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At least 973 records · Page 54Linked to original sources
PubMed2000

The effect of collapsing multinomial data when assessing agreement.

BACKGROUND: In epidemiological studies researchers often depend on proxies to obtain information when primary subjects are unavailable. However, relatively few studies have performed formal statistical inference to assess agreement among proxy informants and primary study subjects. In this paper, we consider inference procedures for studies of interobserver agreement characterized by two raters and three or more outcome categories. Of particular interest is the consequence of dichotomizing such data on the expected confidence interval width for the kappa coefficient. The effect of dichotomization on sample size requirements for testing hypotheses concerning kappa is also evaluated. METHODS: Simulation studies were used to compare coverage levels and widths for constructing confidence intervals. Sample size requirements were compared for multinomial and dichotomous data. We illustrate our results using a published data set on drinking habits that assesses agreement among primary and proxy respondents. RESULTS: Our results show that when multinomial data are treated as dichotomous, not only do the expected confidence interval widths become greater, but the penalty in terms of larger sample size requirements for hypothesis testing can be severe. CONCLUSION: We conclude that there are clear advantages in preserving multinomial data on the original scale rather than collapsing the data into a binary trait.

Alcohol Drinking↗
PubMed2006

Comparative phylogeographic summary statistics for testing simultaneous vicariance.

Testing for simultaneous vicariance across comparative phylogeographic data sets is a notoriously difficult problem hindered by mutational variance, the coalescent variance, and variability across pairs of sister taxa in parameters that affect genetic divergence. We simulate vicariance to characterize the behaviour of several commonly used summary statistics across a range of divergence times, and to characterize this behaviour in comparative phylogeographic datasets having multiple taxon-pairs. We found Tajima's D to be relatively uncorrelated with other summary statistics across divergence times, and using simple hypothesis testing of simultaneous vicariance given variable population sizes, we counter-intuitively found that the variance across taxon pairs in Nei and Li's net nucleotide divergence (pi(net)), a common measure of population divergence, is often inferior to using the variance in Tajima's D across taxon pairs as a test statistic to distinguish ancient simultaneous vicariance from variable vicariance histories. The opposite and more intuitive pattern is found for testing more recent simultaneous vicariance, and overall we found that depending on the timing of vicariance, one of these two test statistics can achieve high statistical power for rejecting simultaneous vicariance, given a reasonable number of intron loci (> 5 loci, 400 bp) and a range of conditions. These results suggest that components of these two composite summary statistics should be used in future simulation-based methods which can simultaneously use a pool of summary statistics to test comparative the phylogeographic hypotheses we consider here.

Classification↗
PubMed1997

Epidemiological study of the genetics and environment of asthma, bronchial hyperresponsiveness, and atopy: phenotype issues.

The Epidemiological Study of the Genetics and Environment of Asthma (EGEA) combined a case-control study and a family study. The total sample of 1,854 consisted of 348 patients with asthma selected through chest clinics and 416 control subjects and nuclear families ascertained through the cases. The protocol included standardized questionnaires, bronchial responsiveness, allergen skin-prick tests according to international protocols, total serum immunoglobulin E (IgE) level measurements, and blood eosinophilia. Criteria used to select subjects with asthma and determine asthma status of relatives for affected sibling pair linkage analysis are described. Based on figures from the 348 asthma cases of the EGEA study, issues relative to the definition of severe asthma and intermediate phenotypes such as bronchial responsiveness and allergic markers are discussed. Given the phenotypic heterogeneity involved, relevant phenotypes that may lead to the detection of genetic factors will depend on the hypothesis tested. Standardization of primary data and subphenotypes is a prerequisite for pooling data, which will be needed in the future to better understand the genetics and environmental factors of asthma.

Adolescent↗
PubMed1987

Aging, alcoholism, and performance on the Luria-Nebraska Neuropsychological Battery.

The Luria-Nebraska Neuropsychological Battery (LNNB) was used to test the hypothesis that alcoholism produces premature or accelerated aging. This hypothesis was tested by the interaction between age (in individuals ranging from 30 to 76 years of age) and alcoholism (nonalcoholics vs. alcoholics). No evidence was observed in support of the hypothesis. Additionally, the differences in LNNB performance found among age groups were different from those found between the alcoholics and nonalcoholics. This suggests that different mechanisms may be associated with age-related psychological deficits as compared with psychological dysfunction associated with alcoholism.

