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At least 1,405 records · Page 78Linked to original sources
PubMed2006

Methodological uncertainties in multi-regression analyses of middle-atmospheric data series.

Multi-regression analyses have often been used recently to detect trends, in particular in ozone or temperature data sets in the stratosphere. The confidence in detecting trends depends on a number of factors which generate uncertainties. Part of these uncertainties comes from the random variability and these are what is usually considered. They can be statistically estimated from residual deviations between the data and the fitting model. However, interferences between different sources of variability affecting the data set, such as the Quasi-Biennal Oscillation (QBO), volcanic aerosols, solar flux variability and the trend can also be a critical source of errors. This type of error has hitherto not been well quantified. In this work an artificial data series has been generated to carry out such estimates. The sources of errors considered here are: the length of the data series, the dependence on the choice of parameters used in the fitting model and the time evolution of the trend in the data series. Curves provided here, will permit future studies to test the magnitude of the methodological bias expected for a given case, as shown in several real examples. It is found that, if the data series is shorter than a decade, the uncertainties are very large, whatever factors are chosen to identify the source of the variability. However the errors can be limited when dealing with natural variability, if a sufficient number of periods (for periodic forcings) are covered by the analysed dataset. However when analysing the trend, the response to volcanic eruption induces a bias, whatever the length of the data series. The signal to noise ratio is a key factor: doubling the noise increases the period for which data is required in order to obtain an error smaller than 10%, from 1 to 3-4 decades. Moreover, if non-linear trends are superimposed on the data, and if the length of the series is longer than five years, a non-linear function has to be used to estimate trends. When applied to real data series, and when a breakpoint in the series occurs, the study reveals that data extending over 5 years are needed to detect a significant change in the slope of the ozone trends at mid-latitudes.

Aerosols↗
PubMed1988

Signal processing strategies for enhancement of signal-to-noise ratio of thermodilution measurements.

Thermodilution measurements of clinically important cardiac parameters, such as cardiac output and stroke volume, are subject to many sources of error. The temperature fluctuations (thermal noise) normally found in the pulmonary artery constitute one of these sources of errors. To improve the signal-to-noise ratio of thermodilution flow measurements rather than increase the signal level, we investigated four signal processing strategies designed to reduce the thermal noise power. We applied the noise reduction strategies to thermal noise data, containing simulated thermodilution curves, obtained in a mock circulatory loop. We compared the accuracy and reproducibility of the curve area estimates produced by the algorithms to the area estimates obtained by numerical integration of the thermal signal. Our results show that a bandpass (BP) integration technique combined with a noise canceler can improve thermodilution curve area estimate reproducibility and accuracy. The BP integration technique improved the reproducibility of cardiac output measurements by roughly 16 dB and is directly applicable to most thermodilution hardware currently in use. The more accurate noise cancelers, combined with the BP integration technique, provided correspondingly improved signal-to-noise ratios, with the improvement ranging up to 50 dB.

Algorithms↗
PubMedDate not supplied

Measurement of aortic blood flow by Doppler echocardiography: temporal, technician, and reader variability in normal subjects and the application of generalizability theory in clinical research.

Although Doppler echocardiographic measurements of aortic flow have been found to correlate with stroke volume, the reliability of this technique is unknown. The purpose of this study was to measure the reliability of Doppler estimates of cardiac output by identifying and estimating the magnitude of different sources of error. We measured the reliability of Doppler estimates of cardiac output by identifying the magnitude of sources of error in 11 subjects with studies performed by two technicians and read by two readers. Analysis with generalizability theory demonstrated that the largest portion of the total variance was from differences among patients, with a smaller contribution due to day-to-day variability. Variability due to technician was low for continuous wave Doppler (2.0%), but high for pulsed wave (23.2%). Thus continuous wave, but not pulsed wave Doppler measurements, can be used to detect serial changes in cardiac output due to an intervention.

