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At least 397 records · Page 22Linked to original sources
PubMed1999

Variation over time of the effects of prognostic factors in a population-based study of colon cancer: comparison of statistical models.

The authors compare the performance of different regression models for censored survival data in modeling the impact of prognostic factors on all-cause mortality in colon cancer. The data were for 1,951 patients, who were diagnosed in 1977-1991, recorded by the Registry of Digestive Tumors of Côte d'Or, France, and followed for up to 15 years. Models include the Cox proportional hazards model and its three generalizations that allow for hazard ratio to change over time: 1) the piecewise model where hazard ratio is a step function; 2) the model with interaction between a predictor and a parametric function of time; and 3) the non-parametric regression spline model. Results illustrate the importance of accounting for non-proportionality of hazards, and some advantages of flexible non-parametric modeling of time-dependent effects. The authors provide empirical evidence for the dependence of the results of piecewise and parametric models on arbitrary a priori choices, regarding the number of time intervals and specific parametric function, which may lead to biased estimates and low statistical power. The authors demonstrate that a single, a priori selected spline model recovers a variety of patterns of changes in hazard ratio and fits better than other models, especially when the changes are non-monotonic, as in the case of cancer stages.

Aged↗
PubMed1999

Statistical method to evaluate management strategies to decrease variability in operating room utilization: application of linear statistical modeling and Monte Carlo simulation to operating room management.

BACKGROUND: Operating room (OR) managers seeking to maximize labor productivity in their OR suite may attempt to reduce day-today variability in hours of OR time for which there are staff but for which there are no cases ("underutilized time"). The authors developed a method to analyze data from surgical services information systems to evaluate which management interventions can most effectively decrease variability in underutilized time. METHODS: The method uses seven summary statistics of daily workload in a surgical suite: daily allocated hours of OR time, estimated hours of elective cases, actual hours of elective cases, estimated hours of add-on cases, actual hours of add-on cases, hours of turnover time, and hours of underutilized time. Simultaneous linear statistical equations (a structural equation model) specify the relationship among these variables. Estimated coefficients are used in Monte Carlo simulations. RESULTS: The authors applied the analysis they developed to two OR suites: a tertiary care hospital's suite and an ambulatory surgery center. At both suites, the most effective strategy to decrease variability in underutilized OR time was to choose optimally the day on which to do each elective case so as to best fill the allocated hours. Eliminating all (1) errors in predicting how long elective or add-on cases would last, (2) variability in turnover or delays between cases, or (3) day-to-day variation in hours of add-on cases would have a small effect. CONCLUSIONS: This method can be used for decision support to determine how to decrease variability in underutilized OR time.

Humans↗
PubMed2005

Statistical modeling of shape and appearance using the continuous medial representation.

We describe a novel approach to combining shape and appearance features in the statistical analysis of structures in medical images. The continuous medial representation is used to relate these two types of features meaningfully. The representation imposes a shape-based coordinate system on structure interiors, in a way that uses the boundary normal as one of the coordinate axes, while providing an onto and nearly one-to-one parametrization. This coordinate system is used to sample image intensities in the context of shape. The approach is illustrated by the principal components analysis of the shape and appearance of the hippocampus in T1-weighted MRI from a schizophrenia study.

Algorithms↗
PubMed2005

A hierarchical statistical modeling approach for the unsupervised 3-D biplanar reconstruction of the scoliotic spine.

This paper presents a new and accurate three-dimensional (3-D) reconstruction technique for the scoliotic spine from a pair of planar and conventional (postero-anterior with normal incidence and lateral) calibrated radiographic images. The proposed model uses a priori hierarchical global knowledge, both on the geometric structure of the whole spine and of each vertebra. More precisely, it relies on the specification of two 3-D statistical templates. The first, a rough geometric template on which rigid admissible deformations are defined, is used to ensure a crude registration of the whole spine. An accurate 3-D reconstruction is then performed for each vertebra by a second template on which nonlinear admissible global, as well as local deformations, are defined. Global deformations are modeled using a statistical modal analysis of the pathological deformations observed on a representative scoliotic vertebra population. Local deformations are represented by a first-order Markov process. This unsupervised coarse-to-fine 3-D reconstruction procedure leads to two separate minimization procedures efficiently solved in our application with evolutionary stochastic optimization algorithms. In this context, we compare the results obtained with a classical genetic algorithm (GA) and a recent Exploration Selection (ES) technique. This latter optimization method with the proposed 3-D reconstruction model, is tested on several pairs of biplanar radiographic images with scoliotic deformities. The experiments reported in this paper demonstrate that the discussed method is comparable in terms of accuracy with the classical computed-tomography-scan technique while being unsupervised and while requiring only two radiographic images and a lower amount of radiation for the patient.