Adult↗
PubMed2007

Time preferences for health in northern Tanzania: an empirical analysis of alternative discounting models.

AIM: The discounted utility (DU) model has dominated economic evaluation for almost 7 decades, despite the fact that important assumptions of the model are commonly found to be violated. This paper formally explores whether the key assumption of stationarity is violated in a sample of the general population of Northern Tanzania. Furthermore, three hyperbolic discounting models are fitted to the data, and whether they perform better than the DU model in predicting individuals' time preferences is tested using nonlinear least squares regression. METHOD: The data were collected from 450 households by trained enumerators. The individual data on time preferences were collected by structured interviews using an open-ended stated preference methodology. Respondents marked a rating scale to indicate the maximum number of days they would be willing to suffer a nonfatal disease if the outbreak of the disease could be delayed to a point further into the future. Households were randomised to answer questions framed to elicit either a private or social time preference. RESULTS: Hypothesis testing confirmed decreasing time aversion and a magnitude effect, suggesting that the DU model is inappropriate as a descriptive tool. When the DU model was compared with the three hyperbolic discounting models by analysing the discount factor using nonlinear least squares regression, the most important findings were that a variable for starting point was nonsignificant only for the Loewenstein and Prelec (L&P) and the Mazur models, and that people in this setting generally discounted future health far more than suggested by current discounting practice in economic evaluations. CONCLUSION: The time preferences of our sample are better represented by the L&P and the Mazur models (which allow relaxation of the stationarity assumption through a modification of the expression for the discount factor) and less well reflected by the Harvey (a modification of the L&P model that assigns more importance to the future than standard utility discounting) and DU models. This implies that, from the point of view of a consumer sovereignty-friendly economist, the Mazur and the L&P models are preferable for discounting of future health in economic evaluations. However, from the point of view of other value bases for discounting the choice of discounting model is of less importance.

Aged↗
PubMed1979

III. Profile, expectation and satisfaction of outpatients (1970-1972).

The present study deals with patients' assessment of the OPD and the experimental changes in terms of "patient satisfaction". The hypothesis tested, stated that the level of satisfaction of clients of the OPD at Machakos hospital would increase after the introduction of changes in the operational and technical procedures. Data on clients' profiles and satisfaction were collected by means of structured interview schedule; 24 background variables and 17 satisfaction indicators were selected for final analysis to compare profiles and satisfaction of 324 baseline and 367 evaluation respondents. Profiles and satisfaction differed significantly at the 5% level. The significant difference in satisfaction could not be explained by the difference in profiles; the hypothesis was confirmed. A detailed analysis showed that total clinic time (the total time patients spent in the clinic) was significantly correlated with some of the 24 background variables and with some of the 17 satisfaction indicators; its duration was one of the key determinants for patient satisfaction, with a highly discriminative value. Another key variable was the answer to the question: "Did you get the treatment you wanted?". OPD patients discriminated various aspects and facets of OPD process and procedures for their assessment.

Consumer Behavior↗
PubMed1992

A retrospective assessment of the 75/75 rule in bioequivalence.

The 75/75 rule was originally proposed by the U.S. Food and Drug Administration (FDA) as an alternative means of testing the bioequivalence of two formulations of a pharmaceutical agent. The rule specified that the ratio of test-to-reference formulation of a bioavailability measure arising in a bioequivalence study must be between 75 and 125 per cent of unity in at least 75 per cent of subjects to declare two formulations bioequivalent. The rule has garnered criticism in the literature and the FDA no longer uses the rule formally in assessing bioequivalence. The basis, however, for all criticism of the rule has been simulation arguments. In this paper, we derive the sampling model implied by the rule and place the rule in the framework of a statistical hypothesis test. We show how the significance level of the test depends upon variability of the formulations, and thus why the rule performs in the way that has received criticism.

Evaluation Studies as Topic↗
PubMed2001

Vagal nerve stimulation does not unkindle seizures.

