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At least 1,639 records · Page 91Linked to original sources
PubMed1989

Quality control of validity of data collected in clinical trials. EORTC Study Group on Data Management (SGDM).

In a study initiated by the EORTC Study Group on Data Management, 15 site visits to main participating centers in ongoing cancer clinical trials have been carried out over a 1 year period. The aim was to evaluate the quality level of EORTC clinical trial data, to find out the order of magnitude of possible problems encountered and to test a technique to objectively assess the quality of data. The process of data collection and the quality of data transfer from hospital charts to EORTC case report forms (CRF) were checked. The data quality was scored and the causes of incorrectness were evaluated. Percentages of correct data ranged from 78% up to 98%; 11/15 centers had greater than 90% correct data. The median rate of error encountered in key data was 2.8% (range 0.5-7%). The main source of error was incorrect transfer of the information recorded in the patient chart to the CRF. Equally good overall results have been observed in the centers where data managers fill in the forms (DM) and those centers without an administrative trial structure (PH). The mean percentage of correct data for both types of centers is 91.4%. The wider range in percentage for incorrect data (DM mean value 3.0%, range 0.5-7%; PH mean value 2.3%, range 1.4-3.1) suggests the important impact of the knowledge and experience of the people involved in data management. The data quality evaluation was hampered by the impossibility of checking part of the data present on the CRF, 0.4-14.5%. Besides knowledge and experience, the main aspects influencing good data quality appeared to be the efficacy of the internal organization and good local data monitoring. The importance of the design of CRFs was also highlighted. As this study was run for on-going protocols, the site visiting team had the opportunity to point out and report to the trial coordinator all shortcomings and controversial points that could thus be corrected during the course of the trial.

Clinical Trials as Topic↗
PubMed2006

Intraocular lens exchange due to incorrect lens power.

PURPOSE: To evaluate patients who had intraocular lens (IOL) exchange for unexpected postoperative refractive errors, determine the sources associated with the errors, and derive an empiric approach to estimating the power for IOL exchange. DESIGN: Retrospective review of interventional case series. PARTICIPANTS: Twenty-two eyes that underwent IOL exchange for correcting unexpected refractive errors after cataract surgery with IOL implantation were reviewed. INTERVENTION: All the IOLs for IOL exchange were placed in-the-bag. The same type of IOLs was used for original IOL implantation and IOL exchange in 91% (20/22) of eyes. MAIN OUTCOME MEASURES: Uncorrected visual acuity, best spectacle-corrected visual acuity (BSCVA), refraction, and reasons for IOL exchange. The equation derived from refractive change and change in IOL power was developed to calculate IOL power for exchange. RESULTS: Of the 22 cases, the identified reasons were keratometry errors in 5 (23%) and incorrect axial length (AL) determination in 3 (14%). In 3 other cases, a wrong IOL was implanted. After IOL exchange, 82% (18/22) of eyes were within +/-0.50 diopters (D) and 86% (19/22) within +/-1.00 D of emmetropia. Uncorrected visual acuity was 20/40 or better in 82% of eyes, and BSCVA was 20/40 or better in 95% (21/22) of eyes. The correlation between change of refraction and IOL power was significant (P<0.002). CONCLUSIONS: Among the identified causes, incorrect corneal power determination was the most frequent reason for incorrect IOL power implantation, followed by error in AL measurement and inserting a wrong IOL. The pre-exchange refraction can be used theoretically to calculate the IOL power for exchange.

Cataract Extraction↗
PubMed2000

Comparison of an in-shoe pressure measurement device to a force plate: concurrent validity of center of pressure measurements.

The purpose of this study was to assess the concurrent validity of center of pressure (COP) measurements during walking, comparing the Parotec System (an in-shoe pressure measurement device) to an Advanced Mechanical Technology Inc. (AMTI) force plate. Pearson correlation coefficients comparing COP displacement in the medial-lateral (ML) and anterior-posterior (AP) direction calculated from the two systems were greater than 0.70 for 52/67 trials (78%) in the ML direction and were greater than 0.90 for 67/67 trials (100%) in the AP direction. The mean root mean square (RMS) error for COP displacement in the ML direction was 0.56+/-0.3 cm and in the AP direction was 1.37+/-0.59 cm. Overall, the Parotec System showed good correlation and small RMS errors when compared to the AMTI force plate. Additional analysis to investigate sources of error pointed to the methods used for calculating the position and orientation of the subject's foot on the force plate. Subsequent analysis to eliminate some of these errors, compared force calculated from the Parotec System and the AMTI plate and showed good to excellent correlation (>0.70) for 62/67 trials (93%). Additional data were collected that allowed for elimination of methodological errors. Again force calculated from the two systems was compared using Pearsons. These data showed excellent correlation (>0.90) for 20/20 trials (100%). This study provides evidence of the validity of the Parotec System for measuring COP during ambulation.

