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PubMed2002

An adaptive test of significance for a subset of regression coefficients.

An F-test for a subset of regression coefficients is often used in order to compare two nested linear models. An adaptive test is proposed that has higher power than this F-test for many non-normal distributions of error terms. The adaptive test uses a weighted least squares procedure with weights determined from the Studentized deleted residuals from a linear model. A permutation method is then used so that the resulting test maintains its size near the nominal value. Results from several simulation studies are used to compare the power of the adaptive test to the F-test. The adaptive test is recommended as a way of increasing the power of many common tests when used with models that have few parameters whenever the distribution of errors is non-normal and the number of observations exceeds 20.

Computer Simulation↗
PubMed1976

[Changes of the tuberculosis of the central nervous system since the introduction of prophylactic BCG vaccination and tuberculostatic therapy (author's transl)].

From 1924 to 1938, 9920 autopsies were carried out in our institute; from 1955 to 1969 there were 37 784 dissections. Among the former 132 cases of tuberculous meningitis (tb. m.) were observed (1.33%), among the latter there were only 74 (0.20%). This decrease in frequency to about 1/7 is statistically highly significant; by standardization to 0.62 it has come down to 0.20% but still remains significant. The marked predominance of tb. m. in males as compared to females (2:1) that was noted in former autopsies is no longer noticeable. With regard to age distribution, a statistically warrented decrease exists only for the age group from 0 to 9 years. From the mortality graph (the dying age being identical with that of affection before the era of chemotherapy) it can be seen that while the peak during childhood has now disappeared, it is only slightly demonstrable during early adult age (as a result of the retarded primary infection). The rise in the curve indicates the deteriorating prognosis with increasing age, with the peak between 60 and 70 years accounting for the importance of miliary tuberculosis of old age for tb. m. Due to therapeutic measures, there is now an increase in the number of chronic tb. m. cases which are productive in their histological picture. As previously, tb. m. at present develops in about half of the cases in the course of miliary tuberculosis. The frequency of early generalization has significantly decreased, while that of miliary tuberculosis of old age has increased. In our examination material tb. m. without further manifestations of tuberculosis has become provable only since the introduction of chemotherapy. Errors in clinical diagnoses have increased from 12.9 to 54%. The decrease of tuberculomata of the central nervous system from 0.17 to 0.013% is also statistically highly significant in our examination material. Apart from the change of the examination material, this decrease in frequency is largely due to prophylactic BCG vaccination and tuberculo-static therapy. Both of them have brought on the rapid advances in tuberculosis epidemiology that started with the turn of the century and resulted mainly in shifting tuberculous primary infection to higher ages and, at the same time, markedly reducing the contagion of tuberculosis.

Acute Disease↗
PubMed1992

Assessing overdispersion and dose-response in the male dominant lethal assay.

In dominant lethal studies the primary variables of interest are typically expressed as discrete counts or proportions (e.g., live implants, resorptions, percent pregnant). Simple statistical sampling models for discrete data such as binomial or Poisson generally do not fit this type of data because of extra-binomial or extra-Poisson departures from variability predicted under these simple models. Extra-variability in the fetal response may originate from parental contributions. These can lead to over- or under-dispersion seen as, e.g., extra-binomial variability in the proportion response. Utilizing a large control database, we investigated the relative impact of extra-variability from male or female contributions on the endpoints of interest. Male-related effects did not seem to contribute to overdispersion in our database; female-related effects were, however, evidenced. Various statistical methods were considered to test for significant treatment differences under these forms of sampling variability. Computer simulations were used to evaluate these methods and to determine which are most appropriate for practical use in the evaluation of dominant lethal data. Our results suggest that distribution-free statistical methods such as a nonparametric permutation test or rank-based tests for trend can be recommended for use.

Analysis of Variance↗
PubMed2005

Using Bayesian networks to model expected and unexpected operational losses.

This report describes the use of Bayesian networks (BNs) to model statistical loss distributions in financial operational risk scenarios. Its focus is on modeling "long" tail, or unexpected, loss events using mixtures of appropriate loss frequency and severity distributions where these mixtures are conditioned on causal variables that model the capability or effectiveness of the underlying controls process. The use of causal modeling is discussed from the perspective of exploiting local expertise about process reliability and formally connecting this knowledge to actual or hypothetical statistical phenomena resulting from the process. This brings the benefit of supplementing sparse data with expert judgment and transforming qualitative knowledge about the process into quantitative predictions. We conclude that BNs can help combine qualitative data from experts and quantitative data from historical loss databases in a principled way and as such they go some way in meeting the requirements of the draft Basel II Accord (Basel, 2004) for an advanced measurement approach (AMA).

Journal Article↗
PubMed2003

Analytical solution of the evolution dynamics on a multiplicative-fitness landscape.