The purpose of this study was to investigate a mechanism of action for the effect of vagal nerve stimulation on reducing seizures in patients with complex partial epilepsy. The hypothesis tested was that vagal nerve stimulation has an antikindling effect on epilepsy. The databases of two large clinical trials (E03, E05) were accessed, and statistical methods were applied using logarithmic transforms and regression analysis. Two parameters--duration of a patient's epilepsy before entering the clinical trial and the patient's seizure density before entering the clinical trial--were used as markers of subsequent seizure control during vagal nerve stimulation. In general, there was not a good fit to the regression lines, and the slope of the lines did not conform to the hypothesis. The hypothesis that vagal nerve stimulation may unkindle epileptic seizures was not supported.

Epilepsy, Complex Partial↗
PubMed2003

Optimal data selection: revision, review, and reevaluation.

Since it first appeared, there has been much research and critical discussion on the theory of optimal data selection as an explanation of Wason's (1966,1968) selection task (Oaksford & Chater, 1994). In this paper, this literature is reviewed, and the theory of optimal data selection is reevaluated in its light. The information gain model is first located in the current theoretical debate in the psychology of reasoning concerning dual processes in human reasoning. A model comparison exercise is then presented that compares a revised version of the model with its theoretical competitors. Tests of the novel predictions of the model are then reviewed. This section also reviews experiments claimed not to be consistent with optimal data selection. Finally, theoretical criticisms of optimal data selection are discussed. It is argued either that the revised model accounts for them or that they do not stand up under analysis. It is concluded that some version of the optimal data selection model still provides the best account of the selection task. Consequently, the conclusion of Oaksford and Chater's (1994) original rational analysis (Anderson, 1990), that people's hypothesis-testing behavior on this task is rational and well adapted to the environment, still stands.

Cognition↗
PubMed2005

Testing "species pair" hypotheses: evolutionary processes in the lichen-forming species complex Porpidia flavocoerulescens and Porpidia melinodes.

Pairs of taxa are commonly found in lichen-forming ascomycetes that differ primarily in their reproductive modes: one taxon reproduces sexually, the other vegetatively. The evolutionary processes underlying such "species pairs" are unknown. The species pair formed by Porpidia flavocoerulescens (sexual) and Porpidia melinodes (vegetative) was chosen to investigate four previously proposed hypotheses. These hypotheses posit that species pairs are either two monophyletic, independently evolving species with contrasting reproductive mode; a single outcrossing species polymorphic with regard to its reproductive modes; a sexual mother lineage frequently giving rise to asexual spin-offs; or a complex of cryptic species. The phylogenetic patterns observed within the species pair in the present study were analyzed using stringent hypothesis testing and visualizations of relationships and conflict based on tree and network reconstructions. DNA sequences at the three analyzed loci revealed the same four to five deeply divergent lineages. A detailed analysis of DNA-sequence variability revealed closely linked gene loci, but high levels of conflict within each of the gene fragments, as well as between observed genetic lineages. The observed patterns of phylogenetic relationships, linkage, and conflict are not congruent with any of the previously proposed species pair hypotheses. Rather, it is proposed that the observed results can be explained by conflicting reproductive and nutritional requirements imposed by an obligate symbiotic lifestyle. These interacting constraints produce recurring selective sweeps within predominantly vegetatively reproducing lineages and are the main forces that shape the evolution within the investigated species pair.

Base Sequence↗
PubMed2006

Determining the degree of cortical bone asymmetry in bilateral, nonpathological, human femur pairs.

When testing the effects of a femoral component on cortical bone following total hip arthroplasty, the patient's implanted femur is often compared with his/her contralateral nonimplanted femur, with differences attributed to the femoral component. However, if normal anatomical differences exist between bilateral femurs, they need to be quantified in order to validate whether the differences between implanted and nonimplanted bilateral femurs are due to the implant or possibility due to intrinsic differences before implantation. This study quantified the geometric properties of cortical bone shape between seven pairs of bilateral, cadaveric, human femurs. The null hypothesis tested stated that the bilateral femurs would not be significantly different in cortical bone geometry. Digitized images of cortical bone cross-sections taken at percent biomechanical lengths (levels 1-8) were used to calculate bone geometry measurements. The paired t-test showed that the only significant difference was in the location of principal axes at the most proximal location, level 1 (p = 0.015). All other measurements and levels were not significant with percent differences less than 6.6%. In conclusion, the data supports attributing cortical bone shape differences between implanted and contralateral nonimplanted femurs in levels 2-8 to the presence of the implant when the significant differences are greater than 6.6%.