Adult↗
PubMed1999

Effects of lesions to the cerebellar vermis and hemispheres on timing and counting in rats.

The effects of lesions to the cerebellum on numerical and temporal discrimination were examined in rats using a psychophysical choice procedure. Lesions to the cerebellar hemispheres but not the cerebellar vermis produced performance deficits in a numerical discrimination task (2-8 events) and a milliseconds temporal discrimination task (0.2-0.8 s). However, temporal discriminations in the seconds range (2-8 s) were unaffected by either type of lesion. Using W. H. Meck and R. M. Church's (1983) mode-control model of timing and counting, these findings suggest that damage to the cerebellar hemispheres influences a source of constant variability (e.g., switch processes) because constant variability is a prominent source of error during both milliseconds timing and counting but is masked by other sources of variability when timing longer durations (>2 s).

Animals↗
PubMed2005

Is there a general factor in ratings of job performance? A meta-analytic framework for disentangling substantive and error influences.

A database integrating 90 years of empirical studies reporting intercorrelations among rated job performance dimensions was used to test the hypothesis of a general factor in job performance. After controlling for halo error and 3 other sources of measurement error, there remained a general factor in job performance ratings at the construct level accounting for 60% of total variance. Construct-level correlations among rated dimensions of job performance were substantially inflated by halo for both supervisory (33%) and peer (63%) intrarater correlations. These findings have important implications for the measurement of job performance and for theories of job performance.

Databases, Factual↗
PubMed2002

Why misinformation is more likely to be recognised over time: A source monitoring account.

Although memory for actual events tends to be forgotten over time, memory for misinformation tends to be retrieved at a stable rate over long delays or at a rate greater than that found immediately after encoding. To examine whether source monitoring errors contribute to this phenomenon, two experiments investigated subjects' memory for the source of misinformation at different retention intervals. Subjects viewed a slide presentation, read a narrative containing misinformation, and, either 10 minutes or 1 week later, completed a recognition test about details seen in the slides and about the source of these details. After the longer retention interval in both experiments, participants were more likely to agree that they had seen misleading information and were also more likely to incorrectly associate the misinformation with the slide event. Theoretical implications of these findings are considered.

Adult↗
PubMed1955

A microphotometric study of the syntheses of desoxyribonucleic acid and nuclear histone.

1. The fast green stain of Alfert and Geschwind for nuclear basic protein is shown to obey the Beer-Lambert laws when used on purified histone. Interference from acid substances other than nucleic acids as a possible source of error is indicated. 2. Use of this technique after a modified Feulgen stain enables determination of relative amounts of desoxyribonucleic acid and histone in the same individual cells. 3. DNA and histone are shown to have the same distribution in formalin-fixed nuclei. 4. The syntheses of DNA and histone proceed simultaneously resulting in the doubling of both these substances prior to cell division. 5. The standard error for histone values is greater than that for DNA; however, the source of this variability is not known.

Cell Nucleus↗
PubMed1990

Direct estimation of aberrating delays in pulse-echo imaging systems.

Nearfield fluctuations in wave propagation velocity and system timing errors are among the sources of focusing aberrations in pulse-echo imaging systems. For situations in which the source of these errors can be modeled by a stationary phase aberrator placed in front of the transmitter and receiver aperture, appropriate electronic delays might be applied to the signals associated with each array element in order to restore the system to focus. A method is described and evaluated for estimating the set of aberrating delays in a linear array utilizing data from a single two-dimensional scan. The underlying principle is analogous to that of phase closure used for one-way passive interferometry and readily generalizes to two-dimensional arrays. Although the following theory is developed in the context of acoustic imaging, the general approach is applicable to other pulse-echo systems, such as radar.

Acoustics↗
PubMed1991

Mathematical modelling of competitive labelled-ligand assay systems. Theoretical re-evaluation of optimum assay conditions and precision data for some experimentally established radioimmunoassay systems.

A mathematical theory of competitive labelled-ligand assays was developed with the intention of theoretically re-evaluating the optimal assay conditions and precision data of assay systems established by experiment. Our theory is based upon the assumptions of a simple bimolecular reaction mechanism, homogeneous reactants, as well as kinetically indistinguishable labelled and non-labelled ligands. The general case of two-step (non-equilibrium) assay was considered including the one-step (equilibrium) assay as a special case. The solution of the system of corresponding kinetic differential equations was used to mathematically construct standard curves. Furthermore, intraassay precision profiles and indices as well as detection limits were calculated considering solely the pipetting error, epsilon, as a source of experimental error. A procedure was outlined to mathematically determine the optimal incubation conditions for any assay system targeted to a given analyte concentration, P, at which the standard deviation of assay results is to be minimized. Estimates of both the content of binding sites and the equilibrium constant, K, of the specific binding agent are necessary, and these can be derived from Scatchard plots. For six RIA systems, of which three were one-step and three were two-step assays, experimental assay conditions and precision data were compared with theoretical predictions. Experimentally determined antibody binding site concentrations agreed fairly well with those independently evaluated by mathematical optimization. Mean precision indices, defined as constituting an average over the complete precision profile, were found to be within the theoretically predicted range, i.e. two- to threefold the pipetting error.(ABSTRACT TRUNCATED AT 250 WORDS)

Binding Sites, Antibody↗
PubMed1975

Interlaboratory proficiency, intermethod comparison, and calibrator suitability in assay of serum aspartate aminotransferase activity.