In an infinite population the frequency distribution of individuals carrying a given number of mutations obeys a set of recursion equations, the equilibrium solution of which describes the mutation-selection balance. Although this solution is well-known in the case of a multiplicative-fitness landscape, where it is assumed that all mutations are deleterious and that each new mutation reduces the fitness of the individual by the same fraction, we are not aware of the existence of an analytical solution for the full dynamics. Using the generating function approach, we present here an explicit analytical solution for the frequency distribution recursion equations valid for all generations and initial conditions.

Alleles↗
PubMed1999

A simple way to estimate the median time and compare survival distributions in analgesic trials under informative censoring.

We discuss the problem of estimating the median time and comparison of survival curves when data are nonrandomly censored in analgesic trials. In these trials patients experience post-surgical pain at the time of randomization. Time to onset of analgesia is measured by patient-administered stopwatches. An effective analgesic is one for which the median time to onset is "short." The study design allows patients to remedicate if their pain persists, and this remedication prior to pain relief censors the time-to-onset measures. The time to onset for patients who remedicate is nonrandomly censored. Assuming noninformative censoring yields misleading results with the Kaplan-Meier method (for estimation of median time) and the log-rank test (for comparison of survival curves). This assumption can also obscure the superior effect of an effective analgesic over an ineffective one. We propose a simple and intuitive way to handle the nonrandomly censored data in analgesic trials in order to (a) estimate the median time to pain relief and (b) compare the survival distributions between treatments. The method proposed is applied to data collected from an acute pain clinical trial, and the results are discussed.

Analgesics↗
PubMed2000

Continuous probability distributions from finite data.

Recent approaches to the problem of inferring a continuous probability distribution from a finite set of data have used a scalar field theory for the form of the prior probability distribution. This paper presents a more general form for the prior distribution that has a geometrical interpretation and can improve the specificity of likely solutions. It is also demonstrated that a numerical sampling of the posterior probability distribution can be used as an alternative to a histogram for visualization and to make probabilistic inferences from the data.

Fourier Analysis↗
PubMed2005

A new, reliable, and simple-to-use method for the analysis of a population of values of a random variable using the Weibull probability distribution: application to acrylic bone cement fatigue results.

In cases where the Weibull probability distribution is being investigated as a possible fit to experimentally obtained results of a random variable (V), there is, currently, no accurate and reliable but simple-to-use method available for simultaneously (a) establishing if the fit is of the two- or three-parameter variant of the distribution, and/or (b) estimating the minimum value of the variable (V(0)), in cases where the three-parameter variant is shown to be applicable. In the present work, the details of such a method -- which uses a simple nonlinear regression analysis -- are presented, together with results of its use when applied to 4 sets of number-of-cycles-to-fracture results from fatigue tests, performed in our laboratory, using specimens fabricated from 3 different acrylic bone cement formulations. The key result of the method is that the two- or three-parameter variant of the probability distribution is applicable if the estimate of V(0) obtained is less than or greater than zero, respectively.

Algorithms↗
PubMed2005

Particle distribution in a microporous material: theoretical concept.

Particle distribution and exchange equilibria in a microporous host material, built up of equivalent particle sites, which are grouped in larger subsets, are described. Simplified descriptions evolve from the exact formulae in the thermodynamic limit. We find that, for example, a single zeolite A nanocrystal consisting of about 1000 pseudo-unit-cells fixes a lower limit for the use of the approximate formula describing particle distribution. A rational selectivity coefficient, which is approximately constant over the whole exchange range, only results if a single zeolite crystal consists of one million pseudo-unit-cells or more, or if a sufficiently large number of smaller crystals is considered. On the basis of the statistical particle distribution model, a closed, simple formula for the ion-exchange isotherm is then derived, which is valid for systems involving a variable number of coupled-exchange reactions. Its similarity to the Langmuir isotherm is discussed. The theory on ion-exchange equilibria is used to derive formulae for the change of free-energy, enthalpy, and entropy occurring in coupled ion-exchange reactions. The findings, though applicable to virtually any particle exchanging system with the structural properties described above, are applied to zeolite A, since this material can be treated as a nearly ideal model. The results derived can straightforwardly be used to evaluate experimental data quantitatively, since the common inequivalence of the host sites can be taken into account.

Models, Chemical↗
PubMed1993

Statistical approaches to pharmacodynamic modeling: motivations, methods, and misperceptions.