Adult↗
PubMed2001

Reliability and validity testing of the revised 12-item Short-Form Health Survey in older adults.

The SF-12 Short Form Health Survey was developed to describe mental and physical health status of adults, and to measure the outcomes of health care services. The purpose of this study was to test the reliability and validity of this measure in a sample of 185 older adults living in a continuing care retirement community. The mean age of the participants was 86 + 6.1 and the majority were female (82%), unmarried (82%), Caucasian (99%), and all had at least a high school education. Initial testing of the SF-12 measurement model as proposed by Ware, Kosinski, and Keller (1995) demonstrated that the model did not fit the data (chi-squared/df = 7.8). The model was revised based on modification indices and the revised model provided evidence of reliability (alpha coefficients of .70, and .89; the majority of the R2 values were greater than .50), and validity as there was improved fit of the model to the data (chi 2 difference of 123, df difference of 1.0, p < .05). There was also evidence of construct validity, based on hypothesis testing, with a statistically significant correlation between physical health and number of chronic illnesses (r = .33, p < .05) and mental health and chronic illnesses (r = .27, p < .05), and a statistically significant difference in physical health (F = 30.5, p < .05) and mental health (F = 18.5, p < .05) between those who exercise regularly and those who do not. The revised SF-12 is a reliable and valid measure of health status in independent living older adults, and has the potential for use as either a predictor or an outcome measure.

Activities of Daily Living↗
PubMed2003

Statistical analysis of noninferiority trials with a rate ratio in small-sample matched-pair designs.

Testing of noninferiority has become increasingly important in modern medicine as a means of comparing a new test procedure to a currently available test procedure. Asymptotic methods have recently been developed for analyzing noninferiority trials using rate ratios under the matched-pair design. In small samples, however, the performance of these asymptotic methods may not be reliable, and they are not recommended. In this article, we investigate alternative methods that are desirable for assessing noninferiority trials, using the rate ratio measure under small-sample matched-pair designs. In particular, we propose an exact and an approximate exact unconditional test, along with the corresponding confidence intervals based on the score statistic. The exact unconditional method guarantees the type I error rate will not exceed the nominal level. It is recommended for when strict control of type I error (protection against any inflated risk of accepting inferior treatments) is required. However, the exact method tends to be overly conservative (thus, less powerful) and computationally demanding. Via empirical studies, we demonstrate that the approximate exact score method, which is computationally simple to implement, controls the type I error rate reasonably well and has high power for hypothesis testing. On balance, the approximate exact method offers a very good alternative for analyzing correlated binary data from matched-pair designs with small sample sizes. We illustrate these methods using two real examples taken from a crossover study of soft lenses and a Pneumocystis carinii pneumonia study. We contrast the methods with a hypothetical example.

Biometry↗
PubMed1999

Multiresolution analysis of emission tomography images in the wavelet domain.

This article develops a theoretical framework for the use of the wavelet transform in the estimation of emission tomography images. The solution of the problem of estimation addresses the equivalent problems of optimal filtering, maximum compression, and statistical testing. In particular, new theory and algorithms are presented that allow current wavelet methodology to deal with the two main characteristics of nuclear medicine images: low signal-to-noise ratios and correlated noise. The technique is applied to synthetic images, phantom studies, and clinical images. Results show the ability of wavelets to model images and to estimate the signal generated by cameras of different resolutions in a wide variety of noise conditions. Moreover, the same methodology can be used for the multiscale analysis of statistical maps. The relationship of the wavelet approach to current hypothesis-testing methods is shown with an example and discussed. The wavelet transform is shown to be a valuable tool for the numerical treatment of images in nuclear medicine. It is envisaged that the methods described here may be a starting point for further developments in image reconstruction and image processing.

Algorithms↗
PubMed1998

Growth hormone, fatigue, poor sleep, and disability in HIV infection.

