are other assumptions about populations and samples that must be satisfied. These include: (i) normality of the population distributions; (ii) equal variance in those normal populations; and (iii) statistical independence of the samples. This review offers advice to investigators on how to recognize breaches of the assumptions of normality and equality of variance, and how to deal with them by modifying the usual t-test or by transforming the experimental data. The sample-size also has an important bearing on statistical inferences: (i) if it is too small, the risk of Type II error is inflated; and (ii) inequality of sample size exaggerates the effects of inequality of variance. The assumption of independence is breached if repeated measurements are made serially rather than in random order, but adjustments to analysis of variance can be made to correct for the inflated risk of Type I error. The review also considers the problem of making multiple comparisons of means, and recommends solutions.

Analysis of Variance↗
PubMed1994

Levels and correlates of maternal nutritional status in urban Bangladesh.

OBJECTIVE: To determine the levels and correlates of maternal nutritional status. DESIGN: Cross-sectional maternal weight, height, and mid-upper arm circumference (MUAC) data were correlated with sociodemographic data. SETTING: Slums of Dhaka, Bangladesh. SUBJECTS: Weight, height and MUAC were collected from a representative sample of 2417 nonpregnant mothers. Socioeconomic data such as age, education, religion and household economic status was collected from 2048 mothers; data on reproductive experiences such as number of pregnancies and number of children born alive now dead was available from 1314 mothers; and both sets of data from 1185 mothers. METHODS: Using weight, height, MUAC and body mass index (BMI) data, the levels of maternal nutritional status were estimated. Bivariate and multivariate relationships of maternal nutritional status with socioeconomic and reproductive experiences variables were examined. RESULTS: Mothers' mean weight, height, MUAC and BMI were 41.8 kg, 148.8 cm, 232.5 mm, and 18.8 respectively. In multivariate regression analyses, mothers' weight, BMI and MUAC were significantly positively correlated with mothers' years of schooling (P < 0.05) and household economic status (P < 0.01). Mothers' height was significantly positively correlated with years of schooling (P < 0.05), but not with household economic status. Maternal height and weight were significantly negatively correlated with number of child deaths (P < 0.05). CONCLUSIONS: The findings suggest a possible inter-generational effect; thus, improvement of the nutritional status of girls of this generation would improve child survival in the next generation. This, however, will require complex and long-term planning. As an interim measure, shorter mothers should be targeted for appropriate antenatal and obstetric services.

Adult↗
PubMed2004

Multiple testing. Part I. Single-step procedures for control of general type I error rates.

The present article proposes general single-step multiple testing procedures for controlling Type I error rates defined as arbitrary parameters of the distribution of the number of Type I errors, such as the generalized family-wise error rate. A key feature of our approach is the test statistics null distribution (rather than data generating null distribution) used to derive cut-offs (i.e., rejection regions) for these test statistics and the resulting adjusted p-values. For general null hypotheses, corresponding to submodels for the data generating distribution, we identify an asymptotic domination condition for a null distribution under which single-step common-quantile and common-cut-off procedures asymptotically control the Type I error rate, for arbitrary data generating distributions, without the need for conditions such as subset pivotality. Inspired by this general characterization of a null distribution, we then propose as an explicit null distribution the asymptotic distribution of the vector of null value shifted and scaled test statistics. In the special case of family-wise error rate (FWER) control, our method yields the single-step minP and maxT procedures, based on minima of unadjusted p-values and maxima of test statistics, respectively, with the important distinction in the choice of null distribution. Single-step procedures based on consistent estimators of the null distribution are shown to also provide asymptotic control of the Type I error rate. A general bootstrap algorithm is supplied to conveniently obtain consistent estimators of the null distribution. The special cases of t- and F-statistics are discussed in detail. The companion articles focus on step-down multiple testing procedures for control of the FWER (van der Laan et al., 2004b) and on augmentations of FWER-controlling methods to control error rates such as tail probabilities for the number of false positives and for the proportion of false positives among the rejected hypotheses (van der Laan et al., 2004a). The proposed bootstrap multiple testing procedures are evaluated by a simulation study and applied to genomic data in the fourth article of the series (Pollard et al., 2004).

Journal Article↗
PubMed1997

An asymptotic analysis of the logrank test.

Asymptotic expansions for the null distribution of the logrank statistic and its distribution under local proportional hazards alternatives are developed in the case of iid observations. The results, which are derived from the work of Gu (1992) and Taniguchi (1992), are easy to interpret, and provide some theoretical justification for many behavioral characteristics of the logrank test that have been previously observed in simulation studies. We focus primarily upon (i) the inadequacy of the usual normal approximation under treatment group imbalance; and, (ii) the effects of treatment group imbalance on power and sample size calculations. A simple transformation of the logrank statistic is also derived based on results in Konishi (1991) and is found to substantially improve the standard normal approximation to its distribution under the null hypothesis of no survival difference when there is treatment group imbalance.

