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At least 1,729 records · Page 96Linked to original sources
PubMed2000

Effects of measurement error and sampling resolution on estimates of atrial tissue recovery parameters.

We studied the effect of sampling resolution and measurement error on estimates of tissue recovery parameters using experimental and simulated data. Action potential duration (APD) was estimated from monophasic action potentials recorded at 250 sites (delta x = 3.5 mm) on the endocardium of the canine right atrium (n = 8) during control and acetylcholine perfusion. APD distributions were also simulated using a random number generator, then scaled and filtered to physiological values. The following parameters were estimated at increasing APD sampling interval and measurement error: mean APD, standard deviation of APD, mean APD gradient, standard deviation of APD gradient, APD wavelength, and APD correlation length. We found that large errors can result from APDs collected at inadequate sampling intervals and adequate sampling intervals may be 3-6 times less than the Nyquist interval. Large parameter errors also resulted from data with relatively low levels of measurement error. The effect of measurement error was dependent upon the standard deviation of APD, sampling resolution, and APD wavelength. Inadequate sampling resolution was the largest source of error in experimental parameter estimates. Estimates of mean and standard deviation of APD gradient decreased with spacing as estimates of correlation length and wavelength increased. Careful selection of spacing interval, taking into account the spatial complexity of recovery, as well as considerably low measurement errors will produce accurate estimates of gradients, correlation length, and wavelength.

Acetylcholine↗
PubMed1976

The optical quality of the monochromatic retinal image as a function of focus.

Changes in retinal image quality with focus for an aberration-free eye with a 5-mm pupil are discussed in terms of both geometrical and physical optics. Further modulation transfer function curves illustrate the effect of spherical aberration. These theoretical results are compared with analogous experimental data obtained at different wavelengths with quasi-monochromatic illumination, using a double-pass photo-electric, scanning instrument. The comparison shows that the approximations of geometrical optics predict ocular performance quite well, provided that the errors of focus are reasonably large (greater than or equal to 0.5 DS for a 5-mm entrance pupil). Possible sources of error in the experimental measurements are indicated.

Ocular Physiological Phenomena↗
PubMed1985

Alternative dual system network estimators.

"When there are two or more data systems and none of them enumerates the population at an acceptable completeness level, concern about the bias due to underenumeration suggests an estimator which makes joint use of data compiled by the combined imperfect data systems. Conventional dual system estimators are based on the existence of two separate data collection systems. Dual system network estimators assume a main survey and a follow-up quality check survey. The main survey adopts a multiplicity counting rule that combines two mutually exclusive partial counting rules." Three dual system network estimators are presented. "One was previously proposed by Sirken (1979) and is the natural analogue of the conventional dual system estimator. The two other estimators are proposed as potential improvements, although neither of them is the natural analogue of the conventional dual system estimator. The design effects of the three estimators are compared analytically and empirically with one another, and with those of the single system conventional and network estimators." (summary in FRE)

Data Collection↗
PubMedDate not supplied

Comparative analysis of population age-sex distribution from sample and complete enumeration: 1970 and 1980 census data.

"This paper will attempt to analyze the degree of distortion introduced by sampling, if any, on the age and sex distribution of the population based on the information taken [in the 1970 census] from the sample households. Specifically, the 1970 age and sex distribution of the Philippine population based on the 5-percent sample households will be compared with that obtained from the information supplied by all households, i.e., the actual age and sex distribution of the Philippines....Findings...suggest a failure of the estimation procedures used in past censuses to correct errors attributable to sampling. In particular, the type of sampling error highlighted in this paper is the distortion introduced by sampling in age-sex distribution."

Age Distribution↗
PubMed1980

Mass spectrometry in anesthesia. Problems in using mass spectrometer for monitoring anesthetic gases and vapours.

The use of mass spectrometers for total gas monitoring during anesthesia presents a number of problems which stem from the Cracking Pattern of the gases present, inlet design, viscosity variations, water vapour and machine deterioration. Some of these have been adequately solved, by making mechanical modifications to the inlet system, and by using more complex electronic circuitry to process the signals obtained. Remaining problems, however, still limit accuracy and ease of operation. CO2 measurements are particularly prone to error, especially when sampling halothane-containing gas which leads to significant internal production CO2. A study of the principal sources of error remaining in the machine suggests that the only definitive solution is to develop a microprocessor system which could cope with complex Cracking Patterns and non-linearities, and which would allow complete automation of the calibration procedure.