Adult↗
PubMed2005

A generally applicable validation scheme for the assessment of factors involved in reproducibility and quality of DNA-microarray data.

BACKGROUND: In research laboratories using DNA-microarrays, usually a number of researchers perform experiments, each generating possible sources of error. There is a need for a quick and robust method to assess data quality and sources of errors in DNA-microarray experiments. To this end, a novel and cost-effective validation scheme was devised, implemented, and employed. RESULTS: A number of validation experiments were performed on Lactococcus lactis IL1403 amplicon-based DNA-microarrays. Using the validation scheme and ANOVA, the factors contributing to the variance in normalized DNA-microarray data were estimated. Day-to-day as well as experimenter-dependent variances were shown to contribute strongly to the variance, while dye and culturing had a relatively modest contribution to the variance. CONCLUSION: Even in cases where 90% of the data were kept for analysis and the experiments were performed under challenging conditions (e.g. on different days), the CV was at an acceptable 25%. Clustering experiments showed that trends can be reliably detected also from genes with very low expression levels. The validation scheme thus allows determining conditions that could be improved to yield even higher DNA-microarray data quality.

Analysis of Variance↗
PubMed2005

Stitch error effect on group-delay ripple for long chirped fiber Bragg gratings.

Different sources of stitch errors are identified and simulated in a 200 mm long chirped grating. In each case, the effect on the group-delay ripple and transmission spectrum is investigated. The anticipated response in a typical 10 Gbit/s transmission system is specifically considered. From the simulations, it is clear that information about error source and magnitude can be gained directly from the transmission spectrum.

Journal Article↗
PubMed2005

Rectifying system-specific errors in NMR relaxation measurements.

15N spin relaxation parameters provide a powerful tool for probing the internal dynamics and thermodynamics of proteins. The biological insight provided by these experiments often involves interpretation of small changes in relaxation parameters. This, in turn, requires careful data analysis, especially in the identification and treatment of systematic error. While progress continues on reduction of experiment-specific errors associated with pulse sequences, system-specific sources of error have received far less attention. The impact of these errors varies between facilities, spectrometers, and biological samples. We demonstrate that performing a series of control experiments along with relaxation measurements can help identify, quantify, and isolate sources of system-specific error, and, in some cases, correct for systematic changes. We further demonstrate that control experiments can be performed without significant loss of spectrometer time, and lead to more accurate relaxation parameter values.

Nuclear Magnetic Resonance, Biomolecular↗
PubMed1990

Observer reliability in CT and MRI of the abdomen/pelvis.

The purpose of this research was to evaluate two sources of error in the performance of computerized tomography (CT) and magnetic resonance imaging (MRI) of the abdomen/pelvis. The sources of error assessed were inter- and intra-observer reliability. Thirty abdomen/pelvis CT scans were randomly selected from each of three hospitals (university, VA, military) with different CT scanners. Two radiologists were recruited from each site to be CT observers. Forty-five abdomen/pelvis MRI scans were randomly selected from two institutions with different MRI scanners. Four observers were recruited to read the MRI scans. All scans were read blind without clinical information or patient identification. Overall inter-observer and intra-observer diagnostic agreement was significantly higher for MRI compared to CT. Inter-observer diagnostic agreement rates were also significantly higher for MRI when the etiologies of neoplastic vascular and metabolic/toxic were assigned. Observer experience in CT (range: 5-9 yr) or MRI (range: 2-4 yr) was not statistically associated with improved diagnostic agreement. This research addresses many of the criticisms of the MRI literature and compares MRI favorably to CT.

Abdomen↗
PubMed2003

Auditory hallucinations, source monitoring, and the belief that "voices" are real.

