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At least 1,621 records · Page 90Linked to original sources
PubMed2003

Systematic error in automated in-tube solid-phase microextraction.

The widely employed configuration for automated in-tube solid-phase microextraction (SPME) involves modification of a commercial liquid chromatographic autosampler into an automated extraction device. This popular configuration is demonstrated to result in an inherent systematic error in the quantitation of analyte in a given matrix. The source of error is traced to the accumulation of analyte in the extraction and the pre-extraction segment (i.e., sample loop, metering valve and tubing prior to the metering valve) of the autosampler where the analyte comes in contact with the residual mobile phase. This results in cross-contamination due to sample/mobile phase mixing. The quantity of analyte accumulated in these segments is shown to consistently increase with the increasing number of draw/eject cycles. As a result of the accumulation, the amount of analyte recorded leads to inaccurate quantitative information, leading to overestimation of the limit of detection and limit of quantitation, when automated in-tube SPME is employed as an approach for sample enrichment. Insertion of a 100-microl air plug prior to extraction step was able to significantly minimize sample/mobile phase mixing of analyte with the residual mobile phase in the pre-extraction and extraction step, thus minimizing the systematic error.

Automation↗
PubMed2003

Source monitoring and memory confidence in schizophrenia.

BACKGROUND: The present study attempted to extend previous research on source monitoring deficits in schizophrenia. We hypothesized that patients would show a bias to attribute self-generated words to an external source. Furthermore, it was expected that schizophrenic patients would be overconfident regarding false memory attributions. METHOD: Thirty schizophrenic and 21 healthy participants were instructed to provide a semantic association for 20 words. Subsequently, a list was read containing experimenter- and self-generated words as well as new words. The subject was required to identify each item as old/new, name the source. and state the degree of confidence for the source attribution. RESULTS: Schizophrenic patients displayed a significantly increased number of source attribution errors and were significantly more confident than controls that a false source attribution response was true. The latter bias was ameliorated by higher doses of neuroleptics. CONCLUSIONS: It is inferred that a core cognitive deficit underlying schizophrenia is a failure to distinguish false from true mnestic contents.

Adult↗
PubMed2003

A test for deviation from island-model population structure.

The neutral island model forms the basis for several estimation models that relate patterns of genetic structure to microevolutionary processes. Estimates of gene flow are often based on this model and may be biased when the model's assumptions are violated. An appropriate test for violations is to compare FST scores for individual loci to a null distribution based on the average FST taken over multiple loci. A parametric bootstrap method is described here based on Wright's beta-distribution to generate null distributions of FST for each locus. These null distributions account for error introduced by sampling populations, individuals and loci, and also biological sources of error, including variable alleles/locus and inbreeding. Confidence limits can be obtained directly from these distributions. Significant deviations from the island model may be the result of selection, deviations from the island model's migration pattern, nonequilibrium conditions, or other deviations from island-model assumptions. Only strong biases are likely to be detected because of the inherently large sampling variation of FST. Nevertheless, a coefficient, Nb, describing bias in the spread of the beta-distribution in units comparable to the gene flow parameter, Nm, can be obtained for each locus. In samples from populations of the butterfly Coenonympha tullia, the loci Idh-1, Mdh-1, Pgi and Pgm showed significantly lower FST than expected.

Animals↗
PubMed1993

A comparative assessment of cephalometric errors.

Measuring a cephalometric radiograph is an imprecise task, and the errors associated with this should be quantified and understood. The potential sources of error are outlined and the literature relating to the assessment of measurement errors is reviewed. Three techniques are examined: the error attributable to a single recording; its associated confidence limits; and the coefficient of reliability were the most commonly applied. Data from two error studies were pooled and used to compare the accuracy of 12 skeletal, dental, and soft tissue measurements, using various assessments of each of the three techniques. Within each group, the use of different mathematical criteria led to variation in the results; one method was totally unsuitable. As with all statistical procedures, a knowledge of the technique facilitates interpretation of the results. Some suggestions as to the most satisfactory approach to the estimation of measurement error in cephalometry are given.

Cephalometry↗
PubMed1988

Measuring length of exposure to fluoridated water.

