SEARCH · PubMed Health

Results for “Model performance”

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 1,279 records · Page 71Linked to original sources

PubMed1997

Validation of a regression model for standardizing lifetime racing performances of thoroughbreds.

OBJECTIVE: To determine the relationship between prediction errors of a regression model of racing finish times and earnings or finish position; the relationship between standardized finish times, determined by use of this model, and earnings or finish position; and whether this model was valid when applied to data for horses that underwent surgical treatment. DESIGN: Survey. SAMPLE POPULATION: Records of 6,700 healthy Thoroughbreds racing in Louisiana and of 31 Thoroughbreds with idiopathic left laryngeal hemiplegia that underwent surgical treatment. PROCEDURE: Predicted and standardized finish times were calculated by use of the regression model for healthy horses, and the relationships between prediction error (actual--predicted finish time) and standardized finish times, and earnings and finish position, were examined. Then, the regression model was applied to data for horses with hemiplegia to determine whether the model was valid when used to calculate predicted and standardized finish times for lifetime performance data. RESULTS: Prediction error and standardized finish times were negatively correlated with earnings and positively correlated with finish position and, thus, appeared to be reliable measures of racing performance. The regression model was found to be valid when applied to lifetime performance records of horses with laryngeal hemiplegia. CLINICAL IMPLICATIONS: Prediction error and standardized finish times are measures of racing performance that can be used to compare performances among Thoroughbred racehorses across a variety of circumstances that would otherwise confound comparison.

Animals↗

PubMed1989

Assessment of accuracy of intraesophageal pH probe in a dog model.

We performed an experiment in dogs in order to assess the accuracy of an intraesophageal pH probe and to examine possible mechanisms for any lack of accuracy. Two pH probes were placed in the esophagus of supine anesthetized dogs and acid was infused in small serial volumes from an infusion tube placed at the gastroesophageal junction. Two general patterns of pH probe responses were seen: (1) neither probe recorded a fall in pH with acid infusion, and (2) both probes recorded a fall in pH, but in a nonsynchronous manner. Changing the relationship of the tips of the probes with respect to the adjacent mucosa caused a fall in the pH recorded by each probe to the pH of the intraesophageal infusate. This and other evidence seen in the pH tracings suggest that mucosal abutment of the probe tips causes a falsely negative pH recording and raises the question of whether such a phenomenon occurs in human studies.

Animals↗

PubMed1983

The hypoglucemic effect of Opuntia streptacantha studied in different animal experimental models.

Studies performed with Opuntia streptacantha sap in three different animal species using several experimental conditions are described. The Opuntia sap induced hypoglucemic effects when orally administered to intact animals under induced states of moderate increase of blood sugar. In normoglucemic and pancreatectomized animals the effect of the product was not detected. The results validate the popular use of this plant for treatment of Diabetes mellitus symptomatology.

Anesthesia↗

PubMed1998

A method for approximating the density of maximum-likelihood and maximum a posteriori estimates under a Gaussian noise model.

The performance of maximum-likelihood (ML) and maximum a posteriori (MAP) estimates in non-linear problems at low data SNR is not well predicted by the Cramér-Rao or other lower bounds on variance. In order to better characterize the distribution of ML and MAP estimates under these conditions, we derive a point approximation to density values of the conditional distribution of such estimates. In an example problem, this approximate distribution captures the essential features of the distribution of ML estimates in the presence of Gaussian-distributed noise.

Algorithms↗

PubMed2002

Error threshold transition in the random-energy model.

We perform a statistical analysis of the error threshold transition in quasispecies evolution on a random-energy fitness landscape. We obtain a precise description of the genealogical properties of the population through extensive numerical simulations. We find a clear phase transition and can distinguish two regimes of evolution: The first, for low mutation rates, is characterized by strong selection, and the second, for high mutation rates, is characterized by quasineutral evolution.

Journal Article↗

PubMed1999

[Usefulness of arterio-portal shunt in 40% hepatectomized and hepatic artery-occluded model].