The term source monitoring refers to a variety of cognitive processes individuals use to determine whether an experience originated within the self or came from an external source. A belief that auditory hallucinations are real entities independent of the self may be considered an error in source monitoring. The Source Monitoring Framework (SMF) is the most developed and empirically validated model of how ordinary individuals judge whether an event was self-generated or occurred in the outside world. This study of 41 acute inpatients is a first attempt to apply the SMF to autobiographical reports of auditory hallucinations in a clinical setting. Consistent with the SMF, results suggest that similarities between "voices" and real speakers may offer a partial explanation of why patients believe the voices are real. While the SMF provides a useful conceptual background for examining the phenomenology of these voices, the types of source monitoring errors typically encountered in normal individuals do not fully account for this belief as it occurs in psychotic individuals.

Cognition Disorders↗
PubMed1977

The mescaline or strychnine spikes as a tool for physiological and pharmacological studies.

The all-or-nothing spikes elicited by antidromic, afferent, or direct electrical stimulation of a cortical area treated with mescaline or strychnine were studied. These potentials are most probably originated in nonsynaptic dentritic membranes and their changes in amplitude and latency can be an index of the excitability of these structures. Several factors are considered as sources of error when measurements of such parameters are made to follow their changes in different experimental conditions. When these sources of error can be eliminated, the frequency, amplitude and latency of those spikes can be taken as an index of the variations in cortical excitability under some physiological conditions. Another use of these giant potentials can be for the study of humoral and pharmacological agents.

Animals↗
PubMed2004

Quality assurance in radiotherapy: evaluation of errors and incidents recorded over a 10 year period.

BACKGROUND AND PURPOSE: To establish an incident reporting system to (1) record and classify incidents, (2) assess the impact of incidents on patients in terms of dose errors, and (3) evaluate the effectiveness of the quality assurance checking program implemented at the Radiation Treatment Program at the Northeastern Ontario Regional Cancer Centre (NEORCC). MATERIALS AND METHODS: An 'incident' is defined as an event or a series of events that has led to, or would have led to if undiscovered, dose errors to a patient undergoing radiation therapy treatment. The incidents reported between November 1992 and December 2002 were analyzed according to their source of error, stage of discovery and dose errors. RESULTS: Between November 1992 and December 2002, 13385 patients have undergone radiation treatment at the NEORCC. Over this period of time, 624 'incidents' were reported. Source of error: the majority of the incidents (42.1%) were related to errors in 'documentation' and most of these could be attributed to 'error in data transfer' or 'inadequate communication'. 'Patient set-up error' accounted for 40.4% of the incidents and about half of these errors were related to shielding. Errors in 'treatment planning' accounted for 13.0% of the incidents. Stage of discovery: independent checks by another dosimetrist/physicist and checking during patient first set-up and port film were effective in detecting documentation errors and errors in treatment planning. The use of portal imaging (Siemens Beamview) has enabled us to detect and correct for more than 85% of reported shielding errors in patient set-up. Dose errors: 40% of the incidents were discovered before the first treatment with no dose error to patients. Overall 97.9% of the incidents had dose error of <5%. CONCLUSIONS: Human errors occur during the various stages of the complex process of radiation therapy. If uncorrected, these could lead to substantial dose errors to patients. The implementation of a quality assurance checking program can substantially reduce these human errors but never totally eliminate them.

Communication↗
PubMed1995

Reliability of stomach temperature changes in determining feeding characteristics of seabirds

We examined the accuracy of stomach temperature archival units (STAUs), which are typically used to determine feeding activity in marine endotherms, with regard to determination of the time of prey ingestion as well as the number of prey items ingested and their masses. Units were deployed in nine species of free-living seabirds, where feeding conditions were uncontrolled, eight species of captive seabirds, where feeding conditions could be partially controlled, and in laboratory stomach simulations, where variables could be strictly controlled. The quality of data obtained on the timing of feeding, the mass ingested and the number of prey items ingested was subject to two main sources of error (i) those induced by changes in animal activity and (ii) those resulting from the physical form of the STAUs themselves. Animal activity factors considered important included the following: variability in (a) body temperature, (b) stomach blood perfusion, (c) consistency of stomach contents and (d) stomach churning and changes in body orientation. The physical form (size and buoyancy) of the STAU affected the location of the unit within the stomach, and thus the likelihood that ingested prey comes into contact with the sensor. The timing of prey ingestion can generally be determined accurately; however, considerable errors in mass estimates can occur if data acquired using STAUs are not critically assessed. An understanding of these sources of errors will allow researchers to construct STAUs appropriate to the species being studied and to analyze data critically so that errors are reduced.