Algorithms↗
PubMed2005

Clinical evidence for the influence of uric acid on hypertension, cardiovascular disease, and kidney disease: a statistical modeling perspective.

This article critically evaluates the clinical evidence regarding the influence of uric acid on hypertension, cardiovascular disease, and kidney disease. Data on these relationships are largely observational and exceedingly complex. The complexity is owing to indirect and direct relations, and bidirectional influences, simultaneously operating on multiple outcomes. Limitations of previous analyses include inadequate statistical methods using only bivariate correlations or poorly specified multiple regression models. As a result, great controversy developed as to whether uric acid is an independent predictor of important outcomes. An example of such analytic limitations is including hypertension as an independent variable, together with uric acid, in a multivariate model for predicting cardiovascular disease. Hypertension may predict significant variance in cardiovascular disease, but the contribution of uric acid may not be recognized if uric acid exerts its influence indirectly through hypertension. Path analysis, which can model direct and indirect influences on outcomes simultaneously, would address this substantive question. Studies of uric acid in relation to hypertension, cardiovascular disease, and kidney disease using a path-analytic approach would help specify such conditions as well as optimize design of clinical trials to determine if decreasing uric acid levels improves outcomes.

Cardiovascular Diseases↗
PubMed1993

A statistical model validating triage for the peer review process: keeping the competitive applications in the review pipeline.

Triage of grant application at the National Institutes of Health (NIH) is a process whereby an initial screening of applications by a scientific peer review group eliminates applications that are not competitive for awards. The process of application triage has been limited to those applications submitted to the NIH in response to an RFA (Request for Applications). A hypergeometric model was developed to determine the extent to which five, six, seven, or eight member triage teams or subsets of 12-to-20 member full committees could provide a statistically defensible triage decision. Although the intent of triage is to remove from review those applications that are noncompetitive, the model was weighted in favor of the applicant to minimize the likelihood that highly competitive applications would be eliminated. Within the assumptions and rules developed, it was determined that there was little likelihood that the latter would occur. For example, in the worst case scenario, the greatest probability that a highly competitive application would be knocked out of competition is P < or = 0.014 in the case of a five-member triage subset of a 20-member committee. Using the latter case, the model was tested on a set of 73 applications that were submitted to the National Cancer Institute for action at the February 1993 National Cancer Advisory Board. The model selected for triage required that each application be assigned to five reviewers, that each reviewer be blinded to the review assignments of the other reviewers, and that four noncompetitive votes be registered to triage out an application. Each of 19 applications received four to five noncompetitive votes, and were triaged out of the review process. The remaining 54 applications were then reviewed according to the usual NIH review process. Four of the applications received three noncompetitive triage votes each and were either rated as not recommended for further consideration (NRF, n = 2)) or received priority scores > or = 250 (n = 2) (The smaller the priority score the better the technical merit). Thirteen of the 53 applications received two noncompetitive votes. Of the latter, two were not recommended for further consideration and the remaining 11 received priority scores between in excess of 200. The distribution of competitive applications was such that funding was limited to those applications with priority scores of less than 190. Thus, the data suggest that the conservative model is valid such that the likelihood of eliminating a highly competitive application from consideration for funding is remotely small.(ABSTRACT TRUNCATED AT 400 WORDS)

Competitive Bidding↗
PubMed1991

A statistical model for in-vitro assessment of patient sensitivity to cytotoxic drugs.

