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 1,225 records · Page 68Linked to original sources
PubMed2006

Pharmacodynamic models for agents that alter production of natural cells with various distributions of lifespans.

Indirect pharmacodynamic response (IDR) models were developed for agents which alter the generation of cell populations with arbitrary lifespan distributions. These models extend lifespan based IDR models introduced previously [J. Pharmacokinet. Biopharm. 27: 467, 1999] for cell populations with the same lifespan ("delta" distribution). Considered are cell populations exhibiting time-invariant lifespan distributions described by the probability density function l(tau). It is assumed that cell response (R) is produced at a zero-order rate (kin(t)) and is eliminated from the population when the cell lifespan expires. The cell loss rate is calculated as kin*l(t), where '*' denotes the convolution operator. Therapeutic agents can stimulate or inhibit production rates according to the Hill function: 1 +/- H(C(t)) where H(C(t)) contains the capacity (Smax) and potency (SC50) parameters and C(t) is a pharmacokinetic function. The production rate is kin(t) = kin.[ 1+/-H(C(t))]. The operational model is dR/dt = kin(t)-kin*l(t) with the baseline condition R0 = kin.TR, where TR is the mean lifespan. Single populations as well as populations with precursors were examined by simulation to establish the role of lifespan distribution parameters (mean and standard deviation) in controlling the response vs. time profile. Estimability of parameters was assessed. Numerical techniques of solving differential equations with the convolution integral were proposed. In addition, the models were applied to literature data to describe the stimulatory effects of single doses of recombinant human erythropoietin on reticulocytes in blood. The estimates of Smax and SC50 for these agents were obtained along with means and standard deviations for reticulocyte lifespan distributions. The proposed models can be used to analyze the pharmacodynamics of agents which alter natural cell production yielding parameters describing their efficacy and potency as well as means and standard deviations for cell lifespan distributions.

Algorithms↗
PubMed2000

Tests of proportional hazards and proportional odds models for grouped survival data.

In this paper, we derive score test statistics to discriminate between proportional hazards and proportional odds models for grouped survival data. These models are embedded within a power family transformation in order to obtain the score tests. In simple cases, some small-sample results are obtained for the score statistics using Monte Carlo simulations. Score statistics have distributions well approximated by the chi-squared distribution. Real examples illustrate the proposed tests.

Biometry↗
PubMed2000

Evaluation of the extent of nonlinearity in reportable range studies.

OBJECTIVES: To extend the polynomial method for evaluating linearity in 2 ways. First, we developed a screen to ascertain whether the data were precise enough to permit a reliable evaluation of linearity and therefore eliminate findings of linearity due to low statistical power. Second, we assessed whether the degree of nonlinearity detected by the polynomial method was clinically relevant using a statistically rigorous method. METHODS: Because we assessed linearity relative to a clinically determined level of importance instead of the default value of zero, we used sampling theory based on the noncentral chi(2) distribution. Using statistical power calculations, we incorporated a screen for imprecision that guarantees that the probability of correctly identifying nonlinear methods is at least 80%. RESULTS: With the described methods, we achieved a sensitivity of at least 80% and a specificity of at least 95%. When the data were too imprecise to achieve a sensitivity of 80%, no determination of linearity was made. This procedure mimics the practice in manual inspection of flagging data that appear imprecise by visual inspection and halting the evaluation. CONCLUSIONS: Formal statistical tests for precision and amount of nonlinearity are advantageous because they allow us to quantify and limit classification errors. By formalizing these various aspects of linearity assessment, we maintain some of the complex features of manual methods while making the linearity assessment feasible to apply to a high volume of assessments and removing the between-analyst variability.

Humans↗
PubMed2000

National Inventory of Clinical Laboratory Testing Services (NICLTS). Development and test distribution for 1996.