Anesthetics↗
PubMed1980

The measurement of blood pressure and its interpretation.

The auscultatory or indirect technique of measuring blood pressure has been proved reliable in multiple clinical settings and has received wide application. In clinical studies, special efforts are used to train observers in the measurement of blood pressure and to ensure properly functioning equipment. Similar diligence in providing quality control is necessary in everyday practice to guard against potential errors. With proper care it is possible to obtain readings that classify the pressure to the nearest 5 mm Hg. Table 1 lists some of the sources of error that are possible with the indirect technique and the magnitude of these errors. It should be emphasized that many factors affect blood pressure variation and that any single reading is of limited value. A number of specific sources of information are useful to the practicing physician and his staff for maintaining equipment and for standardizing technique and procedure. Application of these principles to primary care practice should improve the accuracy of health maintenance and allow proper clinical application of the results of the many clinical studies of mild hypertension that are presently being conducted around the world.

Adult↗
PubMed2004

Quality control and reliability of reported doses.

Results of performance tests verifying the dosimetric properties of dosimetric systems are published in various reports (e.g. IAEA and EURADOS). However, there is hardly any information in the open literature relating to the uncertainty in a dose measurement or in the annual dose, which is increased by failure of the evaluation or data management system, damage of the dosemeter itself or by the loss of dosemeter. In this article, an attempt is made to estimate the importance of the above-mentioned conditions. This is achieved by sending questionnaires to about 200 approved dosimetric services in Europe. In total 88 questionnaires were returned and analysed. In the questionnaires, the frequency of occurrence of the various error conditions were investigated. Participants were also asked to evaluate the impact of the error condition from a dosimetric point of view and what countermeasures are taken. The article summarises all responses and compares different sources of errors according to their impact on the uncertainty of the resulting dose and gives a comprehensive overview on quality control actions and reliability on reported doses from European dosimetric services.

Body Burden↗
PubMed2002

Spatiotemporal EEG/MEG source analysis based on a parametric noise covariance model.

A method is described to incorporate the spatiotemporal noise covariance matrix into a spatiotemporal source analysis. The essential feature is that the estimation problem is split into two parts. First, a model is fitted to the observed noise covariance matrix. This model is a Kronecker product of a spatial and a temporal matrix. The spatial matrix models the spatial covariances by a function dependent on sensor distance. The temporal matrix models the temporal covariances as lag dependent. In the second part, sources are estimated given this noise model, which can be done very efficiently due to the Kronecker formulation. An application to real electroencephalogram (EEG) data shows that the noise model fits the data very well. Simulation results show that the resulting source estimates are more precise than those obtained from a standard analysis neglecting the noise covariance. In addition, the estimated standard errors of the source parameter estimates are far more precise than those obtained from a standard analysis. Finally, the source parameter standard errors are used to investigate the effects of temporal sampling. It is shown that increasing the sampling by a factor x, decreases the standard errors of all source parameters with the square root of x.

Computer Simulation↗
PubMed2004

How to track and assess genotyping errors in population genetics studies.

Genotyping errors occur when the genotype determined after molecular analysis does not correspond to the real genotype of the individual under consideration. Virtually every genetic data set includes some erroneous genotypes, but genotyping errors remain a taboo subject in population genetics, even though they might greatly bias the final conclusions, especially for studies based on individual identification. Here, we consider four case studies representing a large variety of population genetics investigations differing in their sampling strategies (noninvasive or traditional), in the type of organism studied (plant or animal) and the molecular markers used [microsatellites or amplified fragment length polymorphisms (AFLPs)]. In these data sets, the estimated genotyping error rate ranges from 0.8% for microsatellite loci from bear tissues to 2.6% for AFLP loci from dwarf birch leaves. Main sources of errors were allelic dropouts for microsatellites and differences in peak intensities for AFLPs, but in both cases human factors were non-negligible error generators. Therefore, tracking genotyping errors and identifying their causes are necessary to clean up the data sets and validate the final results according to the precision required. In addition, we propose the outline of a protocol designed to limit and quantify genotyping errors at each step of the genotyping process. In particular, we recommend (i) several efficient precautions to prevent contaminations and technical artefacts; (ii) systematic use of blind samples and automation; (iii) experience and rigor for laboratory work and scoring; and (iv) systematic reporting of the error rate in population genetics studies.

