SEARCH · PubMed Health

Results for “Statistics, Nonparametric”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 739 records · Page 41Linked to original sources

PubMed2001

Application of nonparametric models for calculating odds ratios and their confidence intervals for continuous exposures.

Calculating odds ratios and corresponding confidence intervals for exposures that have been measured using a continuous scale presents important limitations in the traditional practice of analytical epidemiology. Approximations based on linear models require making arbitrary assumptions about the shape of the relation curve or about its breakpoints. Categorical analyses generally have low statistical efficiency, and cutpoints for the categories are in most cases arbitrary and/or opportunistic. The use of logistic generalized additive models to calculate odds ratios does not require these assumptions and allows great flexibility and adequate statistical efficiency. Based on the asymptotic normality of the logarithm of the odds ratio, the authors propose the use of an approximate analytical expression for the corresponding covariance matrix, which will allow the construction of confidence intervals for odds ratios that can be interpreted as in the classical parametric context. The authors illustrate this procedure by examining the relation between glycemia and risk of postoperative infection, using data obtained from a cohort study of patients undergoing surgery in Santiago, Spain (January 1996--March 1997). The authors found that glycemia values below 75 mg/dl and above 130 mg/dl were associated with increased risk of postoperative infection.

Blood Glucose↗

PubMed1997

Probabilistic diagnosis in linkage analysis of bipolar disorder: putting weights on the fringe.

We explored the utility of probabilistic weighting of fringe phenotypes in linkage analysis of bipolar disorder for the GAW10 chromosome 18 data. Four liability classes were assigned probabilistic weights based on the estimated probability that the case was a true bipolar. The weights were incorporated in parametric and nonparametric, single and multipoint analyses.

Bipolar Disorder↗

PubMed2005

Nonparametric adjustment techniques for binary covariates.

Though a variety of reasons are often articulated for adjusting analyses for covariates, these reasons often fall into one of two general objectives, specifically to increase precision or to decrease bias. In practice, one does not generally choose between these objectives, because the methods that address one tend to address the other, as well. Because of this, no distinction is made in the methods used to correct for a baseline imbalance with respect to a prognostic covariate versus to ensure a fair comparison across treatment groups by making the comparisons within the levels of a prognostic covariate. Yet the literal translation of these two uses of covariate adjustment will lead to two distinct adjustment methods. We illustrate this divergence in the simplest case of a single binary covariate, a binary outcome, and two treatments, and we note that it is possible to combine the two approaches to derive yet a third approach. Each of these approaches is nonparametric and exact, and so it is the precise reason for adjusting that should dictate which would be used in any given situation.

Analysis of Variance↗

PubMed2004

Maxillary morphology in obstructive sleep apnea: a cephalometric and model study.

The relationship between maxillary constriction and the etiology of obstructive sleep apnea (OSA) is not clear. This prospective case-control study compared maxillary morphology in 94 dentate subjects (47 OSA and 47 control subjects), using upright lateral cephalograms and study models. Each subject had height, weight, and neck circumference measurements recorded and underwent an orthodontic examination. An upright lateral cephalogram and dental impressions were obtained. All data were analyzed using the SPSS statistical package applying nonparametric tests at the 5% level of significance. Male and female subjects were examined separately, and statistically significant differences were found between the cephalometric measurements for OSA and the control subjects. The palatal angle was more obtuse in male OSA subjects (P < .05). The PNS-posterior pharyngeal wall was shorter (P < .05) and the soft palate longer in female OSA subjects (P < .05). Minimum palatal airway widths were significantly reduced in both male (P < .01) and female (P < .001) subjects. In the comparison of study model measurements, palatal heights in OSA subjects were greater (P < .05). Thus, maxillary morphological differences do exist between OSA and control subjects, supporting their role as a etiological factor.

Adult↗

PubMed2002

Adjusting pairwise nonparametric equivalence hypothesis tests and confidence intervals for period effects in 3 x 3 crossover trials.

