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At least 343 records · Page 19Linked to original sources
PubMed1997

The role of the plant properties in point-to-point arm movements: a robustness study.

In recent papers we demonstrated by means of a modeling study that the smoothness of hand paths and the bell-shaped character of hand velocity profiles which have been experimentally observed in point-to-point arm movements can be largely attributed to the biomechanical properties of the arm rather than to specific planning by the central nervous system. In this paper we present a study of the robustness of our earlier results comprising two goals: (i) the determination of the range of model parameters for which such observations remain valid, (ii) the identification of possible relationships between model parameters and kinematic variables. The results of this study imply three conclusions: (i) the valid range of the tested model parameters (namely the main muscle parameters) is large, (ii) the modeled phenomena are well behaved in that parametric changes do not give rise to bifurcations or other behavioral discontinuities in the analyzed ranges, (iii) there exist precise relationships between certain muscle parameters and the time course of the hand velocity. These results point out that the phenomena observed in our previous work are indeed robust and can lead to useful insights into the mechanisms comprising the regulatory action of the central nervous system as well as into the design principles for biologically inspired artificial arms.

Arm↗
PubMed2005

Functional groups based on leaf physiology: are they spatially and temporally robust?

The functional grouping concept, which suggests that complexity in ecosystem function can be simplified by grouping species with similar responses, was tested in the Florida scrub habitat. Functional groups were identified based on how species regulate exchange of carbon and water with the atmosphere as indicated by both instantaneous gas exchange measurements and integrated measures of function (%N, delta13C, delta15N, C:N ratio) in fire-maintained Florida scrub, which was considered the natural state for scrub habitat. Using cluster analysis, five distinct physiologically based functional groups were identified in the fire-maintained scrub and were determined to be distinct clusters and not just arbitrary divisions in a continuous distribution by the non-parametric multivariate analysis of similarities (ANOSIM; R=0.649, P=0.005). These functional groups were tested for robustness spatially, temporally, and with management regime using ANOSIM. The physiological functional groups remained distinct clusters in this broader array of sites (R=0.794, P=0.001) and were not altered by plot differences, primarily, water table depth (R=-0.115, P=0.893) or by the three different management regimes: prescribed burn, mechanically treated and burned, and fire-suppressed (R=0.018, P=0.349). The physiological groupings also remained robust between the two climatically different years, with 1999 being a much wetter year than 2000 (R=-0.027, P=0.725). Easy-to-measure morphological characteristics, if they indicate the same functional groups, would be more practical for scaling and modeling ecosystem processes than detailed gas exchange measurements; therefore, we tested a variety of morphological characteristics as functional indicators. A combination of non-parametric multivariate techniques were used to compare the ability of life form, leaf thickness (LT), and specific leaf area (SLA) classifications to identify the physiologically based functional groups. Life form classifications (ANOSIM; R=0.629, P=0.001) were able to depict the physiological groupings more adequately than either SLA (ANOSIM; R=0.426, P=0.001) or LT (ANOSIM; R=0.344, P=0.001). The ability of life forms to depict the physiological groupings was improved by separating the parasitic Ximenia americana from the shrub category (ANOSIM; R=0.794, P=0.001). Therefore, a life form classification including parasites was determined to be a good indicator of the physiological processes of scrub species and would be a useful method of grouping species for scaling physiological processes to the ecosystem level.

Carbon↗
PubMed2004

Hipocrates: a robust system for the control of neuromuscular blockade.

OBJECTIVE: Development of an automatic system (software package Hipocrates) for the control of neuromuscular blockade by continuous infusion of the non-depolarising types of muscle relaxant drugs presently used in anaesthesia, namely atracurium, cisatracurium, vecuronium and rocuronium. METHODS: Hipocrates incorporates control strategies based upon classical, adaptive and robust control, as well as a wide range of noise reduction techniques and on-line adaptation to patient-specific characteristics. Therefore, the system provides strong robustness to inter- and intra-individual variability of the patients responses or unexpected circumstances and adaptation to the individual requirements. RESULTS: The control system is easy to set up and to use in a clinical environment. It consists of a portable PC computer, a Datex AS/3 NMT sensor and a B/Braun compact perfusion pump. In the simulation mode the software package incorporates sophisticated generation of pharmacokinetic/pharmacodynamic models driven by simulated drug administration regimes (bolus, continuous infusion and a combination of both). CONCLUSIONS: Hipocrates is an advanced standalone application for the control of neuromuscular blockade with a friendly graphic interface. It has been extensively validated, and it can be used on patients undergoing surgery as well as for simulation studies. Therefore Hipocrates also provides an excellent environment for education and training purposes.