Poor sleep, daytime fatigue, and loss of cognitive ability exist during all stages of HIV infection, worsening with disease progression. These symptoms contribute to disability and poor quality of life. Data from several research groups support a role of somnogenic inflammatory process peptides elevated in HIV infection, e.g. TNF alpha. Though the literature is in conflict regarding an effect of HIV infection on growth hormone (GH) secretion, GH axis dysregulation and treatment with GH may be important in HIV infection, e.g. in the wasting syndrome. It has long been known that GH varies with changes in sleep. The hypothesis tested in the current study was that the relationship between delta frequency (0.5-4.0 Hz) sleep EEG amplitude (square root of power from frequency analysis) and GH secretion would differ between HIV positive (HIV+) and HIV negative (HIV-) subjects. In 14 subjects (6 HIV+ and 8 HIV-, none with current or past AIDS-defining illness) a linear relationship change across the night's sleep was found in the coupling between delta frequency sleep EEG amplitude and GH secretion. The phase coupling change was in opposite directions in HIV+ versus HIV- subjects. This difference supports the hypothesis that the brain-based coordination of sleep and sleep-related physiology deteriorates early in HIV infection, and that GH dysregulation may contribute to this sleep pathology.

Adult↗
PubMed1999

The distribution of Schistosoma bovis Sonsino, 1876 in relation to intermediate host mollusc-parasite relationships.

Schistosoma bovis is a digenean platyhelminth that is responsible for a parasitic disease called schistosomiasis or bilharziasis in bovines. It has a natural wide mollusc intermediate host spectrum and is compatible, experimentally, with a wide range of species. Our working hypothesis is that the Mediterranean Sea and the Sahara were two physical barriers that could have separated the populations of S. bovis in three parts and may have played a role in gene flow. Experimental data were collected from earlier published studies, and the different intermediate host spectra and the mollusc-parasite geographical compatibilities were compared between the North Mediterranean zone, the South Mediterranean zone and the South Saharan zone. From our results, the three major groups of S. bovis populations that could be determined were the Iberian, the Mediterranean and the South Saharan populations. Our tested hypothesis was thus not confirmed concerning the Mediterranean sea barrier but was confirmed with the Saharan one. A paleogeographical scenario of S. bovis is proposed following three major steps from a South Saharan origin to a possible local adaptation of the parasite in the Iberian Peninsula.

Africa↗
PubMed1986

Energy balance in low birth weight infants fed formula of high or low medium-chain triglyceride content.

We tested the effect on energy balance of the partial substitution of medium-chain for long-chain triglycerides in the diet of growing low birth weight infants. Fifteen infants were studied in a randomized double-blind crossover clinical trial in which each infant was fed each of two formulas, which were of equal gross energy and protein content but differed in fat composition. The high medium-chain triglyceride (MCT) formula contained medium- and long-chain triglycerides in a weight/weight ratio of 46:54; in the low MCT formula the ratio was 4:96. The hypothesis tested was that under conditions of equal gross energy intake the two diets would differ in their digestible and metabolizable energy contents and would produce differences in the infants' rates of energy expenditure and energy storage. Gross energy intakes averaged 562 and 555 kJ/kg/day (134 and 133 kcal/kg/day) with the high and low MCT formulas, respectively. With each diet, coefficients of energy digestibility (0.93) and metabolizability (0.91) were identical; the rates of energy expenditure were 262 (high MCT) and 265 (low MCT) kJ/kg/day (63 kcal/kg/day for both diets), and of energy storage were 246 (high MCT) and 239 (low MCT) kJ/kg/day (59 and 57 kcal/day). These differences were not significant. There were also no significant differences between the two diets in coefficients of nitrogen retention (mean 0.70) or in rate of weight gain (mean 21.5 gm/kg/day). The use of high MCT content in infant formula neither provided a nutritional advantage in energy digestibility or metabolizability nor resulted in an increased rate of energy expenditure or of energy storage.

Body Weight↗
PubMed2000

Testing the significance of microorganism identification by mass spectrometry and proteome database search.

We derive and validate a simple statistical model that predicts the distribution of false matches between peaks in matrix-assisted laser desorption/ionization mass spectrometry data and proteins in proteome databases. The model allows us to calculate the significance of previously reported microorganism identification results. In particular, for deltam = +/-1.5 Da, we find that the computed significance levels are sufficient to demonstrate the ability to identify microorganisms, provided the number of candidate microorganisms is limited to roughly three Escherichia coli-like or roughly 10 Bacillus subtilis-like microorganisms (in the sense of having roughly the same number of proteins per unit-mass interval). We conclude that, given the cluttered and incomplete nature of the data, it is likely that neither simple ranking nor simple hypothesis testing will be sufficient for truly robust microorganism identification over a large number of candidate microorganisms.

Bacillus subtilis↗