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Substantial Errors (substantial + error)
Selected AbstractsComparison of 1- and 2-Marker Techniques for Calculating System Magnification Factor for Angiographic Measurement of Intracranial VesselsJOURNAL OF NEUROIMAGING, Issue 4 2005A. A. Divani PhD ABSTRACT Background and Purpose. Accurate estimation of an intracranial vessel size is crucial during a diagnostic or therapeutic angiography procedure. The use of 1 or 2 external markers of known size is previously proposed to manually estimate the magnification factor (MF) of an intracranial vessel. The authors evaluated the use of different external marker techniques commonly used during angiographic measurements. Methods. Forty-three intracranial vessels in 17 patients were measured using 1-and 2-marker techniques. To obtain the MF, 2 metallic markers were attached to the frontal-temporal regions. The MFs for the targeted vessels were obtained from the x-ray films by measuring the image sizes of the markers and their positions with respect to the target vessel. Results. Using a phantom, the errors resulted from (a) linear interpolation of MFs, (b) linear interpolation of inverse MFs, and (c) using the MFs of 1 marker, which were 1.23% to 2.23%, 0.8% to 1.55%, and 3.85% to 14.62%, respectively. A similar trend was observed for the measurement of cerebral arteries. Conclusion. The use of 2 markers can result in a more accurate estimation of the vessel size. The use of only 1 external marker can lead to substantial error based on the location of the target vessel. Optimizing image acquisition is also crucial for accurate determination of vessel size. [source] Patient variation in veterinary medicine: part I. Influence of altered physiological statesJOURNAL OF VETERINARY PHARMACOLOGY & THERAPEUTICS, Issue 3 2010M. MARTINEZ Martinez, M., Modric, S. Patient variation in veterinary medicine: part I. Influence of altered physiological states. J. vet. Pharmacol. Therap.33, 213,226. In veterinary medicine, the characterization of a drug's pharmacokinetic (PK) properties is generally based upon data that are derived from studies that employ small groups of young healthy animals, often of a single breed. These are also the data from which population predictions are often generated to forecast drug exposure characteristics in the target population under clinical conditions of use. In veterinary medicine, it is rare to find information on the covariates that can influence drug exposure characteristics. Therefore, it is important to recognize some of the factors that can alter the outcome of PK studies and therefore potentially alter the pharmacological response. Some of these factors are easily identified, such as breed, gender, age, and body weight. Others are less obvious, such as disease, heritable traits, and environmental factors. This manuscript provides an overview of the various stressors (such as disease, inflammation, pregnancy, and lactation) that can substantially alter drug PK. Part II of this series provides an overview of the potential impact of physiological variables such as age, weight, and heritable traits, on drug PK. Ultimately, failure to identify appropriate covariates can lead to substantial error when predicting the dose,exposure relationship within a population. [source] Brief communication: A probabilistic approach to age estimation from infracranial sequences of maturation,AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY, Issue 4 2010Hélène Coqueugniot Abstract Infracranial sequences of maturation are commonly used to estimate the age at death of nonadult specimens found in archaeological, paleoanthropological, or forensic contexts. Typically, an age assessment is made by comparing the degree of long-bone epiphyseal fusion in the target specimen to the age ranges for different stages of fusion in a reference skeletal collection. While useful as a first approximation, this approach has a number of shortcomings, including the potential for "age mimicry," being highly dependent on the sample size of the reference sample and outliers, not using the entire fusion distribution, and lacking a straightforward quantitative way of combining age estimates from multiple sites of fusion. Here we present an alternative probabilistic approach based on data collected on 137 individuals, ranging in age from 7- to 29-years old, from a documented skeletal collection from Coimbra, Portugal. We then use cross validation to evaluate the accuracy of age estimation from epiphyseal fusion. While point estimates of age can, at least in some circumstances, be both accurate and precise based on the entire skeleton, or many sites of fusion, there will often be substantial error in these estimates when they derive from one or only a few sites. Because a probabilistic approach to age estimation from epiphyseal fusion is computationally intensive, we make available a series of spreadsheets or