We performed an experimental study in dogs with 40% hepatectomy to prove the efficacy and safety of arterio-portal shunt (APS). Forty-two adult mongrel dogs were used for the experiments. After dearterialization of all the collateral arteries to the liver, the dogs were divided into 3 groups. In Group 1, dearterialization of the hepatic artery, 40% hepatectomy and APS were performed; in Group 2, only 40% hepatectomy was performed, and in Group 3, dearterialization of the hepatic artery and 40% hepatectomy were performed. APS was constructed by anastomosing the common hepatic artery with the portal vein. The hepatic hemodynamics, oxygen delivery and consumption were evaluated, and biochemical and histological studies were carried out in each group. The study consisted of a short-term (48-hour; Experiment 1) and a medium-term (4-weeks; Experiment 2) observations. In Experiment 2, liver regeneration was additionally examined. In Experiment 1, Group 1 and Group 2 showed almost the same findings in all the 4 examination points. However, in Group 3, postoperative oxygen delivery and consumption significantly decreased, and biochemical and histological examinations evidenced the significant liver failure. These results show that APS is capable of preventing postoperative liver failure during the short-term observation period and can substitute for reconstruction of the hepatic artery. Experiment 2 was intended for a longer follow-up of Experiment 1. However, Group 3 did not survive longer than 1 week and was thus excluded. In Group 1 and Group 2, findings on the 4 examinations remained alike for p.o. 4 weeks. And, the two groups regained the preoperative liver weight, respectively, at p.o. 4 weeks. The number of PCNA (proliferating cell nuclear antigen)-positive hepatocytes significantly increased at p.o. 1 week in both groups. These findings revealed that regeneration of the liver in Group 1 with APS was equivalent to that in Group 2 with mere hepatectomy for p.o. 4 weeks at least. These results indicate that liver failure can be averted by constructing APS in hepatectomy cases where reconstruction of the hepatic artery is impossible.

Animals↗

PubMed1992

A long-term study on defect filling and bone ingrowth using a canine fiber metal total hip model.

When performing primary and revision total hip arthroplasty (THA), bone defects are often encountered. At present, grafting osseous defects with autogeneic bone is a common means of treatment. In this study, defects in bone were created in the femora and acetabula of dogs being treated with cementless THA with a fiber metal implant (Group A) or a hydroxyapatite tricalcium phosphate (HA/TCP) sprayed implant (Group B). The following methods of defect filling were compared: (1) leaving defects unfilled, (2) filling with autogeneic bone graft, (3) filling with a 50:50 mixture of autograft and a biphasic ceramic composed of HA/TCP, and (4) filling with a collagen-HA/TCP-bone marrow mixture. Analysis of defect healing and the extent of ingrowth into the overlying fiber metal, at defect sites and sites distant from defects, was made at six, 12, and 24 weeks postimplantation. Defect healing was enhanced at six and 12 weeks in all grafted groups when compared with ungrafted controls. Bone ingrowth into the porous fiber metal overlying the defects was not significantly affected by grafting the defects, compared with the ungrafted defects. The extent of bone ingrowth into the fiber metal acetabular implant at sites away from the defects increased during the entire study. In contrast, the extent of bone ingrowth on the femoral side was maximal at 12 weeks. The HA/TCP coating enhanced ingrowth into the acetabular component at 12 weeks, compared with the uncoated prosthesis, but did not enhance ingrowth on the femoral side. The data from this study demonstrate that defect healing is enhanced with graft materials. However, this does not necessarily result in increased ingrowth into porous surfaces overlying osseous defects. General bone ingrowth and ingrowth at defect sites at 12 weeks postimplantation can be enhanced on the acetabular side with the use of HA/TCP-sprayed implants. However, no positive effect is seen with the use of an HA/TCP-sprayed femoral implant.

Acetabulum↗

PubMed1993

[The interstitial organ in weightlessness: a clinico-physiologic model].

We performed an experiment within the 1991 Austromir mission, 3 months preflight, on the 6th inflight and 4th postflight days, respectively. Primary goal was to measure changes in venous blood and plasma sound velocity (BSV, PSV) with lower body suction (LBNP). Contrasting earth-based findings, there was no increase in BSV nor PSV with LBNP inflight. Postflight, the elevation in both values was greater than expected. Taken together with LBNP data from the German 1992 Mir cosmonaut, we propose the hypothesis that the dynamics of tissue fluid exchange is profoundly influenced by m-g adaptation. A further goal of the experiment was to determine volume- and stress-sensitive hormones. The results need validation by additional flight data and might be considered as an important contribution to our understanding of the normal functioning and pathophysiology of the interstitial organ.