Journal Article↗
PubMed2002

Estimation and implications of random errors in whole-body dosimetry for targeted radionuclide therapy.

For targeted radionuclide therapy, the level of activity to be administered is often determined from whole-body dosimetry performed on a pre-therapy tracer study. The largest potential source of error in this method is due to inconsistent or inaccurate activity retention measurements. The main aim of this study was to develop a simple method to quantify the uncertainty in the absorbed dose due to these inaccuracies. A secondary aim was to assess the effect of error propagation from the results of the tracer study to predictive absorbed dose estimates for the therapy as a result of using different radionuclides for each. Standard error analysis was applied to the MIRD schema for absorbed dose calculations. An equation was derived to describe the uncertainty in the absorbed dose estimate due solely to random errors in activity-time data, requiring only these data as input. Two illustrative examples are given. It is also shown that any errors present in the dosimetry calculations following the tracer study will propagate to errors in predictions made for the therapy study according to the ratio of the respective effective half-lives. If the therapy isotope has a much longer physical half-life than the tracer isotope (as is the case, for example, when using 123I as a tracer for 131I therapy) the propagation of errors can be significant. The equations derived provide a simple means to estimate two potentially large sources of error in whole-body absorbed dose calculations.

Computer Simulation↗
PubMed2005

Accuracy of a PDA-based dietary assessment program.

OBJECTIVE: Study objectives were to assess the accuracy of a food record delivered on a personal digital assistant (PDA) and to examine sources of error from the PDA-based food record. METHODS: Thirty-nine adults recruited with a newspaper advertisement were trained to record food intake using DietMatePro, a dietary assessment program delivered on a PDA. After 3 d of use, subjects returned for a follow-up visit in which a 24-h recall was conducted. Subjects also were timed while recording an observed, weighed lunch. Recalled and actual food intakes were compared with estimates recorded by the subjects when using the PDA. Paired sample t tests and Pearson's correlations assessed means and measurements of association between DietMatePro data compared with the 24-h recall data and observed meal data. Bland-Altman plots were used to assess bias in food recording. Sources of error were quantified by using calories as the unit for comparison. RESULTS: There were no significant differences in daily totals for calories and macronutrients between DietMatePro data and comparison measurements. Pearson's correlations of associations between DietMatePro data and the comparison measurement ranged from 0.505 to 0.797 (P < 0.005, n = 28) for the 24-h recall and from 0.419 to 0.786 (P < 0.005, n = 33) for the observed lunch, depending on the nutrient measured. The largest source of absolute error in caloric estimation was attributable to portion size estimation error (49%). CONCLUSIONS: DietMatePro, a PDA-based dietary assessment program, provides a method of assessing energy and macronutrient intakes comparable to the 24-h recall in samples lacking dietary restrictions.

Adult↗
PubMed2005

Target definition in prostate, head, and neck.