In pharmacokinetic and pharmacodynamic 3 x 3 crossover trials, average bioequivalence and noninferiority between treatments need to be only assessed pairwise in most cases. Due to the restricted number of subjects in such trials, normal distribution assumptions cannot be checked and frequently outliers are encountered, so that a nonparametric approach is more adequate. Therefore, to assess average bioequivalence or noninferiority, a new method is proposed to derive period adjusted nonparametric confidence intervals for pairwise treatment differences.

Chemistry, Pharmaceutical↗

PubMed1997

Flexible modeling of the effects of serum cholesterol on coronary heart disease mortality.

Current understanding of the impact of lipids and other risk factors on coronary heart disease is largely based on the results of parametric multiple regression analyses of large prospective studies. To assess the potential impact of the a priori assumption of linearity of continuous risk factors on the results of parametric analyses, the authors completed a secondary analysis of the Lipid Research Clinics Prevalence and Follow-up Studies (1972-1987) data using an assumption-free nonparametric modeling approach. The effects of total serum cholesterol and the ratio of total serum cholesterol to high density lipoprotein cholesterol, adjusted for common risk factors, were estimated using a smoothing spline method available in the generalized additive model extension of the multiple logistic regression. The data set included 2,512 men in the random sample of the Lipid Research Clinics study who did not take lipid-lowering medications. During the median follow-up of 12.6 years, 94 coronary heart disease deaths occurred. The generalized additive model fits the effects of total serum cholesterol (p < 0.01) and the ratio of total serum cholesterol to high density lipoprotein cholesterol (p < 0.02) significantly better than the parametric logistic regression. Validation studies confirmed that, among new observations arising from the same population, generalized additive model estimates predicted outcomes better than the parametric estimates. Nonlinear effects of both lipid measures were robust and may be clinically important. The authors conclude that the linearity assumption inherent in parametric models may result in biased estimates of the effects of total serum cholesterol on coronary heart disease mortality and recommend that their findings be verified in a nonparametric analysis of data from another large prospective study.

Cholesterol↗

PubMed2006

Assessing isotropy for spatial point processes.

A common assumption while analyzing spatial point processes is direction invariance, i.e., isotropy. In this article, we propose a formal nonparametric approach to test for isotropy based on the asymptotic joint normality of the sample second-order intensity function. We derive an L(2) consistent subsampling estimator for the asymptotic covariance matrix of the sample second-order intensity function and use this to construct a test statistic with a chi(2) limiting distribution. We demonstrate the efficacy of the approach through simulation studies and an application to a desert plant data set, where our approach confirms suspected directional effects in the spatial distribution of the desert plant species.

Anisotropy↗

PubMed2003

Microleakage in ceramic inlays luted with different resin cements.

PURPOSE: The objective of this in vitro study was to evaluate the microleakage in ceramic inlays using different resin cements with margins in enamel and cementum/dentin interfaces. MATERIALS AND METHODS: Standard Class II MOD inlay cavities were prepared in 32 noncarious human premolars. The cavities were randomly divided into 4 groups (n = 8): CONTROL GROUP: cavities were treated with Single Bond and incrementally filled with a composite resin (P60); Enforce group: feldspathic ceramic inlays were luted using Prime & Bond 2.1 and Enforce; RelyX group: inlays were cemented with Single Bond and RelyX ARC; Resin Cement group: ceramic inlays were bonded using Single Bond and Resin Cement. Ceramic inlays were previously treated with 10% hydrofluoric acid for 2 min, followed by silane application. After 7 days of storage in distilled water, teeth were submitted to thermocycling. After applying nail varnish, specimens were immersed in 2% aqueous solution of methylene blue for 8 h. After washing, teeth were cut into three sections through the restorations, and the leakage was assessed using a standardized score. RESULTS: Data were submitted to statistical analysis using nonparametric tests (Mann-Whitney and Kruskal-Wallis). Dye leakage at margins in enamel was statistically lower (p < 0.01) than at cementum/dentin interfaces. RelyX ARC performed better (p < 0.05) than resin cement (enamel) and composite restorations (cementum/dentin). No other statistical differences were observed. CONCLUSIONS: Both the material and the substrate interface influenced microleakage of the ceramic inlays.