Animals↗
PubMed1997

Three-dimensional reconstruction of curves from pairs of projection views in the presence of error. II. Analysis of error.

We have previously described an approach to 3D intracerebral vascular reconstruction that uses an MRA as a reconstruction base. Additional vessels seen only by angiography are added by segmenting 2D curves from projection angiograms and reconstructing these curves into 3D, building upon the MRA. This paper is the second of two that discuss the specific problem of reconstructing a 3D curve from a given pair of 2D curves in the presence of error. The method presented is capable of detecting and handling many errors produced by misregistration, image distortion, or misdefinition of 2D curves. The first paper gives an algorithm. The current paper discusses factors affecting the accuracy of a reconstructed curve, with emphasis upon registration error. We analyze the spatial accuracy of a reconstructed point in terms of the relationships between pixel size, relative viewing angle, 3D point location, and registration error. We provide a theoretical framework that, given the known error properties of a registration algorithm, allows optimization of the viewing geometry so as to produce the highest precision of point reconstruction. A major focus is the effect of registration error upon the reconstruction of a curve. We subdivide registration error into two types, one of which produces smoothly continuous point placement errors and the other of which produces pixel pairing errors. We test our ability to reconstruct a 3D curve in the presence of both. Finally, we summarize approaches to other sources of error. We conclude with a list of recommendations to optimize reconstruction accuracy. When projection points are associated by the rules of epipolar geometry, viewplane point displacements should not exceed 1.5-2 mm along the axis perpendicular to epipolar planes.

Biometry↗
PubMed1991

Reliability of six pulse oximeters in chronic obstructive pulmonary disease.

Six pulse oximeters with finger probes were studied in three groups of 17 hypoxemic patients with COPD aged 50 to 75 years. Transcutaneous arterial oxygen saturation (SpO2) was measured with the Nellcor N101 (oximeter 1a), the Ohmeda Biox III (oximeter lb), the Nellcor N200 (oximeter 2a), the Critikon Oxyshuttle (oximeter 2b), the Radiometer Oxi100 (oximeter 3a), and the Ohmeda Biox 3700 (oximeter 3b). The SpO2 was compared with SaO2 measured in simultaneously withdrawn samples of arterial blood (Radiometer OSM2) at three 20-minute steady-state levels of FIO2 ranging from 0.21 to 0.40 (SaO2, 62 to 100 percent). The bias (mean SpO2-SaO2 difference) and the error in precision (SD of the differences) were both below 4 percent for instruments 1a and 1b and remained below 1.2 and 3 percent, respectively, for the others. A good agreement between SpO2 and SaO2, as reflected by the Bartko intraclass correlation coefficient, was observed in instruments 2a, 3a, and 3b. The individual relationships between SpO2-SaO2 differences and SaO2 appeared to be linear and parallel. With four instruments (1a, 1b, 2a, and 2b), the mean slope of this relationship was negative, showing a systematic instrumental error: the lower the SaO2, the larger the overestimation of SaO2. The scattering of the data (precision) principally reflects a subject source of error. In most instruments a technical adjustment could greatly improve instrumental errors and accuracy. The correction of the errors due to between-subject variation would require a system of calibration adjustable by the users to each individual.

Aged↗
PubMedDate not supplied

Accuracy and potential pitfalls of fluoroscopy-guided acetabular cup placement.

Using a total of 30 cadaveric hips, the accuracy of a fluoroscopy-based computer navigation system for cup placement in total hip arthroplasty (THA) was investigated and an error analysis was carried out. The accuracy of placing the acetabular component within a predefined safe zone using computer guidance was compared to the precision that could be achieved with a freehand approach. Accurate control measurements of the implanted cup were obtained using fiducial-based matching to a pre-operative CT scan with respect to the anterior pelvic plane. A significantly higher number of cups were placed in the safe zone with the help of the navigation system. The variability of cup placement could be reduced for cup abduction but not substantially for cup version. An error analysis of inaccurate landmark reconstruction revealed that the registration of the mid-pubic point with fluoroscopy was a potential source of error. Keeping this pitfall in mind, fluoroscopy-based navigation in THA is a useful tool for registration of the pelvic coordinate system, particularly those points that cannot be reached by direct pointer digitization with the patient in the lateral decubitus position.

Acetabulum↗
PubMedDate not supplied

Avoiding errors in spirometry.

Routine measurements of vital capacity, forced expired volume in one second, and forced expiratory flow are easy to make, and so are errors in these measurements. Even if the technologist observes all the rules in performing the tests, there is still a potential source of error in the machine.