Adaptation, Physiological↗

PubMed1996

Serial changes in proximal urethral function after transurethral balloon laser hyperthermia of the prostate (TUBAL-H) in a canine model.

We performed transurethral balloon laser hyperthermia of the prostate (TUBAL-H) in 6 mongrel dogs. To evaluate the changes in proximal urethral function after TUBAL-H, the urethral cross sectional area (CSA, cm2) and urethral pressure (Pu, cmH2O) were measured using a balloon probe that allows their simultaneous measurement, and the urethral compliance (Comp, cm3/cmH2O) was calculated from these parameters and serially evaluated. In addition, the changes in Pu, CSA and Comp after administration of an alpha-adrenoceptor antagonist were evaluated before and 16 weeks after TUBAL-H. Before TUBAL-H, the mean Comp was 0.013, and the mean maximum CSA (MCSA) was 0.66. Eight weeks after TUBAL-H, the mean Comp was 0.038, and the mean MCSA was 1.39, showing a significant increase (p < 0.01). Sixteen weeks, after TUBAL-H, the mean Comp was 0.026, and the mean MCSA was 1.21, being lower than those at 8 weeks after TUBAL-H, but significantly higher than those before TUBAL-H (p < 0.05). After administration of the alpha-adrenoceptor antagonist, phentolamine (1 mg/kg), before TUBAL-H, the mean Comp significantly increased to 0.046 (p < 0.05), and the mean MCSA to 1.40 (p < 0.01). The mean Comp was significantly increased to 0.033 (p < 0.05), by phentolamine administration 16 weeks after TUBAL-H, but no other changes were observed. After TUBAL-H, urethral elasticity increased, and this increase persisted for 4 months. The responses of Comp and MCSA to alpha-adrenoceptor antagonist administration before and 16 weeks after TUBAL-H suggested that part of the effects of TUBAL-H is due to damage to alpha-adrenoceptors.

Adrenergic alpha-Antagonists↗

PubMed2005

Accuracy of rate estimation using relaxed-clock models with a critical focus on the early metazoan radiation.

In recent years, a number of phylogenetic methods have been developed for estimating molecular rates and divergence dates under models that relax the molecular clock constraint by allowing rate change throughout the tree. These methods are being used with increasing frequency, but there have been few studies into their accuracy. We tested the accuracy of several relaxed-clock methods (penalized likelihood and Bayesian inference using various models of rate change) using nucleotide sequences simulated on a nine-taxon tree. When the sequences evolved with a constant rate, the methods were able to infer rates accurately, but estimates were more precise when a molecular clock was assumed. When the sequences evolved under a model of auto-correlated rate change, rates were accurately estimated using penalized likelihood and by Bayesian inference using lognormal and exponential models of rate change, while other models did not perform as well. When the sequences evolved under a model of uncorrelated rate change, only Bayesian inference using an exponential rate model performed well. Collectively, the results provide a strong recommendation for using the exponential model of rate change if a conservative approach to divergence time estimation is required. A case study is presented in which we use a simulation-based approach to examine the hypothesis of elevated rates in the Cambrian period, and it is found that these high rate estimates might be an artifact of the rate estimation method. If this bias is present, then the ages of metazoan divergences would be systematically underestimated. The results of this study have implications for studies of molecular rates and divergence dates.

Algorithms↗

PubMed2002

Repeatability of Model Genetic Coefficients Derived from Soybean Performance Trials across Different States.

Crop model testing in diverse environments is essential if modelers wish to make applications or extrapolations to those environments. A recent study demonstrated the effectiveness of optimization techniques for deriving cultivar coefficients for the CROPGRO-Soybean model from typical information provided by soybean performance tests. The objectives of this study were (i) to explore the extent to which cultivar coefficients developed by these approaches from crop performance tests are stable across different regions, (ii) to test the CROPGRO-Soybean model's ability to predict phenology and seed yield using cultivar coefficients that were developed in different regions, and (iii) to investigate whether 3 yr of crop performance data are adequate for developing stable genetic coefficients. A stepwise procedure was applied to derive cultivar coefficients for 10 common cultivars grown in different environments in Georgia and North Carolina. Regarding the transportability of cultivar coefficients across states, we found that the critical daylength coefficients were the most reliable cultivar traits. We found less stability of the cultivar traits that control genetic differences in seed yield potential. The estimated cultivar coefficients developed in Georgia enabled CROPGRO to predict yield and harvest maturity in North Carolina within 3.8% and 3.5 d, respectively, from the observed averages. Using the cultivar coefficients developed from North Carolina environments allowed us to simulate the actual mean yield and harvest maturity in Georgia to within 2.5% and 2.0 d. Furthermore, the model's ability to predict seed yield and maturity with cultivar coefficients developed from 3 yr of data was nearly as good as that derived from much larger data sets.