Acid Etching, Dental↗

PubMed1997

Empirical evaluation of genome scans for linkage of a quantitative trait associated with a complex disorder.

Nonparametric sib-pair analysis (Haseman-Elston) was used to search for evidence of linkage between a putative locus for a complex quantitative trait Q1 and genome-wide markers (367 markers from 10 chromosomes) for the first 100 replicates of nuclear family data. The characteristics of the statistically positive linkage results [the magnitude of p-values (p), the number of supporting flanking markers, and the percentage of positive replicates] were compared for true linkage (major and minor genes) and false positive evidence for linkage. Discriminant analysis was used to evaluate which characteristics of these statistically positive linkage results are good indicators to discriminate true linkage from false positive evidence for linkage. Sensitivity and false positive rates of several proposed criteria for linkage, as well as the criteria based on our results were evaluated. The relationship between the map location of the marker with the lowest p-value and the map location of the true underlying gene was also evaluated, which provided useful information for fine mapping and replication studies.

Age Distribution↗

PubMed2005

A new approach to correct the QT interval for changes in heart rate using a nonparametric regression model in beagle dogs.

INTRODUCTION: Over- and/or under-correction of QT intervals for changes in heart rate may lead to misleading conclusions and/or masking the potential of a drug to prolong the QT interval. This study examines a nonparametric regression model (Loess Smoother) to adjust the QT interval for differences in heart rate, with an improved fitness over a wide range of heart rates. METHODS: 240 sets of (QT, RR) observations collected from each of 8 conscious and non-treated beagle dogs were used as the materials for investigation. The fitness of the nonparametric regression model to the QT-RR relationship was compared with four models (individual linear regression, common linear regression, and Bazett's and Fridericia's correlation models) with reference to Akaike's Information Criterion (AIC). Residuals were visually assessed. RESULTS: The bias-corrected AIC of the nonparametric regression model was the best of the models examined in this study. Although the parametric models did not fit, the nonparametric regression model improved the fitting at both fast and slow heart rates. DISCUSSION: The nonparametric regression model is the more flexible method compared with the parametric method. The mathematical fit for linear regression models was unsatisfactory at both fast and slow heart rates, while the nonparametric regression model showed significant improvement at all heart rates in beagle dogs.

Administration, Oral↗

PubMed2004

Analyzing psychopathology items: a case for nonparametric item response theory modeling.

The authors discuss the applicability of nonparametric item response theory (IRT) models to the construction and psychometric analysis of personality and psychopathology scales, and they contrast these models with parametric IRT models. They describe the fit of nonparametric IRT to the Depression content scale of the Minnesota Multiphasic Personality Inventory--2 (J. N. Butcher, W. G. Dahlstrom, J. R. Graham, A. Tellegen, & B. Kaemmer, 1989). They also show how nonparametric IRT models can easily be applied and how misleading results from parametric IRT models can be avoided. They recommend the use of nonparametric IRT modeling prior to using parametric logistic models when investigating personality data.

Adult↗

PubMed2003

Estimating the accuracy of polymerase chain reaction-based tests using endpoint dilution.

Polymerase chain reaction (PCR)-based tests for various microorganisms or target DNA sequences are generally acknowledged to be highly "sensitive," yet the concept of sensitivity is ill-defined in the literature on these tests. We propose that sensitivity should be expressed as a function of the number of target DNA molecules in the sample (or specificity, when the target number is 0). However, estimating this "sensitivity curve" is problematic, since it is difficult to construct samples with a fixed number of targets. Nonetheless, using serially diluted replicate aliquots of a known concentration of the target DNA sequence, we show that it is possible to disentangle random variations in the number of target DNA molecules from the underlying test sensitivity. We develop parametric, nonparametric, and semiparametric (spline-based) models for the sensitivity curve. The methods are compared on a new test for M. genitalium.