We have attempted to outline the fundamental statistical aspects of pharmacodynamic modeling. Unexpected yet substantial variability in effect in a group of similarly treated patients is the key motivation for pharmacodynamic investigations. Pharmacokinetic and/or pharmacodynamic factors may influence this variability. Residual variability in effect that persists after accounting for drug exposure indicates that further statistical modeling with pharmacodynamic factors is warranted. Factors that significantly predict interpatient variability in effect may then be employed to individualize the drug dose. In this paper we have emphasized the need to understand the properties of the effect measure and explanatory variables in terms of scale, distribution, and statistical relationship. The assumptions that underlie many types of statistical models have been discussed. The role of residual analysis has been stressed as a useful method to verify assumptions. We have described transformations and alternative regression methods that are employed when these assumptions are found to be in violation. Sequential selection procedures for the construction of multivariate models have been presented. The importance of assessing model performance has been underscored, most notably in terms of bias and precision. In summary, pharmacodynamic analyses are now commonly performed and reported in the oncologic literature. The content and format of these analyses has been variable. The goals of such analyses are to identify and describe pharmacodynamic relationships and, in many cases, to propose a statistical model. However, the appropriateness and performance of the proposed model are often difficult to judge. Table 1 displays suggestions (in a checklist format) for structuring the presentation of pharmacodynamic analyses, which reflect the topics reviewed in this paper.

Data Interpretation, Statistical↗
PubMed1987

The impact of advanced prehospital emergency care on the mortality of severely brain-injured patients.

The mortality of 128 consecutive severely brain-injured patients (Glasgow Coma Score less than or equal to 8) treated by a land advanced life support system and transported to a trauma center was compared to the mortality of 104 consecutive severely brain-injured patients treated during the same time period by an advanced care rotorcraft aeromedical emergency service and transported to the same trauma center. The two patient groups had statistically similar distribution of CNS lesions and Glasgow Coma Scores. The mortality of the patients treated by the land ALS system was 40%. The mortality of the patients treated by the aeromedical service was 31%. The 9% reduction in mortality in the patients treated by the aeromedical service was statistically significant (p less than 0.001). There was also a statistically significant (p less than 0.05) reduction in the Glasgow Outcome Scores of the patients treated by the aeromedical service compared with patients treated by the land advanced life support system.

Adult↗
PubMed1978

An economic analysis of migration in Mexico.

This paper analyzes internal migration in Mexico over the 1960-70 period. A model of the determinants of migration is specified and estimated for aggregated interstate migration flows. Results show that distance serves as a significant deterrent to migration, that higher destination earning levels are attractive to migrants, and that regions with high unemployment rates experience lower rates of inmigration. An unanticipated finding is that regions with higher earning levels have greater rates of outmigration. The data are disaggregated to examine separate migration relationships for each state. The results are that distance is a lesser deterrent for those migrants with more accessible alternatives, that higher earning levels reduce the deterring effects of distance, and that regions with higher earning levels have lower associated elasticities of migration. It is concluded that economic factors have played a crucial role in internal migration and thus in the changing occupational and geographic structure of the Mexican labor force.

Americas↗
PubMedDate not supplied

Ultrastructural immunostaining of infiltrating ductal breast carcinomas with the monoclonal antibody H: a comparative study with cytokeratin 8.

The monoclonal antibody H (mAbH) detects an epitope consisting of an O-linked N-acetyl glucosamine (O-GlcNAc) and neighboring amino acids. This epitope has been found by using extracts from the MCF-7 human breast carcinoma cell line in immunoblotting experiments, on cytokeratin 8 (CK8) and 5 other polypeptides. In the present study, a double immunogold method was applied for the colocalization of CK8 and mAbH epitope on epoxy thin sections in 18 cases of infiltrating ductal breast carcinomas (IDBC) and in 6 cases of fibroadenomas, to study the accurate subcellular distribution of CK8 in breast cancer cells, as compared to the 5 polypeptides, recognized by mAbH. Furthermore, a detailed quantitative evaluation of the double immunolocalization over the cellular compartments of cancer cells was undertaken with the aid of a computerized image analysis system and the results were assessed statistically. The distribution pattern of CK8 and the mAbH epitope in the neoplastic mammary epithelial cells was similar in IDBC as compared to fibroadenomas, while the gold labeling intensity of these epitopes differed over the cellular compartments between malignant and benign biopsies. The results reveal the significance of the role of CK8 and O-GlcNAc glycosylation in the biology of the neoplastic mammary cells in vivo, determining their malignant potential.

Acetylglucosamine↗
PubMed1995

[Spontaneous intracerebral hemorrhage: case histories].

Spontaneous intracerebral hemorrhage is a significant cause of morbidity and mortality. The disease primarily affects men between the age of 45 to 75 years. Arterial hypertension is the most frequent etiologic factor. Here is presented a case-record of 148 intracerebral hemorrhages observed during a five years period. Several clinical and radiological features have been correlated and analyzed from a statistical point of view. The most important prognostic factor is the state of consciousness. Either deep coma and a normal state of consciousness do not represent surgical indications.

Adult↗