computer programs that implement the approach presented here. Am J Phys Anthropol 142:655,664, 2010. © 2010 Wiley-Liss, Inc. [source] True and Apparent Temperature Dependence of Protein Adsorption Equilibrium in Reversed-Phase HPLCBIOTECHNOLOGY PROGRESS, Issue 6 2002Szabelski The adsorption behavior of bovine insulin on a C8 -bonded silica stationary phase was investigated at different column pressures and temperatures in isocratic reversed-phase HPLC. Changes in the molar volume of insulin (, Vm) upon adsorption were derived from the pressure dependence of the isothermal retention factor ( k,). The values of , Vm were found to be practically independent of the temperature between 25 and 50 °C at ,96 mL/mol and to increase with increasing temperature, up to ,108 mL/mol reached at 50 °C. This trend was confirmed by two separate series of measurements of the thermal dependence of ln( k,). In the first series the average column pressure was kept constant. The second series involved measurements of ln( k,) under constant mobile-phase flow rate, the average column pressure varying with the temperature. In both cases, a parabolic shape relationship was observed between ln( k,) and the temperature, but the values obtained for ln k, were higher in the first than in the second case. The relative difference in ln( k,), caused by the change in pressure drop induced by the temperature, is equivalent to a systematic error in the estimate of the Gibbs free energy of 12%. Thus, a substantial error is made in the estimates of the enthalpy and entropy of adsorption when neglecting the pressure effects associated with the change in the molar volume of insulin. This work proves that the average column pressure must be kept constant during thermodynamic measurements of protein adsorption constants, especially in RPLC and HIC. Our results show also that there is a critical temperature, Tc , 53 °C, at which ln( k,) is maximum and the insulin adsorption process changes from an exothermic to an endothermic one. This temperature determines also the transition point in the molecular mechanism of insulin adsorption that involves successive unfolding of the protein chain. [source] Plant growth functions and possible spatial and temporal scaling errors in models of herbivoryGRASS & FORAGE SCIENCE, Issue 1 2001A. J. Parsons Recent studies have provided detail of the mechanisms by which plants and animals interact, but attempts to apply this knowledge to understand function at the scale of whole fields or grazed ecosystems can be fraught with difficulties. Faced with a plethora of detail, and yet demands to make models more comprehensive, modellers face pressure to revert to simplified accounts for what are assumed to be well-established biological phenomena, for example, for describing plant growth and intake, but this raises the risk that important insights may be lost, or that the analyses may face errors of scaling. The predictions of a previously described spatial model are compared with those of a non-spatial rendition of the same model to identify the differences in predictions and the sources of these differences. In particular, the use of the conventional empirical growth functions and their interaction with temporal and spatial scaling errors are examined. The comparison exposed how substantial errors could be made in predicting yield and stability under grazing. It is proposed that such errors might be avoided by ensuring that the functional responses used capture the insights of more detailed studies, and by recognizing the difficulties of scaling-up from the level of processes to the field scale and beyond. [source] Effects of Differentially Time-Consuming Tests on Computer-Adaptive Test ScoresJOURNAL OF EDUCATIONAL MEASUREMENT, Issue 2 2004Brent Bridgeman Time limits on some computer-adaptive tests (CATs) are such that many examinees have difficulty finishing, and some examinees may be administered tests with more time-consuming items than others. Results from over 100,000 examinees suggested that about half of the examinees must guess on the final six questions of the analytical section of the Graduate Record Examination if they were to finish before time expires. At the higher-ability levels, even more guessing was required because the questions administered to higher-ability examinees were typically more time consuming. Because the scoring model is not designed to cope with extended strings of guesses, substantial errors in ability estimates can be introduced when CATs have strict time limits. Furthermore, examinees who are administered tests with a disproportionate number of time-consuming items appear to get lower scores than examinees of comparable ability who are administered tests containing items that can be answered more quickly, though the issue is very complex because of the relationship of time and difficulty, and the multidimensionality of the test. [source] Anatomical information is needed in