Biometry↗

PubMed2005

A clinical trial of patient satisfaction and prosthodontic needs with ball and bar attachments for implant-retained complete overdentures: three-year results.

STATEMENT OF PROBLEM: Few prospective trials of implant-retained mandibular dentures have evaluated the increase and duration of patient satisfaction, costs of denture maintenance in relation to different methods of attaching overdentures to implants, or the use of a reinforced framework. PURPOSE: This report evaluates subjects' satisfaction and prosthodontic maintenance during a 3-year randomized clinical trial of implant-retained mandibular complete dentures, whether reinforced or not with a cast framework, and attached by bar-clip or 2.25-mm ball-spring matrices to endosteal dental implants. MATERIALS AND METHODS: One hundred edentulous subjects, each having at least 1 year's experience with conventional complete dentures, were selected from respondents to a university dental clinic's request for volunteers. Candidates were examined to verify adequate mandibular bone and medical suitability for implants. Subjects then received 2 implants in the anterior mandible before being stratified by mandibular bone height and gender and assigned randomly to 1 of 4 treatment groups. Every subject received a new maxillary complete denture in addition to an implant-supported mandibular complete denture, with or without a reinforcing framework, connected to implants by either a bar-clip or a ball-spring patrix and matrix. The dentures were adjusted and repaired as needed. Subjects indicated on a visual analogue scale (VAS) satisfaction with conventional dentures prior to the study and then with new dentures at 1 month, 1 year, and 2 years. The results reported here are from the first 68 subjects observed for 3 years after receiving new dentures (19 subjects received new dentures less than 3 years before this analysis, and another 13 subjects were lost to follow-up). VAS scores are presented in simple tables and graphs, and results for different groups were compared using 2-sided nonparametric rank tests and repeated measures ANOVA. With respect to costs and maintenance, t tests were used to compare group means. Sample size and other design considerations used a .05 significance level. RESULTS: After receiving new dentures with mandibular implant supports, improved satisfaction "within subject" was prompt, durable, substantial, and statistically significant, regardless of the attachment mechanism, and with or without a reinforcing framework. In contrast, there were no notable satisfaction differences between the 2 attachment mechanisms, or with the presence or absence of a reinforcing framework, either at specific intervals after receiving the new dentures, or in repeated measures ANOVA. For both attachment groups, most denture adjustments occurred during the first year. This accounted for 81% of total adjustments during 3 years, when the 34 subjects in the ball-spring group and the 34 in the bar-clip group were combined. The mean numbers of adjustments per subject and associated clinical times did not differ significantly between the 2 groups. Conversely, denture repairs declined more slowly than adjustments. Almost all repairs (90%) occurred in the ball-spring group to correct problems with the attachments, 39% in the first year, and tapering off only slightly in the following 2 years. Over 3 years of follow-up, mean numbers of repairs per subject differed significantly between groups: 6.7 repairs per person in the ball-spring group, compared to 0.8 in the bar-clip group ( P<.001), and mean time per appointment was greater for repairs in the ball-spring group: 18.9 minutes compared to 16.9 ( P<.01). The cast framework had no influence on the satisfaction expressed or on adjustments and repairs. CONCLUSION: Subjects were very satisfied with the new dentures, although the ball-spring attachment tested in this trial required substantially more repairs.

Analysis of Variance↗

PubMed2002

Evaluation of microleakage in cervical margins of various posterior restorative systems.