Sources of variation in assays of aspartate aminotransferase (EC 2.6.1.1) activity were examined in an interlaboratory survey and through an examination of materials used as calibration materials in these assays. Four highly stable lyophilized specimens containing human cytoplasmic enzyme, with activities of 0, 22, 46, and 96 U/liter at 30 degrees C and optimal substrate concentrations, were assayed by 319 laboratories. Mean values obtained on these specimens by laboratories using 2,4-dinitrophenylhydrazine kits varied among manufacturers and deviated from values expected from this procedure. The average coefficient of variation (CV) with these kits was greater than 20%. Automated continuous-flow procedures with use of diazonium salt showed the best precision (av CV, less than 10%). However, the automated continuous-flow malate dehydrogenase/NADH coupled method produced an average CV greater than 20%. Results from each of the automated methods were related to a reference malate dehydrogenase/NADH coupled continuous kinetic assay method by temperature relationships alone. Mean values from manual diazonium salt procedures were 1.7-fold greater than similar reference values (av CV was 18%). The higher results were attributed to the use of poorly-defined units and to an artifact caused by chromophore stabilizers in this procedure when aqueous samples are used. The average CV in continuous kinetic methods varied among kit manufacturers, ranging from 6 to 28% for the specimen of highest activity. Variations in results were much larger at 366 nm than at 340 nm than at 340ity. Variations in results were much larger at 366 nm than at 340 nm. Interassay relationships of these methods are presented. Concentrations of pyruvate in commercially available calibration materials differed between manufacturers, varied in stability, and deviated from the expected concentration. For some colorimetric assays the precision attained on reported absorbance values for the enzyme specimens was of the same order of magnitude as that for pyruvate standards. Other sources of error are revealed by the interlaboratory survey. The value of commercially available sources of enzyme activity as calibration or control materials was assessed by evaluating the following properties: activity at suboptimal concentrations of L-aspartate or 2-oxoglutarate, temperature effects, preincubation lability owing to aspartate and phosphate, pyridoxal phosphate saturation, contamination with glutamate dehydrogenase, and manufacturer's rated activity. These properties are compared to those of human cytoplasmic enzyme in a human serum matrix.

Aspartate Aminotransferases↗
PubMed1989

Estimation of adult male mortality in Turkey by the widowhood method.

"In this paper, the [widowhood] method is applied to Turkish data to estimate adult male mortality. First, the method and its assumptions are briefly discussed. Secondly, the data source and its major characteristics are described and the method is applied; the resulting mortality measures estimated with the method are compared with findings from other sources, and the reliability of the results, the applicability of the method to the Turkish case, and possible biases risked with the application of the method are elaborated in the final section of the paper." (SUMMARY IN TUR)

Asia↗
PubMed1993

Arterial blood pressure measurement technique.

Arterial blood pressure (BP) measurements, which include invasive direct methods and noninvasive indirect methods, provide a picture of the hemodynamic status of the patient. Invasive BP methods measure pressure pulse wave amplitude; noninvasive methods rely on blood flow or arterial wall motion as a basis for the determination of BP values. To obtain the most accurate BP value, the clinician must identify which measurement variables in a specific clinical situation are most contributory to error and, if possible, use a method of measurement for which the sources of error are not parallel. Blood pressure values obtained by different methods cannot be compared without a thorough understanding of the user-related and instrumentation-related limitations associated with each BP measurement technique.

Blood Pressure Monitors↗
PubMed2001

On integrating direct methods and isomorphous-replacement techniques: triplet estimation and treatment of errors.

The method of joint probability distribution functions has been generalized in order to include and treat different sources of error. The probability distributions of the isomorphous pairs (E(p), E(d)) and of the two triples (E(ph), E(pk), E(ph+k), E(dh), E(dk), E(dh+k)) are obtained, on the assumption that the lack of isomorphism and the errors in measurements cumulate on the E(d) variables. The conditional distributions of the two-phase and the three-phase structure invariants are derived, showing how the reliability of the probabilistic estimates depends on the errors.

Journal Article↗
PubMed1993

Analysis of systematic and random error in MR volumetric flow measurements.