ultrasound imaging of muscle to avoid potentially substantial errors in measurement of muscle geometryMUSCLE AND NERVE, Issue 5 2009Menno R. Bénard MSc Abstract This study validates two-dimensional (2D) ultrasound measurements of muscle geometry of the human medial gastrocnemius (GM) and investigates effects of probe orientation on errors in these measurements. Ultrasound scans of GM muscle belly were made both on human cadavers (n = 4) and on subjects in vivo (n = 5). For half of the cadavers, ultrasound scans obtained according to commonly applied criteria of probe orientation deviated 15° from the true fascicle plane. This resulted in errors of fascicle length and fascicle angle up to 14% and 23%, respectively. Fascicle-like structures were detectable over a wide range of probe tilt and rotation angles, but they did not always represent true fascicles. Errors of measurement were either linear or quadratic functions of tilt angle. Similar results were found in vivo. Therefore, we conclude that similar errors are likely to occur for in vivo measurements. For all cadavers, at the distal end of GM, the true fascicle plane was shown to be perpendicular to the distal aponeurosis. Using transverse images of GM to detect the curvature of the deep aponeurosis at the distal end of the muscle belly is a simple strategy to help identify the fascicle plane. For subsequent longitudinal imaging, probe alignment within this plane will help minimize measurement errors of fascicle length, fascicle angle, and muscle thickness. Muscle Nerve, 2009 [source] Twenty-five pitfalls in the analysis of diffusion MRI data,NMR IN BIOMEDICINE, Issue 7 2010Derek K. Jones Abstract Obtaining reliable data and drawing meaningful and robust inferences from diffusion MRI can be challenging and is subject to many pitfalls. The process of quantifying diffusion indices and eventually comparing them between groups of subjects and/or correlating them with other parameters starts at the acquisition of the raw data, followed by a long pipeline of image processing steps. Each one of these steps is susceptible to sources of bias, which may not only limit the accuracy and precision, but can lead to substantial errors. This article provides a detailed review of the steps along the analysis pipeline and their associated pitfalls. These are grouped into 1 pre-processing of data; 2 estimation of the tensor; 3 derivation of voxelwise quantitative parameters; 4 strategies for extracting quantitative parameters; and finally 5 intra-subject and inter-subject comparison, including region of interest, histogram, tract-specific and voxel-based analyses. The article covers important aspects of diffusion MRI analysis, such as motion correction, susceptibility and eddy current distortion correction, model fitting, region of interest placement, histogram and voxel-based analysis. We have assembled 25 pitfalls (several previously unreported) into a single article, which should serve as a useful reference for those embarking on new diffusion MRI-based studies, and as a check for those who may already be running studies but may have overlooked some important confounds. While some of these problems are well known to diffusion experts, they might not be to other researchers wishing to undertake a clinical study based on diffusion MRI. Copyright © 2010 John Wiley & Sons, Ltd. [source] A dynamical model for characterising seasonality effects on eelgrass plastochron intervalsANNALS OF APPLIED BIOLOGY, Issue 1 2010E. Solana-Arellano The plastochron interval is widely used to calculate age and rates of productivity in many plants, including seagrasses. However, plant responses to changing environmental conditions, including seasonal patterns, can introduce substantial errors in methods for calculating age and rates of growth. We propose a generalised method for characterising seasonal variability in eelgrass plastochron values based on a model that consists of a linear combination of a trend, a seasonality component and a stochastic noise component. The model was validated using data obtained biweekly during 1998,2003 in a Zostera marina meadow in a coastal lagoon in northwestern Baja California. Plastochron intervals exhibited marked interannual and seasonal variability as well as in the timing of plastochron interval (PI) minima and maxima. Correlation analyses indicated that sea surface temperature is a fundamental forcing factor for the plastochron interval, whose local variability is influenced by the onset of ,El Niño' and ,La Niña' events. The proposed model provided reliable interpretations that elicited the existence of seasonal processes which are usually masked by multimodal changes in the plastochron interval. Using successive averages of seasonal PI to describe annual cycles resulted in reliable leaf-growth assessments as well as in better determinations of shoot age than those calculated using a single annual mean. [source] | |||||||||||||