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At least 1,045 records · Page 58Linked to original sources
PubMed2004

Validation of MODIS aerosol retrievals and evaluation of potential cloud contamination in East Asia.

MODIS aerosol retrievals onboard Terra/Aqua and ground truth data obtained from AERONET (Aerosol Robtic Network) solar direct radiance measurements are collocated to evaluate the quality of the former in East Asia. AERONET stations in East Asia are separated into two groups according to their locations and the preliminary validation results for each station. The validation results showed that the accuracy of MODIS aerosol retrievals in East Asia is a little worse than that obtained in other regions such as Eastern U.S., Western Europe, Brazil and so on. The primary reason is due to the improper aerosol model used in MODIS aerosol retrieval algorithm, so it is of significance to characterize aerosol properties properly according to long-term ground-based remote sensing or other relevant in situ observations in order to improve MODIS retrievals in East Asia. Cloud contamination is proved to be one of large errors, which is demonstrated by the significant relation between MODIS aerosol retrievals versus cloud fraction, as well as notable improvement of linear relation between satellite and ground aerosol data after potential cloud contamination screened. Hence, it is suggested that more stringent clear sky condition be set in use of MODIS aerosol data. It should be pointed out that the improvement might be offset by other error sources in some cases because of complex relation between different errors. Large seasonal variation of surface reflection and uncertainties associated with it result in large intercepts and random error in MODIS aerosol retrievals in northern inland of East Asia. It remains to be a big problem to retrieve aerosols accurately in inland characterized by relatively larger surface reflection than the requirement in MODIS aerosol retrieval algorithm.

Aerosols↗
PubMed1997

A pragmatic approach to estimating total analytical error of immunoassays.

Several groups have recently commented on the need for more realistic information on analytical performance of laboratory tests. The term "total analytical error" is sometimes used in this context. However, differing opinions have been expressed on how best to obtain estimates of clinical assay error, as it would be perceived by clinicians. We suggest a pragmatic definition of total analytical error for immunoassays and describe our attempts to estimate it by simple designs in the internal quality-control process. We use results over 29 months from a total serum thyroxine RIA. The most important error sources were those related to calibration materials and operator effects, errors not usually captured by short-term or snapshot experiments.

Humans↗
PubMed2002

Grating-based real-time polarization phase-shifting interferometry: error analysis.

A phase Ronchi grating-based real-time polarization phase-shifting method can be efficiently used for dynamic phase measurement in optical interferometry. A thorough error analysis is required for exhibiting how error sources influence phase-measurement results. We analyze the phase-measurement errors that are induced by the retardation error and azimuth angle error of the quarter-wave plate, the azimuth angle error of polarizers, the phase and intensity aberrations of diffractive wave fronts, and pixel mismatch of the interferometric patterns. The results will also be useful for evaluating the phase-measurement accuracy of other similar systems.

Journal Article↗
PubMed2004

Sources of measurement error in an ECG examination: implications for performance-based assessments.

OBJECTIVE: To assess the sources of measurement error in an electrocardiogram (ECG) interpretation examination given in a third-year internal medicine clerkship. DESIGN: Three successive generalizability studies were conducted. (1) Multiple faculty rated student responses to a previously administered exam. (2) The rating criteria were revised and study 1 was repeated. (3) The examination was converted into an extended matching format including multiple cases with the same underlying cardiac problem. RESULTS: The discrepancies among raters (main effects and interactions) were dwarfed by the error associated with case specificity. The largest source of the differences among raters was in rating student errors of commission rather than student errors of omission. Revisions in the rating criteria may have helped increase inter-rater reliability slightly however, due to case specificity, it had little impact on the overall reliability of the exam. The third study indicated the majority of the variability in student performance across cases was in performance across cases within the same type of cardiac problem rather than between different types of cardiac problems. CONCLUSIONS: Case specificity was the overwhelming source of measurement error. The variation among cases came mainly from discrepancies in performance between examples of the same cardiac problem rather than from differences in performance across different types of cardiac problems. This suggests it is necessary to include a large number of cases even if the goal is to assess performance on only a few types of cardiac problems.