Anesthesia, General↗
PubMed2005

Robust population pharmacokinetic experiment design.

The population approach to estimating mixed effects model parameters of interest in pharmacokinetic (PK) studies has been demonstrated to be an effective method in quantifying relevant population drug properties. The information available for each individual is usually sparse. As such, care should be taken to ensure that the information gained from each population experiment is as efficient as possible by designing the experiment optimally, according to some criterion. The classic approach to this problem is to design "good" sampling schedules, usually addressed by the D-optimality criterion. This method has the drawback of requiring exact advanced knowledge (expected values) of the parameters of interest. Often, this information is not available. Additionally, if such prior knowledge about the parameters is misspecified, this approach yields designs that may not be robust for parameter estimation. In order to incorporate uncertainty in the prior parameter specification, a number of criteria have been suggested. We focus on ED-optimality. This criterion leads to a difficult numerical problem, which is made tractable here by a novel approximation of the expectation integral usually solved by stochastic integration techniques. We present two case studies as evidence of the robustness of ED-optimal designs in the face of misspecified prior information. Estimates from replicate simulated population data show that such misspecified ED-optimal designs recover parameter estimates that are better than similarly misspecified D-optimal designs, and approach estimates gained from D-optimal designs where the parameters are correctly specified.

Algorithms↗
PubMed2006

Implications of model misspecification in robust tests for recurrent events.

Chronic disease processes often feature transient recurrent adverse clinical events. Treatment comparisons in clinical trials of such disorders must be based on valid and efficient methods of analysis. We discuss robust strategies for testing treatment effects with recurrent events using methods based on marginal rate functions, partially conditional rate functions, and methods based on marginal failure time models. While all three approaches lead to valid tests of the null hypothesis when robust variance estimates are used, they differ in power. Moreover, some approaches lead to estimators of treatment effect which are more easily interpreted than others. To investigate this, we derive the limiting value of estimators of treatment effect from marginal failure time models and illustrate their dependence on features of the underlying point process, as well as the censoring mechanism. Through simulation, we show that methods based on marginal failure time distributions are shown to be sensitive to treatment effects delaying the occurrence of the very first recurrences. Methods based on marginal or partially conditional rate functions perform well in situations where treatment effects persist or in settings where the aim is to summarizee long-term data on efficacy.

Clinical Trials as Topic↗
PubMed2006

Emergent robustness in competition between autocatalytic chemical networks.

The origin of auto-catalytic networks has been proposed as an initial step in pre-biotic evolution. It is possible to derive simple models where auto-catalytic networks naturally arise from simple chemical mixtures. In order for such a system to develop, there needs to be some degree of stability, what is characterised as ;robustness'. We demonstrate that competing systems generate this robustness as they create a distributed network of catalytic pathways.

Computer Simulation↗
PubMed1985

A robust microcomputer routine for the identification of outlying and influential points in radioimmunoassay standard curves.

This paper describes a suite of robust logit-log optimization routines for the fitting of a linear relation to laboratory data. The microcomputer routines assign a weighting value to each data point corresponding to the relative importance of that point. This procedure ensures that the routines identify and deal with any outlying data points. The routines are designed to generate the results of both a standard, unweighted least-squares algorithm and a robust, weighted algorithm thus providing the laboratory analyst with invaluable information concerning the data and the fitted relation. The routines are thoroughly tested on "perfect" and perturbed laboratory data.

Biometry↗
PubMed1978

Robustness of life table methods in large populations--a study by computer simulation.

The robustness of the product life table estimator of the survival function was studied for large populations under perturbations in the age distribution, changing levels of mortality and changing patterns of fertility. A macrosimulation system, based on a class of stochastic population models called generalised age-dependent branching processes, was used to carry out the numerical investigations. Aside from drastic perturbations in the age distribution and changes in levels of mortality, the product life table estimator of the survival function was found to be robust in large populations, under a variety of conditions.