CONTEXT: A statistically valid inventory of the distribution, both geographic and by laboratory type, of clinical and anatomical laboratory testing in the United States is needed to assess the impact of the Clinical Laboratory Improvements Amendments of 1988 and to provide information for other health care and public health policy decisions. OBJECTIVE: To present initial US laboratory testing volume data compiled by the National Inventory of Clinical Laboratory Testing Services. DESIGN: Stratified random sample of laboratories performing testing in 1996 with data on the number of laboratory tests performed, identified by method and analyte. Data were collected by field tabulators (moderate- or high-complexity laboratories) or through a mail/telephone survey (waived or provider-performed microscopy laboratories) for each site. PARTICIPANTS: Laboratories that were enrolled in the 1996 Online Certification Survey and Reporting System, maintained by the US Health Care Finance Administration, and that performed laboratory testing during 1996. MAIN OUTCOME MEASURE: Laboratory testing distribution for 1996 in the United States by analyte, method, and specimen type. RESULTS: An overall response rate of 79% provided data from 757 moderate- or high-complexity laboratories and 1322 waived or provider-performed microscopy laboratories. The estimated total US testing volume for 1996 was 7.25 +/- 1.09 billion tests. Laboratories performing complex testing, defined as greater than 16 method/analyte/specimen type combinations, comprised 16% of the US laboratories by survey site, but performed 80% (95% confidence limits, 43% to 100%) of the testing volume. Glucose analysis was the most frequently performed test. Automated hematology and chemistry analyzers were the most frequently used methods. CONCLUSIONS: A statistically valid, consistent survey of the distribution of US laboratory testing was obtained. Simple analysis of these data by laboratory type and geographic region can provide insights into where laboratory testing is performed. The study design allows extensions that will facilitate collection of additional data of importance to public health and medical care delivery.

Clinical Laboratory Techniques↗
PubMed1983

The boundary value problem in spatial statistical analysis.

"The primary objective of this paper is to investigate procedures for detecting and handling existing border biasing in spatial statistical analysis." Six conventional solutions to the boundary value problem are criticized, and three alternate statistical solutions are proposed.

Demography↗
PubMedDate not supplied

Population concentration: a consideration of density measures and correlates.

"The purpose of the paper is to explore the nature of association of certain external and internal density measures with each other and with a set of selected variables in four states of India. Pearson's Simple Coefficient of Correlations and Stepwise Multiple Regression were used for analysis of data. The results reinforce the notion of a clear distinction between external and internal density measures and their associative factors in India; the two measures of internal density, although highly correlated, were found to have varied relevance to the dependent variables."

Asia↗
PubMed2005

Parametric modelling of cost data: some simulation evidence.

Recently, commentators have suggested that the distributional form of cost data should be explicitly modelled to gain efficiency in estimating the population mean. We perform a series of simulation experiments to evaluate the usual sample mean and the mean estimator of a lognormal distribution, in the context of both theoretical distributions and three large empirical datasets. The sample mean is always unbiased, but is somewhat less efficient when the population distribution is truly lognormal. However the lognormal estimator can perform appallingly when the true distribution is not lognormal. In practical situations, where the true distribution is unknown, the sample mean generally remains the estimator of choice, especially when limited sample size prohibits detailed modelling of the cost data distribution.

Clinical Trials as Topic↗
PubMed2006

Study of ultrasonic echo envelope based on Nakagami-inverse Gaussian distribution.

A new probability density function (pdf) to describe the envelope of the backscattered echoes from tissues is proposed. The combined effect of multiple scattering and randomness arising from local and global variations in scattering cross-section is captured through Nakagami-inverse Gaussian distribution (NIGD). The proposed distribution has the advantage that it has a closed form unlike the realistic Nakagami-lognormal distribution (NLD). The degree of closeness, as measured in terms of Kullback-Leibler (KL) distance between two probability distributions is computed. It is shown that NIGD much more closely approximates NLD compared to Nakagami-gamma distribution (NGD) used to model such problems.

Humans↗
PubMed2005

Maximum likelihood estimation in survival studies under progressive interval censoring with random removals.

Censoring occurs commonly in clinical trials. This article investigates a new censoring scheme, namely, Type II progressive interval censoring with random removals to cope with the setting that patients are examined at fixed regular intervals and dropouts may occur during the study period. We discuss the maximum likelihood estimation of the model parameters and derive the corresponding asymptotic variances when survival times are assumed to be Weibull distributed. An example is discussed to illustrate the application of the results under this censoring scheme.

Data Interpretation, Statistical↗
PubMed2006

Comparison of the distribution of doses summarised over different periods of time.

Track dosemeters are very useful devices for dose assessment. The low cost and relatively simple procedures of dose calculation allow the mass distribution of track dosemeters in the population of exposed people. Because of the large number of measurements, observations may be made that would not be possible if the number of measurements were low. One such observation is the unexpected difference between the parameters of the annual dose distribution and the distribution of short-term doses. This difference may have a strong impact on lifetime dose or risk calculations. Much epidemiological data are based on the extrapolation of the distribution of short-term doses into the distribution of the lifetime dose. How the distribution of lifetime doses is spread is more complicated. The answer depends on the period of the single measurement.