Journal Article↗

PubMed1998

Prospective evaluation of a teaching model to determine competency in performing flexible sigmoidoscopies.

OBJECTIVE: There are limited objective criteria for determining competency in performing flexible sigmoidoscopies. The authors developed a teaching model using eight objective criteria that measured both cognitive and technical skills in performing flexible sigmoidoscopies. The objective was to determine whether the teaching model was a valid method and then to compare it to a commonly used Overall Competence Score or independent observer method. METHODS: A total of 120 procedures with 10 residents were evaluated using the two methods. Eight objective parameters had been determined before validating the teaching model. Preset criteria determined competency on a single procedure, and competent performance on five procedures in a row determined certification to independently perform procedures. The teaching model was separately tested for validity and then compared with the independent observer method. RESULTS: A total of 120 procedures were graded using the teaching model method: 73 competent and 47 incompetent. Six of the seven objective parameters were associated with competency in the teaching model method. The eighth objective parameter was excluded because of lack of variation. Fifty of the 120 procedures were graded using the independent observer method: 36 competent and 14 incompetent. Two of the seven objective parameters were associated with competency in the independent observer method. Comparison of the two methods revealed seven discrepancies regarding competent and incompetent procedures. The correlation between the two methods was 0.71. CONCLUSION: The teaching model method offers objective criteria to evaluate skill in performing flexible sigmoidoscopies.

Clinical Competence↗

PubMed2004

Amylose formulations for drug delivery to the colon: a comparison of two fermentation models to assess colonic targeting performance in vitro.

The purpose of this study was to develop an enzyme-based fermentation system for the in vitro assessment of colonic digestion of amylose films and coatings, and to compare its performance with a conventional fermentation model inoculated with human faecal bacteria. Amylose and ethylcellulose were mixed in different ratios and cast as isolated films, as well as spray coated onto drug-(5-aminosalicylic acid) loaded pellets. Four commercial amylase enzymes were individually screened for their ability to digest amylose cast films. The enzyme from the bacterium Bacillus licheniformis was found to be the most active against this substrate. Digestion of mixed amylose and ethylcellulose films was also observed, with the extent of digestion being proportional to the quantity of amylose present in the film. In terms of product performance, drug release from coated pellets was accelerated in the presence of the enzyme. The results with the enzyme system were comparable to those obtained from a faecal-based fermentation model, thereby suggesting that such a system has practical potential for in vitro screening of putative amylose formulations for colonic drug delivery.

Amylases↗

PubMed2006

A 3-month preclinical trial to assess the performance of a new TVT-like mesh (TVTx) in a sheep model.

The objective of this study was to evaluate in a sheep model the performance of a new polypropylene mesh (TVTx), which is intended as a less invasive treatment for female stress urinary incontinence. Eight female sheep were used in this study, each one being implanted with eight TVTx samples. At each time-point (weeks 1, 2, 4, and 12) seven TVTx were pulled out, while one TVTx was carefully dissected for histological investigations. One TVTx and one TVT, moreover, were inserted and immediately pulled out for obtaining the initial pullout forces in all sheep. The initial pullout values of TVT and TVTx were overlapping. The pullout forces of TVTx were >5 N (500 g) and increasing from weeks 1 to 12 (p<0.001). Histology revealed good tissue integration of TVTx in the tissue within 12 weeks after implantation. No abnormal histological findings were observed. This data could support the realization of a clinical trial with the TVTx mesh.

Animals↗

PubMed1991

Using generalized linear models (GLMs) to model errors in motor performance.