PURPOSE: The purpose of this in vitro study was to evaluate the microleakage and dye penetration patterns of various restorative systems in the cervical margins of Class II cavities using the dye penetration technique. MATERIALS AND METHODS: Ninety bovine incisors were selected and part of the crown was sectioned producing a standardized occlusogingival margin of 4 mm. Standardized Class II box cavities were designed on the mesial (MO) and distal (OD) surfaces with the gingival margin in dentin. The cavities were randomly divided into six groups (n = 30): group 1 (control), Etch & Prime 3.0 (Degussa AG) + Definite (Degussa AG); group 2, Prime & Bond NT (Dentsply De Trey) + Definite; group 3, Etch & Prime 3.0 + flowable composite resin Wave (SDI) + Definite; group 4 (control), Prime & Bond NT + TPH Spectrum (Denstply De Trey); group 5, Etch & Prime 3.0 + TPH Spectrum; group 6, Prime & Bond NT + Wave + TPH Spectrum. After they were finished and polished, the specimens were subjected to 1500 thermal cycles in baths of 5 degrees+/-2 degrees C and 55 degrees+/-2 degrees C with 60 seconds of dwell time for each bath. After the thermocycling, the specimens were sealed with acid-resistant varnish, leaving a 1-mm window around the cervical margin interface, and immersed in 2% methylene blue buffered solution for 4 hours. Then the specimens were sectioned longitudinally in half and qualitatively evaluated under stereomicroscopy at 45 times magnification, using ranked scores for the dentin cervical margin (0-4). The dye penetration pattern (adhesive or dentinal) was also evaluated. The data were subjected to a reproducibility kappa test, and the mean of the three examiners' scores was analyzed by the Kruskal-Wallis nonparametric test and a multiple comparison nonparametric test. Fisher's exact test was used to evaluate the dye penetration pattern. RESULTS: The rank sums of the groups were expressed in decreasing order: group 4, 67.23a; group 3, 72.58ab; group 6, 78.43ab; group 5, 93.19bc; group 2, 112.97c; group 1, 113.52c. (Statistically significant differences are expressed by different superscript letters.) The results indicate that none of the systems used in this study was able to impede microleakage, and that the use of a flowable resin decreases the values of leakage only for Definite composite resin. The dye penetration pattern for the groups that used Prime & Bond NT was predominantly dentinal, whereas the pattern for Etch & Prime 3.0 groups was adhesive. CLINICAL SIGNIFICANCE: An ormocer-based resin that was developed to improve the physical properties of composite resin and a self-etching primer adhesive system obtained higher values of microleakage compared with a one-bottle adhesive system and a microhybrid composite resin. The flowable composite resin decreased the values of microleakage only for the ormocer-based resin.

Animals↗

PubMed2005

A simple nonparametric multipoint procedure to test for linkage through mothers or fathers as well as imprinting effects in the presence of linkage.

A simple multipoint procedure to test for parent-of-origin effects in samples of affected siblings is discussed. The procedure consists of artificially changing all full sibs to half-sibs, with distinct mothers or fathers depending on the parental origin to be evaluated, then analyzing these families with commonly used statistics and software. The procedure leads to tests for linkage through mothers or fathers and also leads to a test for imprinting effects in the presence of linkage. Moreover, simulations illustrate that in regions unlinked to susceptibility genes this multipoint procedure does not have an inflated type I error if a sex-averaged genetic map is used, even when large differences exist between male-specific and female-specific maps. In regions linked with susceptibility genes, the test of imprinting is biased under the null hypothesis if differences exist between sex-specific maps, irrespective of the map used in the analysis. The procedure is applied to the Collaborative Study on the Genetics of Alcoholism dataset from the Genetic Analysis Workshop 14. Results indicate that brothers categorized as affected according to the DMS-III-R and Feighner classification show evidence of linkage through fathers to the 6q25 region (p = 0.00038) as well as modest evidence of imprinting (p = 0.018). This region harbors OPRM1, a candidate gene for substance dependence.

Chromosome Mapping↗

PubMed2005

Laryngeal biomechanics in Middle Eastern singing.