Data Interpretation, Statistical↗
PubMed2002

Multiscale dissipative particle dynamics.

We present a simplified kinetic derivation of the multiscale Voronoi-based dissipative particle dynamics (DPD) method. The Voronoi tessellation is used to coarse-grain the molecular level of a fluid, resulting in mesoscopic equations of motion for local mass, momentum and energy. The dissipative particles follow the dynamics of extended objects subject to forces including pressure and stresses. The stresses and heat fluxes are computed through constitutive relations, which lead to fluctuating Navier-Stokes hydrodynamics for the solvent. The present formulation is based on the use of statistical mechanical distribution functions and the connection with the underlying molecular description of the fluid is maintained through the pair-distribution function and the intermolecular potential. The main features of this DPD method are the adaptivity of the dissipative particles to the important length-scales of the problem and the explicit role played by the molecular pair-distribution function.

Biological Transport↗
PubMedDate not supplied

The expression of survivin and Ki-67 in meningiomas: correlation with grade and clinical outcome.

Identification of proteins involving in the control of proliferation in normal cells may lead to a better understanding of cellular transformation and proliferation mechanisms. Measurement of proliferative activity may help determine the tumor grade, recurrence and malignancy. Forty meningiomas of various histological grade, diagnosed in Baskent University and Cukurova University Medical Faculty, Departments of Pathology, were graded according to WHO grading system and survivin and Ki-67 monoclonal antibodies were administered by immunohistochemical method. Findings were analyzed with statistical methods; distribution of survivin expression and Ki-67 LI values were determined for different grade meningiomas. The correlation between increasing grades of meningiomas, particularly between grades I and III, survivin and Ki-67 LI values was statistically significant. In addition, we have found a positive correlation between clinical outcome, survivin and Ki-67 LI. For over expression of survivin in some of the grade I meningiomas and no or mild expression in some of the grades II and III meningiomas, however, it has been suggested that apoptosis inhibited by survivin is not the only factor responsible for progression of meningiomas.

Adult↗
PubMed1990

Cross-national models of fertility, family planning, and development: testing for reciprocal effects.

Cross-national models of fertility, family planning, and development commonly assume that there are no reciprocal effect between fertility and other variables in the model and that when path models are used, there are no reciprocal or nonrecursive effects among any set of variables in the model. The present study tests for nonrecursiveness using 2-wave panel data, and finds that nonrecursive effects are present among variables commonly used in model of fertility, family planning, and development. In addition, the pattern of relationships found has implications for the explanation of the relationship between mortality and fertility in demographic transition theory.

Birth Rate↗
PubMed2006

A robust approach to t linear mixed models applied to multiple sclerosis data.

We discuss a robust extension of linear mixed models based on the multivariate t distribution. Since longitudinal data are successively collected over time and typically tend to be auto-correlated, we employ a parsimonious first-order autoregressive dependence structure for the within-subject errors. A score test statistic for testing the existence of autocorrelation among the within-subject errors is derived. Moreover, we develop an explicit scoring procedure for the maximum likelihood estimation with standard errors as a by-product. The technique for predicting future responses of a subject given past measurements is also investigated. Results are illustrated with real data from a multiple sclerosis clinical trial.

Bayes Theorem↗
PubMed2004

Cumulative distribution functions from Dempster-Shafer belief structures.

We discuss the Dempster-Shafer belief structure. We introduce the idea of a cumulative distribution induced by a Dempster-Shafer belief structure. We call these belief-cumulative distribution (B-CDs) functions. We study the properties of these distribution functions and show that they are interval functions. We investigate the possibility of using these distribution functions as a tool for knowledge representation.

Algorithms↗
PubMed2003

Online tool for the discrimination of equi-distributions.

BACKGROUND: For many applications one wishes to decide whether a certain set of numbers originates from an equiprobability distribution or whether they are unequally distributed. Distributions of relative frequencies may deviate significantly from the corresponding probability distributions due to finite sample effects. Hence, it is not trivial to discriminate between an equiprobability distribution and non-equally distributed probabilities when knowing only frequencies. RESULTS: Based on analytical results we provide a software tool which allows to decide whether data correspond to an equiprobability distribution. The tool is available at http://bioinf.charite.de/equifreq/. CONCLUSIONS: Its application is demonstrated for the distribution of point mutations in coding genes.

Amino Acid Sequence↗