Because of differences in design factors, experiments in human motor performance sometimes produce a wide range in variability or consistency in a subject's individual errors. These differences in variation often lead to heterogeneity in the variance-covariance matrices between group factors, which prohibits the use of repeated-measures (RM) ANOVA or MANOVA techniques to analyze the error data. Provided certain conditions are met, however, each subject's individual errors can be collapsed into the summary error measures, constant error (CE) and variable error (VE), which can still provide a more than adequate description of the subjects' performance. This article proposes the appropriate conditions and the corresponding generalized linear models (GLMs) with which a subject's individual errors, recorded in short-term motor memory research, can be combined into the summary measures, CE and VE, which can be analyzed subsequently as the dependent variables in the experimental design. The CE scores can be modeled using GLMs without requiring the assumption of homogeneity of variances. Similarly, the VE scores can be modeled as a GLM, using a log-linear regression model that assumes a gamma distribution, rather than using a traditional analysis of variance (ANOV A) model that assumes an inappropriate normal error distribution for these scores. An example reveals that the analysis of VE scores, unlike the analysis of CE scores, is able to differentiate between group practice methods. These differences tend to be underestimated by traditional ANOVA methods, however. Differences between the ANOV A and GLM analyses of the VE scores are further clarified by simulation. Based on differences like those observed in the example, when simulated VE scores were analyzed, assuming both a normal and a gamma distribution, the power of the gamma tests was found to be superior to the normal analyses in all but a small range of cases, in which such differences were found to be negligible. Hence, it is only by declaring the VE scores to have a GLM with a gamma distribution that the anticipated group practice differences can be properly identified.

Journal Article↗

PubMed1998

Analysis of the performance of a model-based optimal auditory signal processor.

Traditionally, psychophysical data have been predicted either by constructing models of the peripheral auditory system or by applying signal detection theory (SDT). Frequently, the theoretical detection performance predicted by SDT is greater than that observed experimentally and a nonphysiologically based "internal noise" source is often added to the system to compensate for the discrepancy. A more appropriate explanation may be that traditional SDT approaches either incorporate little or no physiology or make simplifying assumptions regarding the density functions describing the physiological data. In the work presented here, an integrated approach, which combines SDT and a physiologically based model of the human auditory system, is proposed as an alternate method of quantifying detection performance. To validate this approach, the predicted detection performance for a simultaneous masking task is compared to predictions obtained from traditional methods and to experimental data. Additionally, the sensitivity of the integrated method is thoroughly investigated. The results suggest that by combining SDT with a physiologically based auditory model, thereby capitalizing on the strengths of each individual method, the previously observed discrepancies can be partially explained as the result of physical processes inherent in the auditory system rather than unspecified "internal noise" and more accurate predictions of psychophysical behavior can be obtained.

Auditory Perception↗

PubMed1996

Error models and quality control performance.

As an alternative to the oversimplified error schemes currently adopted in establishing quality control (QC) strategies, a complex model was assumed implying (a) the distribution of errors (critical error is regarded as a value discriminating between "effective errors" to be detected and "subcritical errors" which do not interfere with the medical decision whose detection is considered as a false-reject signal), and (b) the possibility of simultaneous losses of precision and accuracy. The control data recorded for digoxin radioimmunoassay over a one-year period were used for (1) deriving the probability density functions of random and systematic errors, through a within-run across-level normalisation procedure; (2) obtaining the functional relationships between the critical random or systematic error and the QC performance statistics (sensitivity, specificity, predictive value), weighted for the error prevalences, through integration of the probability density functions and the power functions associated with an exemplifying control rule; and (3) describing the functions which correlate the corrected performance statistics with the allowable error (whose individual values account for all possible combinations of critical random errors and critical systematic errors), by extending to the tridimensional space the above procedures. Analysis of the resulting data shows that it is necessary to revise the criteria for the choice and optimisation of QC schemes.

Analysis of Variance↗

PubMed1992

Human factors with nonhumans: factors that affect computer-task performance.

There are two general strategies that may be employed for "doing human factors research with nonhuman animals." First, one may use the methods of traditional human factors investigations to examine the nonhuman animal-to-machine interface. Alternatively, one might use performance by nonhuman animals as a surrogate for or model of performance by a human operator. Each of these approaches is illustrated with data in the present review. Chronic ambient noise was found to have a significant but inconsequential effect on computer-task performance by rhesus monkeys (Macaca mulatta). Additional data supported the generality of findings such as these to humans, showing that rhesus monkeys are appropriate models human psychomotor performance. It is argued that ultimately the interface between comparative psychology and technology will depend on the coordinated use of both strategies of investigation.

Adolescent↗