The spatial aspects of error in 2D MR cine phase-velocity mapping are considered in order to define acquisition strategies which will minimize error in measuring volumetric flow. Error was separated into two categories: systematic and random. Potential sources of systematic error examined were intravoxel phase dispersion (IVPD), partial volume effects, misalignment of flow axis and flow-encoding gradients, and improper choice of vessel voxels for flux calculations. Random error was addressed using analysis of propagation of variance. Analytical expressions for sources of error were derived; and computer models were used to test the analytical models. Flow phantom studies examining error in MR volumetric flow measurements were performed and compared with error predicted by the analytical models. Expected error in several clinical situations of interest was then derived to find appropriate acquisition strategies. Spatial resolution, signal to noise ratio, velocity sensitivity and the ratio of the modulus of moving isochromats to that of static isochromats were found to be the most important parameters in controlling error and were found to cause competing effects with respect to systematic and random error.

Artifacts↗
PubMed2002

Origins of spontaneous mutations: specificity and directionality of base-substitution, frameshift, and sequence-substitution mutageneses.

Spontaneous mutations are derived from various sources, including errors made during replication of undamaged template DNA, mutagenic nucleotide substrates, and endogenous DNA lesions. These sources vary in their frequencies and resultant mutations, and are differently affected by the DNA sequence, DNA transactions, and cellular metabolism. Organisms possess a variety of cellular functions to suppress spontaneous mutagenesis, and the specificity and effectiveness of each function strongly affect the pattern of spontaneous mutations. Base substitutions and single-base frameshifts, two major classes of spontaneous mutations, occur non-randomly throughout the genome. Within target DNA sequences there are hotspots for particular types of spontaneous mutations; outside of the hotspots, spontaneous mutations occur more randomly and much less frequently. Hotspot mutations are attributable more to endogenous DNA lesions than to replication errors. Recently, a novel class of mutagenic pathway that depends on short inverted repeats was identified as another important source of hotspot mutagenesis.

Base Sequence↗
PubMed2006

Correction of systematic errors in CORE processing of DOSY data.

DOSY data for mixtures are commonly processed either by single channel methods (e.g. HR-DOSY) or multichannel methods (e.g. CORE). Both aim to separate the signals from species of different molecular sizes by their diffusion coefficients; the result is displayed either as a 2D plot (as in HR-DOSY) or as individual spectra (as in CORE). Both types of methods are sensitive to any systematic errors in the experimental data. The effects of, and remedies for, two such sources of error, spatially non-uniform pulsed field gradients (PFGs) and instrument instability, are demonstrated for CORE processing, using a corrected form of the Stejskal-Tanner equation and reference deconvolution, respectively.

Journal Article↗
PubMed2002

Recent advances in image reconstruction, coil sensitivity calibration, and coil array design for SMASH and generalized parallel MRI.

Parallel magnetic resonance imaging (MRI) techniques use spatial information from arrays of radiofrequency (RF) detector coils to accelerate imaging. A number of parallel MRI techniques have been described in recent years, and numerous clinical applications are currently being explored. The advent of practical parallel imaging presents various challenges for image reconstruction and RF system design. Recent advances in tailored SiMultaneous Acquisition of Spatial Harmonics (SMASH) image reconstructions are summarized. These advances enable robust SMASH imaging in arbitrary image planes with a wide range of coil array geometries. A generalized formalism is described which may be used to understand the relations between SMASH and SENSE, to derive typical implementations of each as special cases, and to form hybrid techniques combining some of the advantages of both. Accurate knowledge of coil sensitivities is crucial for parallel MRI, and errors in calibration represent one of the most common and the most pernicious sources of error in parallel image reconstructions. As one example, motion of the patient and/or the coil array between the sensitivity reference scan and the accelerated acquisition can lead to calibration errors and reconstruction artifacts. Self-calibrating parallel MRI approaches that address this problem by eliminating the need for external sensitivity references are reviewed. The ultimate achievable signal-to-noise ratio (SNR) for parallel MRI studies is closely tied to the geometry and sensitivity patterns of the coil arrays used for spatial encoding. Several parallel imaging array designs that depart from the traditional model of overlapped adjacent loop elements are described.

Calibration↗
PubMed1994

Did I unplug the iron or did I only look at it? External source monitoring across the adult life span.

Adult age differences in external source monitoring were examined using three types of materials differing in degree of salience. Young (mean age = 25.2 years), young-old (mean age = 66.0 years), and old adults (mean age = 74.7 years) learned organizable words, objects, and subject-performed tasks, with a blocked or random presentation. Later, subjects were asked to recognize the items and to identify their origins. Results indicated an age-related deficit in both item memory and external source monitoring as reflected by lower recognition performance, and more source confusion errors with increasing age. A strong positive relationship between episodic item memory and source memory was found for all age groups. Overall, the results suggest that age deficits in source monitoring may be determined by the same factors as age deficits in item memory.

Adolescent↗