Age Factors↗
PubMed1996

Algorithms for robust nonlinear regression with heteroscedastic errors.

Nonlinear regression algorithms were compared by Monte-Carlo simulations when the measurement error was dependent on the measured values (heteroscedasticity) and possibly contaminated with outliers. The tested leastsquares (LSQ) algorithms either required user-supplied weights to accommodate heteroscedasticity or the weights were estimated within the procedures. Robust versions of the LSQ algorithms, namely robust iteratively reweighted (IRR) and least absolute value (LAV) regressions, were also considered. The comparisons were based on the efficiency of the estimated parameters and their resistance to outliers. Based on these criteria, among the tested LSQ algorithms, extended least squares (ELSQ) was found to be the most reliable. The IRR versions of these algorithms were slightly more efficient than the LAV versions when there were no outliers but they provided weaker protection to outliers than the LAV variants.

Algorithms↗
PubMed1984

Distribution of mechanical robustness in the human femoral shaft.

The object of the present study was to determine the mechanical robustness of the human femoral shaft. The geometric properties of the cross sections were measured. The compressive strength, the gamma-ray absorption, and the ash density of the compact bone were examined at 40 points along the shaft. The geometric properties connected with the mechanical robustness increased with the body size. The distribution of the mineral density, the material compressive strength, and the endurable bending moment on the surface of the bone were determined. The density and the strength were great in specimens from the postero-lateral side and in those from the antero-medial to the postero-medial side of the shaft. The calculated endurable bending moment was large on the anterior, the postero-lateral, and the postero-medial surfaces of the middle part of the bone. The anterior side was especially able to endure a large bending moment when the bone had a large moment of inertia of the cross-sectional area in the sagittal direction. The distribution of these mechanical properties could be the functional adaptation of the human femur against external bending forces mainly caused by muscle activity.

Aged↗
PubMed1987

Power and robustness of the linkage homogeneity test in genetic analysis of common disorders.

The power and robustness of the admixture test are studied. The power of the test depends on the genetic parameters at the disease locus. In particular, the power decreases drastically with the level of penetrance. The test is robust to errors in genetic parameters provided that the recombination fraction is not fixed a priori. However, misspecifying genetic parameters leads to a bias in the estimates of both the recombination fraction and the proportion of linked families.

Chromosome Mapping↗
PubMed1992

Robustness of perception of heading from optic flow.

Heading discrimination in optic flow stimuli was investigated in 5 humans. Flow patterns consisted of a computer generated motion sequence which showed a mixture of randomly moving and coherently-moving points. The motion of the coherently-moving fiducial points was completely determined by the changing position of the observer's simulated vantage point. Ego-rotation as well as ego-translation was simulated. Noise and fiducial points were confined to the ground plane in most experiments. In one experiment the points formed a cloud with no visible horizon. The results indicate that heading perception is robust against degradation of the flow-field by the presence of noise or by the reduction of the lifetime of the fiducial points. The results suggest that points, which move independently from the reference frame as formed by the fiducial points, are to a large extent removed from the analysis of optic flow by the visual process which derives the heading. The motion of recognizable points at infinity (like the horizon) appears to be essential for robust heading perception in the presence of ego-rotations.

Adult↗
PubMedDate not supplied

Robust alternatives to traditional analysis of variance: Welch W*, James J I*, James J II*, Brown-Forsythe BF*.

Contaminated data sets commonly appear in biomedical sciences. Traditional statistical analyses are based on a 'normality' assumption and homogeneity of variances assumption. If either or both assumptions are violated, traditional statistical procedures may give an inflated type I error rate and are then not robust. Robust alternatives to traditional one-way analysis of variance have better power curves and protect against inflated type I error rates.

Analysis of Variance↗
PubMed1996

The problem of assuring performance robustness of biological life support systems.

A perspective of the process of obtaining performance robustness of space exploration life support systems is presented. Some useful definitions are made, some relevant issues are addressed, and a procedure for establishing performance robustness, so far as it now known, is explained. An example is given to illustrate the procedure.

Air Conditioning↗
PubMed1996

The registration of MR images using multiscale robust methods.