Formation of colonies in semisolid medium is an assay used for the study of stem cell characteristics in hematopoietic and solid tumors. Previous experience with leukemia patients failed to show an association between the reduction in colony formation observed when patient blast cells were exposed to increased concentrations of an anticancer agent, and the subsequent patient response to the agent. By introducing a model that takes into account the possibility of a resistant subpopulation of clonogenic cells, the paper demonstrates that the null result was due to an inadequate summarization of the dose-response curve, and in fact a statistically and biologically significant association exists between one of the parameters of the model and patient response. The properties, implementation, and interpretation of the model are discussed.

Biometry↗
PubMed2004

Mathematical-statistical models of generated hazardous hospital solid waste.

This research work was carried out under the assumption that wastes generated from hospitals in Irbid, Jordan were hazardous. The hazardous and non-hazardous wastes generated from the different divisions in the three hospitals under consideration were not separated during collection process. Three hospitals, Princess Basma hospital (public), Princess Bade'ah hospital (teaching), and Ibn Al-Nafis hospital (private) in Irbid were selected for this study. The research work took into account the amounts of solid waste accumulated from each division and also determined the total amount generated from each hospital. The generation rates were determined (kilogram per patient, per day; kilogram per bed, per day) for the three hospitals. These generation rates were compared with similar hospitals in Europe. The evaluation suggested that the current situation regarding the management of these wastes in the three studied hospitals needs revision as these hospitals do not follow methods of waste disposals that would reduce risk to human health and the environment practiced in developed countries. Statistical analysis was carried out to develop models for the prediction of the quantity of waste generated at each hospital (public, teaching, private). In these models number of patients, beds, and type of hospital were revealed to be significant factors on quantity of waste generated. Multiple regressions were also used to estimate the quantities of wastes generated from similar divisions in the three hospitals (surgery, internal diseases, and maternity).

Environmental Monitoring↗
PubMed1994

Statistical models for the analysis of ordered categorical data in public health and medical research.

In the late 1970s statisticians extended the methods for analysing loglinear and logit models for cross-classified categorical data to incorporate information about the ordinal structure of the categories corresponding to some of the classification variables. In this paper we review one class of such extensions known as association models. We consider association models with and without order restrictions on the parameters and we use these models to answer research questions about several medical examples involving ordered categorical data. We emphasize the interpretation of parameters in the association models and how this relates to the research questions of interest.

Adult↗
PubMed1975

Life games and statistical models.

A set of equations is obtained, which describes the rules of a class of games (life games). These games simulate the processes of growth, death, survival, and competition. The equations are nonlinear difference equations, where the degree of nonlinearity is directly related to the number of interacting neighbors. The time evolution and the development of geometric patterns can be studied starting from these equations. Extensions and generalizations, such as the introduction of stochastic elements, can easily be accommodated in the formalism. Some significant unsolved problems are noted.

Biological Evolution↗
PubMed1984

[Longitudinal study of the evolution of the frequency of dental caries in a school milieu: a statistical model].

Following a 3-year epidemiological study of the appearance of dental caries in a population of children aged 6 to 9 years (and examined every 12 months), a model of the evolution of the frequency of caries based on a Poisson "with zeros" distribution is proposed. The multivariate distribution of the above phenomena appears as a mixture of multiple negative binomial distribution (MNB) of the following form: sigma mjMNB(a;k0jb,k1jb,k2jb) with sigma mj = 1. The experimental data from the sample of 501 children validate the model in its successive stages. Simulations using the bootstrap method show that the estimates of the model parameters remain stable in the neighbourhood of the distribution observed; at the same time they permit establishing the margins of confidence. Finally, it is shown why the total number of dental faces affected during the experimental period remained strongly asymmetrical.

Child↗
PubMed1991

Statistical model in tests for eye irritants.

A test used to classify substances for eye irritancy, as required by the Consumer Product Safety Commission, is performed on 1-3 groups of 6 albino rabbits in a sequential manner. When the statistical implications of the test are realized, it is possible for a substance to be classified as an irritant with fewer reactions than the number required for it to be classified as not an irritant. A procedure is given for correcting the inconsistency in the current test, and an alternative test, which considerably reduces the number of animals required, is proposed. Probability models and expected sample size calculations have been derived.

Animal Testing Alternatives↗