Target definition is a major source of errors in both prostate and head and neck external-beam radiation treatment. Delineation errors remain constant during the course of radiation and therefore have a large impact on the dose to the tumor. Major sources of delineation variation are visibility of the target including its extensions, disagreement on the target extension, and interpretation or lack of delineation protocols. The visibility of the target can be greatly improved with the use of multimodality imaging. Both in the head and neck and the prostate, computed tomography (CT)-magnetic resonance imaging coregistration decreases the target volume and its variability. CT-positron emission tomography delineation is promising for delineation in head and neck cancer. Despite the better visibility, a different interpretation of the target extension remains a major source of error. The use of coregistration of CT with a second modality, together with improved guidelines for delineation and an online anatomical atlas, increases agreement between observers in prostate, lung, and nasopharynx tumors. Delineation errors should not be treated differently from other geometrical errors. Similar margin recipes for the correction of setup errors and organ motion should be adapted to incorporate the effect of delineation errors. A calculation of a 3-dimensional clinical target volume-planning target volume margin incorporating delineation errors for the head and neck is around 6.1 to 9.7 mm. Given the good local control of IMRT with smaller margins and smaller pathological specimens, it is likely that the delineated CTV frequently overestimates the actual volume.

Head and Neck Neoplasms↗
PubMed1993

Laboratory assessment of five glucose meters designed for self-monitoring of blood glucose concentration.

We describe a laboratory assessment of five blood glucose meters. The instruments' analytical characteristics under optimum laboratory conditions and examination of potential sources of errors were intercompared. All glucometers produced precise results, and in all but one meter the CV values varied between 1.5% and 6%. CV's for reproducibility and within-day precision of Glucometer Gx were 10.5% and 7.3%. Sample volume, blood incubation time and colour stability of the strip may influence the results of blood glucose measurements with glucose meters. Underloading the sample strip had statistically significant effects on normal blood glucose values for all meters. One Touch II gave also significantly different results when the strip was overloaded. Incubation times shorter or longer than those recommended by the manufacturer influenced the results of all glucose meters. After colour development of the strip, changes were small for Haemo-Glukotest 20-800R strips and Glucotrix, whereas Accutrend glucose strips had to be read immediately following the prescribed incubation time. Comparison of the glucose meter results with those obtained by the hexokinase method showed good correlation coefficients for Reflolux S (r = 0.992), Accutrend (r = 0.988), One Touch II (r = 0.942), Glucometer Gx (r = 0.986) and Glucocard (r = 0.976). Error grid analysis showed that the results of all meters were clinically correct. Variations in haematocrit are known to be a source of errors when blood glucose is determined with a test strip. In the normoglycaemic range the results obtained with Accutrend and Glucocard were not influenced by even extreme haematocrit values. In the high glucose concentration range there was a decrease in blood glucose values with increasing haematocrit for all meters. This error was smaller with Accutrend and Glucocard than with the other meters. Although the new versions of the monitors are easier to use, their analytical performance is not superior to earlier models.

Blood Glucose Self-Monitoring↗
PubMedDate not supplied

Investigating induced abortion in developing countries: methods and problems.

Interest in abortion research is reemerging, partly as a result of political changes and partly due to evidence of the contribution of induced abortion to maternal mortality in developing countries. Information is lacking on all aspects of induced abortion, particularly methodological issues. This article reviews the methodological dilemmas encountered in previous studies, which provide useful lessons for future research on induced abortion and its complications, including related deaths. Adverse health outcomes of induced abortion are emphasized, because these are largely avoidable with access to safe abortion services. The main sources of information are examined, and their relevance for assessing rates of induced abortion, complications, and mortality is addressed. Two of the major topics are the problems of identifying cases of induced abortion, abortion complications, and related deaths, and the difficulties of selecting a valid and representative sample of women having the outcome of interest, with an appropriate comparison group. The article concludes with a discussion of approaches for improving the accuracy, completeness, and representativeness of information on induced abortion. Although the prospects for high-quality information seem daunting, it is essential that methodological advances accompany program efforts to alleviate this important public health problem.

Abortion, Induced↗
PubMed1989

Spherical lens decentration errors by Prentice's rule.

Sources and magnitude of errors in applying Prentice's rule in calculating decentration for spherical lenses were investigated. It was found that spherical aberration can be the only source of error in applying the rule. The overall magnitude of errors in effective prismatic effect is, however, clinically insignificant.

Eyeglasses↗