Analysis of Variance↗
PubMed2000

American College of Cardiology/European Society of Cardiolgoy International Study of Angiographic Data Compression Phase II: the effects of varying JPEG data compression levels on the quantitative assessment of the degree of stenosis in digital coronary angiography. Joint Photographic Experts Group.

OBJECTIVES: This report describes whether lossy Joint Photographic Experts Group (UPEG) image compression/decompression has an effect on the quantitative assessment of vessel sizes by state-of-the-art quantitative coronary arteriography (QCA). BACKGROUND: The Digital Imaging and Communications in Medicine (DICOM) digital exchange standard for angiocardiography prescribes that images must be stored loss free, thereby limiting JPEG compression to a maximum ratio of 2:1. For practical purposes it would be desirable to increase the compression ratio (CR), which would lead to lossy image compression. METHODS: A series of 48 obstructed coronary segments were compressed/decompressed at CR 1:1 (uncompressed), 6:1, 10:1 and 16:1 and analyzed blindly and in random order using the QCA-CMS analytical software. Similar catheter and vessel start- and end-points were used within each image quartet, respectively. All measurements were repeated after several weeks using newly selected start- and end-points. Three different sub-analyses were carried out: the intra-observer, fixed inter-compression and variable inter-compression analyses, with increasing potential error sources, respectively. RESULTS: The intra-observer analysis showed significant systematic and random errors in the calibration factor at JPEG CR 10:1. The fixed inter-compression analysis demonstrated systematic errors in the calibration factor and recalculated vessel parameter results at CR 16:1 and for the random errors at CR 10:1 and 16:1. The variable inter-compression analysis presented systematic and random errors in the calibration factor and recalculated parameter results at CR 10:1 and 16:1. Any negative effect at CR 6:1 was found only for the calibration factor of the variable inter-compression analysis, which did not show up in the final vessel measurements. CONCLUSIONS: Compression ratios of 10:1 and 16:1 affected the QCA results negatively and therefore should not be used in clinical research studies.

Adult↗
PubMed2000

American College of Cardiology/ European Society of Cardiology international study of angiographic data compression phase II. The effects of varying JPEG data compression levels on the quantitative assessment of the degree of stenosis in digital coronary angiography.

OBJECTIVES: This report describes whether lossy Joint Photographic Experts Group (JPEG) image compression/decompression has an effect on the quantitative assessment of vessel sizes by state-of-the-art quantitative coronary arteriography (QCA). BACKGROUND: The Digital Imaging and Communications in Medicine (DICOM) digital exchange standard for angiocardiography prescribes that images must be stored loss free, thereby limiting JPEG compression to a maximum ratio of 2:1. For practical purposes it would be desirable to increase the compression ratio (CR), which would lead to lossy image compression. METHODS: A series of 48 obstructed coronary segments were compressed/decompressed at CR 1:1 (uncompressed), 6:1, 10:1 and 16:1 and analyzed blindly and in random order using the QCA-CMS analytical software. Similar catheter and vessel start- and end-points were used within each image quartet, respectively. All measurements were repeated after several weeks using newly selected start- and end-points. Three different sub-analyses were carried out: the intra-observer, fixed inter-compression and variable inter-compression analyses, with increasing potential error sources, respectively. RESULTS: The intra-observer analysis showed significant systematic and random errors in the calibration factor at JPEG CR 10:1. The fixed inter-compression analysis demonstrated systematic errors in the calibration factor and recalculated vessel parameter results at CR 16:1 and for the random errors at CR 10:1 and 16:1. The variable inter-compression analysis presented systematic and random errors in the calibration factor and recalculated parameter results at CR 10:1 and 16:1. Any negative effect at CR 6:1 was found only for the calibration factor of the variable inter-compression analysis, which did not show up in the final vessel measurements. CONCLUSIONS: Compression ratios of 10:1 and 16:1 affected the QCA results negatively and therefore should not be used in clinical research studies.

Cardiology↗
PubMed2006

Deviation of optical constants extracted in terahertz transmission spectroscopy.