Diagnostic Errors↗
PubMed2005

On the concept of model structural error.

A consideration of model structural error leads to some particularly interesting tensions in the model calibration/conditioning process. In applying models we can usually only assess the total error on some output variable for which we have observations. This total error may arise due to input and boundary condition errors, model structural errors and error on the output observation itself (not only measurement error but also as a result of differences in meaning between what is modelled and what is measured). Statistical approaches to model uncertainty generally assume that the errors can be treated as an additive term on the (possibly transformed) model output. This allows for compensation of all the sources of error, as if the model predictions are correct and the total error can be treated as "measurement error." Model structural error is not easily evaluated within this framework. An alternative approach to put more emphasis on model evaluation and rejection is suggested. It is recognised that model success or failure within this framework will depend heavily on an assessment of both input data errors (the "perfect" model will not produce acceptable results if driven with poor input data) and effective observation error (including a consideration of the meaning of observed variables relative to those predicted by a model).

Environment↗
PubMed1980

[Possibilities for misinterpretation of direct fetal electrocardiography].

Direct foetal electrocardiography doses help the experienced examiner to detect cardiac arrhythmia. It can be an early signal of imminent danger. However, errors may occur to the interpreter of foetal electrocardiograms. They may be caused by materno-foetal ECG conduction in a case of intra-uterine foetal death, appearance of maternal pulses in the foetal curve, errors in pole connection of the scalp electrode, and superposition of alternating current on the ECG curve. Knowledge of those potential sources of error is essential, if erroneous decisions are to be avoided.

Arrhythmias, Cardiac↗
PubMed2003

Systematic and statistical error in histogram-based free energy calculations.

A common technique for the numerical calculation of free energies involves estimation of the probability density along a given coordinate from a set of configurations generated via simulation. The process requires discretization of one or more reaction coordinates to generate a histogram from which the continuous probability density is inferred. We show that the finite size of the intervals used to construct the histogram leads to quantifiable systematic error. The width of these intervals also determines the statistical error in the free energy, and the choice of the appropriate interval is therefore driven by the need to balance the two sources of error. We present a method for the construction of the optimal histogram for a given system, and show that the use of this technique requires little additional computational expense. We demonstrate the efficacy of the technique for a model system, and discuss how the principles governing the choice of discretization interval could be used to improve extended sampling techniques.

Journal Article↗
PubMed2000

Three-dimensional myocardial strain analysis based on short- and long-axis magnetic resonance tagged images using a 1D displacement field.

A robust algorithm to estimate three-dimensional strain in the left-ventricular heart wall, based on magnetic resonance (MR) grid-tagging in two sets of orthogonal image planes, is presented. Starting-point of this study was to minimize global interpolation and smoothing. Only the longitudinal displacement was interpolated between long-axis images. Homogeneous strain analysis was performed using small tetrahedrons. The method was tested using a stack of short-axis images and three long-axis images in six healthy volunteers. In addition, the method was subjected to an analytical test case, in which the effect of noise in tag point position on the observed strains was explored for normally distributed noise (0.5 mm RMS). In volunteers, the error in the longitudinal displacement due to interpolation between the long-axis image planes was -0.10 +/- 0. 48 mm (mean +/- SD). The resulting error in the longitudinal strain epsilon(l) was -0.003 +/- 0.02. The analytical test case was used to quantify the effects of three sources of errors on the observed strain. The SD of the difference between homogeneous strain and true strain was 0.06 for epsilon(r.) The error due to the 3-D reconstruction was 0.004 for epsilon(r.) The error in epsilon(r) resulting from simulated noise in the tag point position was 0.10. Equivalent results were obtained for all other strain parameters; thus, the error resulting from noise in the tag point position dominates the error introduced by approximations in the method. Because the proposed method uses a minimum of global interpolation and smoothing, it offers the prospect to detect small regions of aberrant contraction.

Adult↗
PubMed2004

Dorsal anterior cingulate cortex shows fMRI response to internal and external error signals.

In our event-related functional magnetic resonance imaging (fMRI) experiment, participants learned to select between two response options by trial-and-error, using feedback stimuli that indicated monetary gains and losses. The results of the experiment indicate that error responses and error feedback activate the same region of dorsal anterior cingulate cortex, suggesting that this region is sensitive to both internal and external sources of error information.

Adult↗