Biometry↗
PubMed1993

Evidence of commingling in human eating behavior.

This investigation tested whether distributions of certain aspects of eating behavior were consistent with the notion of a "mixture model;" that is, two or more distinct commingled component distributions, consistent with the possibility of major gene action. Undergraduates (n=901) completed self-report trait measures of hunger, disinhibition, and dietary restraint. Variables were residualized for gender and age and transformed to remove skewness. Residualized transformed distributions were tested for departure from unimodality with Hartigan's dip statistic. The distributions of all three aspects of eating behavior were significantly non-unimodal. Next, component multivariate normal distributions were estimated via maximum likelihood. Likelihood ratio tests were employed to compare nested models. A mixture of four distributions with unequal variance-covariance matrices fit significantly better than any more parsimonious model. In sum, these data strongly suggest that the distributions of several measures of eating behavior are composed of four component distributions. This finding is consistent with the possibility of major gene effects for eating behavior.

Adolescent↗
PubMed2001

A brief history of numbers and statistics with cytometric applications.

A brief history of numbers and statistics traces the development of numbers from prehistory to completion of our current system of numeration with the introduction of the decimal fraction by Viete, Stevin, Burgi, and Galileo at the turn of the 16th century. This was followed by the development of what we now know as probability theory by Pascal, Fermat, and Huygens in the mid-17th century which arose in connection with questions in gambling with dice and can be regarded as the origin of statistics. The three main probability distributions on which statistics depend were introduced and/or formalized between the mid-17th and early 19th centuries: the binomial distribution by Pascal; the normal distribution by de Moivre, Gauss, and Laplace, and the Poisson distribution by Poisson. The formal discipline of statistics commenced with the works of Pearson, Yule, and Gosset at the turn of the 19th century when the first statistical tests were introduced. Elementary descriptions of the statistical tests most likely to be used in conjunction with cytometric data are given and it is shown how these can be applied to the analysis of difficult immunofluorescence distributions when there is overlap between the labeled and unlabeled cell populations.

Cell Count↗
PubMed2006

Image and texture segmentation using local spectral histograms.

We present a method for segmenting images consisting of texture and nontexture regions based on local spectral histograms. Defined as a vector consisting of marginal distributions of chosen filter responses, local spectral histograms provide a feature statistic for both types of regions. Using local spectral histograms of homogeneous regions, we decompose the segmentation process into three stages. The first is the initial classification stage, where probability models for homogeneous texture and nontexture regions are derived and an initial segmentation result is obtained by classifying local windows. In the second stage, we give an algorithm that iteratively updates the segmentation using the derived probability models. The third is the boundary localization stage, where region boundaries are localized by building refined probability models that are sensitive to spatial patterns in segmented regions. We present segmentation results on texture as well as nontexture images. Our comparison with other methods shows that the proposed method produces more accurate segmentation results.

Algorithms↗
PubMed1979

Studies on the dense-core vesicles in the ontogenesis of locus coeruleus. III. Statistical reconstruction of the diameter distribution of the dense-core vesicles in locus coeruleus during rat ontogenesis.

The method for statistical reconstruction of the diameters of dence-core vesicles, described in the previous work, is applied for studying the changes of these vesicles in the perikarya, the axonal boutons and the neuropile of the rat locus coeruleus during ontogenesis. The data obtained show the presence of the population during the embryonal development and of two populations of dense-core vesicles after birth in the structures studied. This finding is interpreted in connection with the different function of the dense-core vesicles and of the transmitters, respectively, in the development and function of the main noradrenergic nucleus in the brain of mammals -- locus coeruleus.

Age Factors↗
PubMed1991

A test based on the exact probability distribution of the chi 2 statistic--incorporation into the MASC method.

The MASC method (Clerget-Darpoux et al. 1988) to test genetic models in HLA associated disease makes use of a test of goodness-of-fit on categorized data. This test is composed of a sum of independent chi 2s, which are applicable only when expectations in each category are not too small. The commonly used approach to solve this problem is to combine classes in order to raise expectations. This, however, means losing some of the information contained in the data and may decrease greatly the power of discrimination between models. A test based on the exact probability distribution of a test statistic, namely the chi 2, has been implemented to avoid this dilemma.

Chi-Square Distribution↗