We present a simulation with a Lorentz oscillator model to evaluate the accuracy of the extracted optical constants in terahertz transmission spectroscopy. The contributions of various error sources including the deflection of the incident angle, the delay-line timing error, and two other kinds of noises are analyzed. It is revealed that the influence of the deflection angle is an approximate parabolic dependence. The delay-line timing uncertainty introduces an exponential frequency dependence in the deviation of the extinction coefficient and a distinct change in the refractive index data in the resonant region. When background and 1/f noises are introduced, the deviation mainly concentrates in the resonant region.

Journal Article↗
PubMed2000

Evaluation of 3D imaging.

Interactive computer-based simulation is gaining acceptance for craniofacial surgical planning. Subjective visualization without objective measurement capability, however, severely limits the value of simulation since spatial accuracy must be maintained. This study investigated the error sources involved in one method of surgical simulation evaluation. Linear and angular measurement errors were found to be within +/- 1 mm and 1 degree. Surface match of scanned objects was slightly less accurate, with errors up to 3 voxels and 4 degrees, and Boolean subtraction methods were 93 to 99% accurate. Once validated, these testing methods were applied to objectively compare craniofacial surgical simulations to post-operative outcomes, and verified that the form of simulation used in this study yields accurate depictions of surgical outcome. However, to fully evaluate surgical simulation, future work is still required to test the new methods in sufficient numbers of patients to achieve statistically significant results. Once completely validated, simulation cannot only be used in pre-operative surgical planning, but also as a post-operative descriptor of surgical and traumatic physical changes. Validated image comparison methods can also show discrepancy of surgical outcome to surgical plan, thus allowing evaluation of surgical technique.

Adult↗
PubMed2002

Uncertainty analysis of predicted disturbance from off-road vehicular traffic in complex landscapes at fort hood.

The US Army Engineering Research Development Center (ERDC) uses a modified form of the Revised Universal Soil Loss Equation (RUSLE) to estimate spatially explicit rates of soil erosion by water across military training facilities. One modification involves the RUSLE support practice factor (P factor), which is used to account for the effect of disturbance by human activities on erosion rates. Since disturbance from off-road military vehicular traffic moving through complex landscapes varies spatially, a spatially explicit nonlinear regression model (disturbance model) is used to predict the distribution of P factor values across a training facility. This research analyzes the uncertainty in this model's disturbance predictions for the Fort Hood training facility in order to determine both the spatial distribution of prediction uncertainty and the contribution of different error sources to that uncertainty. This analysis shows that a three-category vegetation map used by the disturbance model was the greatest source of prediction uncertainty, especially for the map categories shrub and tree. In areas mapped as grass, modeling error (uncertainty associated with the model parameter estimates) was the largest uncertainty source. These results indicate that the use of a high-quality vegetation map that is periodically updated to reflect current vegetation distributions, would produce the greatest reductions in disturbance prediction uncertainty.

Conservation of Natural Resources↗
PubMed2005

Measuring diagnoses: ICD code accuracy.

OBJECTIVE: To examine potential sources of errors at each step of the described inpatient International Classification of Diseases (ICD) coding process. DATA SOURCES/STUDY SETTING: The use of disease codes from the ICD has expanded from classifying morbidity and mortality information for statistical purposes to diverse sets of applications in research, health care policy, and health care finance. By describing a brief history of ICD coding, detailing the process for assigning codes, identifying where errors can be introduced into the process, and reviewing methods for examining code accuracy, we help code users more systematically evaluate code accuracy for their particular applications. STUDY DESIGN/METHODS: We summarize the inpatient ICD diagnostic coding process from patient admission to diagnostic code assignment. We examine potential sources of errors at each step and offer code users a tool for systematically evaluating code accuracy. PRINCIPLE FINDINGS: Main error sources along the "patient trajectory" include amount and quality of information at admission, communication among patients and providers, the clinician's knowledge and experience with the illness, and the clinician's attention to detail. Main error sources along the "paper trail" include variance in the electronic and written records, coder training and experience, facility quality-control efforts, and unintentional and intentional coder errors, such as misspecification, unbundling, and upcoding. CONCLUSIONS: By clearly specifying the code assignment process and heightening their awareness of potential error sources, code users can better evaluate the applicability and limitations of codes for their particular situations. ICD codes can then be used in the most appropriate ways.