UNLABELLED: We would like to describe the muscle tension patterns observed in Middle Eastern singing and correlate these findings with demographic data. MATERIAL: A total of 42 candidates were included in this study. The medical charts and video records of their fiberoptic nasopharyngeal laryngoscopy were reviewed. Demographic information included age, sex, history of smoking, history of reflux, history of voice overuse/abuse, status in singing (professional vs. amateur), and style of singing (classical vs pop). Each frame was analyzed for each of the four muscle tension patterns (MTPs): MTP I, MTP II, MTP III, and MTP IV. STATISTICAL METHOD: Nonparametric Mann-Whitney U test was conducted. RESULTS: Twenty-six subjects were professional singers and half sang classical singing. The mean MTP total score for all subjects was 35.58 +/- 21.98% (+/- standard deviation). The highest muscle tension score was obtained for type III pattern followed by type II. There were no statistical differences in the mean MTP total, MTP II, and MTP III scores among the professional singers versus the amateurs. Similar results were obtained when MTP scores were compared by style of singing. CONCLUSION: The vocal technique and status of singing did not seem to affect the muscle tension score.

Adolescent↗

PubMed2003

The efficacy of dantrolene sodium in controlling exertional rhabdomyolysis in the Thoroughbred racehorse.

REASONS FOR PERFORMING STUDY: Dantrolene sodium (Dantrium) has been used extensively for the treatment of myopathies in man and anecdotal evidence suggests it is of clinical benefit in the control of exercise-induced rhabdomyolysis (ER) in racehorses, although data to support this are currently lacking. OBJECTIVES: To investigate the efficacy of oral dantrolene sodium in controlling ER in a randomised, double-blind, placebo-controlled crossover trial involving 77 Thoroughbred racehorses in Newmarket, UK. METHODS: Horses were treated on 2 occasions 1 week apart, with treatment days coinciding with a return to exercise following 2 days box rest on each occasion. For the first treatment, each horse was randomly selected to receive either 800 mg dantrolene sodium or a colour-matched placebo administered orally 1 h before exercise. This was followed by crossover to the other treatment on the second occasion, with each horse thereby acting as its own control. Degree of ER was assessed using rising serum creatine kinase (CK) levels, by subtracting pre-exercise blood CK levels from those measured in 6 h post exercise blood samples. For each horse, the difference in change between pre- and post exercise CK values between placebo and dantrolene treatments was calculated, with positive values indicating a greater rise with placebo than with dantrolene sodium treatment. RESULTS: The overall mean difference for all horses was +104.8 iu/l and the null hypothesis, that there was no true difference in non-normally distributed post exercise rises in CK values between placebo and dantrolene treatments, was rejected (P = 0.0013) using the nonparametric Wilcoxon signed rank test. Additionally, no horses given dantrolene sodium showed clinical signs of ER, whereas 3 horses given the placebo developed ER following exercise. The incidence of ER in the study was 4% (3/77). CONCLUSIONS: The results confirmed that oral administration of dantrolene sodium, 1 h before exercise, had a statistically significant effect on reducing the difference between pre- and post exercise plasma CK levels compared with a placebo in the same animals, and preventing clinical ER in susceptible individuals. POTENTIAL RELEVANCE: This study suggested that dantrolene sodium is of use in controlling ER in the Thoroughbred racehorse. Further investigation into pre- and post exercise myoplasmic calcium levels and the repeat of the study late in the season when horses receive a much higher energy ration and more strenuous exercise would appear to be warranted.

Administration, Oral↗

PubMed2006

Nonparametric identification of petrogenic and pyrogenic hydrocarbons in aquatic ecosystems.

Novel nonparametric models developed herein discriminated between oiled and nonoiled or pyrogenic and oiled sources better than traditionally used diagnostic ratios and can outperform previously published oil identification models. These methods were compared using experimental and environmental hydrocarbon data (sediment, mussels, water, and fish) associated with the Exxon Valdez oil spill. Several nonparametric models were investigated, one designed to detect petroleum in general, one specific to Alaska North Slope crude oil (ANS), and one designed to detect pyrogenic PAH. These ideas are intended as guidance; nonparametric models can easily be adapted to fit the specific needs of a variety of petrogenic and pyrogenic sources. Oil identification was clearly difficult where composition was modified by physical or biological processes; model results differed most in these cases, suggesting that a multiple model approach to source discrimination may be useful where data interpretation is contentious. However, a combined nonparametric model best described a broad range of hydrocarbon sources, thus providing a useful new analytical assessment tool.

Alaska↗