Acquisition of MR images involves their registration against some prechosen reference image. Motion artifacts and misregistration can seriously flaw their interpretation and analysis. This article provides a global registration method that is robust in the presence of noise and local distortions between pairs of images. It uses a two-stage approach, comprising an optional Fourier phase-matching method to carry out preregistration, followed by an iterative procedure. The iterative stage uses a prescribed set of registration points, defined on the reference image, at which a robust nonlinear regression is computed from the squared residuals at these points. The method can readily accommodate general linear or even nonlinear, registration transformations on the images. The algorithm was tested by recovering the registration transformation parameters when a 256 x 256 pixel T2*-weighted human brain image was scaled, rotated, and translated by prescribed amounts, and to which different amounts of Gaussian noise had been added. The results show subpixel accuracy of recovery when no noise is present, and graceful degradation of accuracy as noise is added. When 40% noise is added to images undergoing small shifts, the recovery errors are less than 3 pixels. The same tests applied to the Woods algorithm gave slightly inferior accuracy for these images, but failed to converge to the correct parameters in some cases of large-scale-shifted images with 10% added noise.

Algorithms↗
PubMed1995

A robust transcutaneous electro-muscle stimulator (RTES): a multi-modality tool.

This paper introduces a transcutaneous electro-muscle stimulator design that has a wide range of output capabilities. Because of this, the unit is referred to as a robust transcutaneous electro-muscle stimulator (RTES). The RTES is a constant current stimulator that is designed to be capable of generating significant tetanic contractions from large muscle groups, such as the quadriceps. It is capable of generating complex current pulse profiles and has been tested at pulse frequencies greater than 7500 Hz. It is routinely used to generate rectangular, bi-phasic pulses in muscle-modelling studies in ranges of widths from 3 to 1000 microseconds, amplitudes from -50 to +50 mA and frequencies from 10 to 60 Hz. The design extrema on pulse width and amplitude, are 1000 microseconds and +/- 100 mA respectively. Because of the stimulator's robust output characteristics, it is suitable for many types of electro-stimulation studies including pain management, edema reduction and more.

Adult↗
PubMed2006

The validity of the letter memory test as a measure of memory malingering: robustness to coaching.

The letter memory test (LMT) is a computerized forced-choice test of malingering detection including two face valid difficulty manipulations: increase in target stimulus length and increase of response foils. Prior research suggests the LMT shows promise as a malingering detection measure. In the present study, the utility of the LMT in the identification of malingering was further explored, using a counterbalanced design in a simulated malingering sample. Prior work was extended by assessing the robustness of the LMT to coaching and assessing the effectiveness of an additional scoring method, utilizing the face valid difficulty manipulations. Results were consistent with prior research on the LMT, with the standard cutoff score yielding high indices of accuracy. The LMT showed no order effects and was superior to the 15-item test in accuracy indices. Both the standard LMT score and the proposed score based on difficulty manipulations were relatively robust to coaching. Overall, findings indicate the LMT is a viable contender among measures of memory malingering.

Case-Control Studies↗
PubMed2026

Mutation rate heterogeneity biases variant effect prediction and reveals genuine mutational robustness.

Variant effect predictors (VEPs) are widely used to interpret the functional consequences of human genetic variation. Because most methods rely on sequence conservation, they implicitly treat conservation as evidence of functional constraint. However, substitution patterns across a phylogeny reflect not only selection but also differences in underlying mutation rates. Here, we show that this creates a systematic confounding: most VEPs capture mutation rate variation and misinterpret it as variation in functional importance. Widely used conservation metrics exhibit a related bias; in particular, phyloP scores correlate strongly with mutation rate even at putatively neutral sites. Consequently, variants at low-mutation-rate sites tend to be predicted as more damaging, and variants at highly mutable sites as more tolerated, than warranted by their true functional impact. We also identify a distinct biological signal in experimental measurements of mutational effects on protein stability: amino acid substitutions that are more likely to arise are, on average, less destabilizing than rarer substitutions. This provides empirical support for mutational robustness in the context of protein stability. However, this relationship is insufficient to explain the mutation-rate dependence observed in current VEP outputs. Together, our findings show that mutation rate heterogeneity systematically biases current variant effect prediction frameworks, highlight the need to model mutation probabilities explicitly in future VEPs, and reveal a genuine biological signal of mutational robustness.

conservation scores↗