Data Collection↗
PubMed2004

The regional elastic properties analysis of myocardium based on echocardiographic 3-D reconstruction of the left ventricle.

The present study evaluates the myocardium regional elastic properties on the basis of relative thickness change (DeltaHWT) in the left ventricular (LV) wall during the diastolic filling phase. Two-dimensional (2-D) LV long-axis images were obtained with a Powervision-380 (Toshiba) transesophageal echocardiographic imager. Three-dimensional (3-D) reconstruction of the LV was carried out by rotation of the transducer in calibrated steps. Endocardial and epicardial surfaces were approximated to the shape of heart wall by means of spherical functions. At the beginning of the diastolic filling phase, LV endocardial surface was divided into equal angular segments sized about 4 x 4 mm in a spherical coordinate system. To define the displacement direction of the heart wall surface fragments at every moment (frame) of diastolic filling, a new algorithm was developed. The elastic properties of LV wall regions were represented as regional DeltaHWT maps. A qualitative test of the method was implemented according to data from clinical and instrumental inspections of the patients with ischemic heart disease (IHD). Possible error sources were considered to evaluate the method quantitatively. The method root-mean-square error was about 5.4%, including errors of initial data, approximation and rounding off.

Algorithms↗
PubMed1990

Real-time gait assessment utilizing a new way of accelerometry.

Real-time registration of body segment angles is essential in artificial body position control. A new method is presented for the real-time calculation of the lower extremity angles using data obtained from pairs of two one-dimensional accelerometers. It is shown that, assuming rigid-body dynamics and simple hinge joints, relative angles (i.e. angles between segments) can be calculated without integration, thereby solving the problem of integration drift normally associated with accelerometry. During the stance phase of walking, the relative angles can be transformed to absolute angles (i.e. relative to the gravitational field direction) for the different leg segments. The feasibility of relative angle calculation is demonstrated by calculation of the knee angle of a healthy subject. Stability and resolution were demonstrated with measurements during standing. Measurements during standing up, sitting down and walking showed that shock (heel-strike) and skin movements, due to movements of the underlying muscle tissue, are the main error sources. Additional signal processing, e.g. low-pass filtering, can be used to diminish this error. The accuracy of the knee angle found is shown to be high enough to be used in a feedback controller for functional electrostimulation of the lower extremities.

Biomechanical Phenomena↗
PubMed1996

Accurate recovery of articulator positions from acoustics: new conclusions based on human data.

Vocal tract models are often used to study the problem of mapping from the acoustic transfer function to the vocal tract area function (inverse mapping). Unfortunately, results based on vocal tract models are strongly affected by the assumptions underlying the models. In this study, the mapping from acoustics (digitized speech samples) to articulation (measurements of the positions of receiver coils placed on the tongue, jaw, and lips) is examined using human data from a single speaker: Simultaneous acoustic and articulator measurements made for vowel-to-vowel transitions, /g/ closures, and transitions into and out of /g/ closures. Articulator positions were measured using an EMMA system to track coils placed on the lips, jaw, and tongue. Using these data, look-up tables were created that allow articulator positions to be estimated from acoustic signals. On a data set not used for making look-up tables, correlations between estimated and actual coil positions of around 94% and root-mean-squared errors around 2 mm are common for coils on the tongue. An error source evaluation shows that estimating articulator positions from quantized acoustics gives root-mean-squared errors that are typically less than 1 mm greater than the errors that would be obtained from quantizing the articulator positions themselves. This study agrees with and extends previous studies of human data by showing that for the data studied, speech acoustics can be used to accurately recover articulator positions.

Humans↗
PubMed1998

Micromechanical behavior of cortical bone as inferred from laser speckle data.

We investigated the micromechanical behavior of porcine femoral cortical bone using a novel, nondestructive, noncontacting, laser-based strain measurement technique. The technique is based upon the well-known concept of tracking translating laser speckle with a linear array CCD camera, but employs a unique data-processing scheme based upon a two-dimensional frequency transform of the data. The method proved to be successful in evaluating strain rates in the bone samples. Measured strain rates ranged between 4.61 and 23.84 micro epsilon/s. Total strains recorded were between 3.7 and 19.1 micro epsilon. Estimated Young's moduli averaged 9.01 +/- 3.93 GPa, which, considering the extremely slow strain rates, is an acceptable value for porcine cortical bone. General advantages of the technique include high sensitivity, insensitivity to zero-mean noise sources, compact design, and the fact that it is truly noncontact. A brief discussion of possible error sources within the method is also given.

Algorithms↗
PubMed1987

Vertebral bone mineral analysis: an integrated approach with CT.

In the assessment of vertebral bone mineral density (BMD) with quantitative computed tomography, accuracy is limited by systematic errors caused by beam-hardening effects and unknown marrow and fat concentrations in trabecular bone. Reproducibility depends on precise repositioning of the patient, selection of the region of interest (ROI), and apparatus stability. In an integrated approach, these error sources were taken into account. Dual-energy methods are incorporated and tested to avoid systematic errors. Contour-detection algorithms for automated ROI determination that work reliably and offer improved reproducibility and efficiency are presented. A new design of reference phantom with solid tissue-equivalent materials provides calibration in BMD units and checks apparatus stability. Dual-energy methods are necessary to achieve accuracy of 5% or better. Automated ROI definition and reference phantom methods are mandatory for reproducibility of 2% or better.

Algorithms↗
PubMed1993

The strategy and advantage in use of an electronic probe for attachment measurement.

Attachment level has been used as the "gold standard" for assessment of the progression of periodontal disease. However, the measurement of attachment level by periodontal probing can be subject to a large number of error sources. Recently, we have designed experiments by using an electronic probe to identify the magnitude of error components due to the instrument, gingival tissue condition, position or probing angle, and time interval between replicate probings. Even with a very careful clinical setting, a few percent of uncontrollable large errors or outliers could not be avoided. A previously used 'option-3' probing scheme to reduce the unexpected large error is justified from the mathematical viewpoint.

Analysis of Variance↗
PubMed1996

Morphometric applications in anatomic pathology.

The components of the cell and tissue changes in many diseases are variable and can therefore be quantified. Characterization of these quantitative changes provides data that is useful not only for making a definitive, cell- and tissue-based diagnosis of disease, but also for predicting the course of disease. The spectrum of changes found in malignant tumors, ie, cell grade, architecture, cellularity, extent of invasion, nature and extent of inflammatory reaction, exemplify this range of quantifiable features. The diagnosis and prognosis of nonneoplastic diseases, ie, myopathy and metabolic bone disease, can also be determined by quantitating tissue changes. Morphometry is the quantification of changes in the "objects" of tissues, ie, cells and organelles, and their organization, using quantitative evaluation tools. The principles of morphometry have been known for a century. With the increasing availability of affordable, powerful computer systems and increasingly flexible and user-friendly software has come easier ability to measure these changes. This article discusses the principles of morphometry with illustrations of types of analysis (ie, area fraction, object counting, shape and size analyses, and mutliparametric analyses) using examples of these applications with discussions of error sources and limitations of morphometry.

Cell Size↗
PubMed1989

Allopurinol kinetics in humans as a means to assess liver function: design of a loading test.

A six-compartment model of allopurinol and oxipurinol kinetics, after intravenous allopurinol injection in the human, is studied further to improve the blood and urine specimen collection schedule for clinical use. The effects of various error sources are also investigated by simple techniques like real data set truncation and adding normally distributed random errors to data obtained from simulation of allopurinol and oxipurinol plasma curves with preset parameters. All parameters estimation is performed with the NONLIN parameter estimation program. Main interest was focused on estimation of the fractional rate constant of transport from the central "extracellular" compartment to the metabolically active compartment. This parameter is regarded as a lumped measure of liver perfusion and liver cell membrane transport. The blood sampling schedule can be reduced to six specimens collected over 60 min, without affecting the accuracy and precision of estimated clinical parameters. The maximum allowable coefficient of variation for preanalytical errors and the analytical within-run and between-run errors are around 5, 4, and 5%, respectively. Analytical between-run bias up to 20% does not affect the estimate of the principal parameter, when both allopurinol and oxipurinol are biased in the same direction. Collection and analysis of urine samples was shown to be unnecessary.

Allopurinol↗