Previous studies have measured subjects' exposure to fluoridated water in two ways: number of years exposed to fluoridation and a dummy variable indicating the fluoridation status of the subjects' present community. The former assumes that fluoride concentrations of water supplies are constant across years, while the latter assumes subjects have never changed residences. Measurement error may occur when either assumption is not satisfied. These two sources of error may be reduced in a newly developed measure of lifetime fluoridation exposure (LFE) containing residence history and fluoride level elements. The aim of this paper is to examine the accuracy of the three measures. Results reveal that the number of years measure and LFE are highly correlated (0.98) and have similar effects in a regression model, indicating both are valid measures of fluoridation exposure. LFE also appears to be fairly insensitive to measurement error due to inaccurate recall of residence histories. Measuring fluoridation exposure with a dummy variable is not recommended.

Adolescent↗
PubMed1966

Isotopic method for estimating microbial cell volumes.

Reid, Della F. (University of Hawaii, Honolulu), and Hilmer A. Frank. Isotopic method for estimating microbial cell volumes. J. Bacteriol. 92:639-644. 1966.-A sensitive isotopic method was developed for estimating the mean cell volume in small numbers of bacterial spores by measuring directly the extracellular fluid volume when cells are present in a saturated inulin solution containing inulin-C(14) at low specific activity. The method minimizes errors due to solute adsorption to the cells and avoids errors arising from differences in cell packing. The effects of these sources of error on cell volume and intercellular space estimations made with nonpenetrating solutes are discussed.

Bacillus subtilis↗
PubMed1987

Recurrent aneuploidic and euploidic abortion.

The evaluation of recurrent embryopathy in man is one of the most difficult tasks. The physician must clearly recognize which couples are experiencing recurrent euploidic abortion and which couples are experiencing recurrent aneuploidic abortion. A firm knowledge of the cytogenetic character of the abortion material enables one to make the next step, namely to determine the source of error in gametogenesis. Techniques are gradually becoming available to identify the source of the parental gametogenic area. In addition, new methods to detect molecular mutation in euploidic abortion are gradually evolving. The establishment of normal protein profiles by two-dimensional gel electrophoresis in normal abortions should enable one to screen recurrent embryopathy in man for abnormal proteins. Abnormal proteins, once identified, can enable us to identify the specific DNA mutation that may be at the heart of the problem.

Abortion, Habitual↗
PubMed1984

Use of questionnaires in occupational studies of pregnancy outcome.

Previous studies have demonstrated a high miscarriage risk among women in laboratories and women exposed to anaesthetic gases. Many of these studies have methodological shortcomings. The aim of the investigation was to study the pregnancy outcome, mainly the miscarriage rate in these occupational groups, and to study such potential sources of error as response bias and inaccurate diagnosis. A questionnaire study was performed among women who had worked at a Swedish hospital as well as women employed in laboratories at the University of Gothenburg (U.G.). Reported pregnancies were accepted only if they could be verified in hospital records or if they were anamnestically reported in medical records. Pregnancies among non-respondents were also sought in medical records. All women suffering a miscarriage after exposure to anaesthetic gases reported their miscarriage while one-third of all miscarriages occurring to women who were not exposed during pregnancy were not reported in the questionnaire. Twelve per cent of the miscarriages among laboratory employees at U.G. could not be verified or found anamnestically in medical records. Work with solvents during pregnancy was more frequently reported for these miscarriages as compared to those which were found in medical records. No significantly increased miscarriage risk after exposure to solvents in laboratory work or to anaesthetic gases could be demonstrated after correction for these errors. It is concluded that use of questionnaires as the only source of information in studies of pregnancy outcome could give biased results and should thus be avoided.

Abortion, Spontaneous↗
PubMedDate not supplied

A method for two dimensional multi-segmental kinematic and kinetic analysis of normal and pathological human gait.

The application of an integrated gait analysis system in clinical research is described. With this system it is possible to analyse the walking pattern of various groups of patients with regard to foot-floor contact, kinematics and kinetics, using a Selspot system. In order to study various groups of patients an extensive body-segment model adapted to the variety in patient groups is dealt with and is related to data from the literature. Special attention has been paid to the analysis or errors in the quantities used in kinematics and kinetics. The various sources of errors leading to inaccuracy of the data describing the walking pattern are discussed.

Biomechanical Phenomena↗
PubMed1993

Human errors in the calculation of monitor units in clinical radiotherapy practice.

Human mistakes are an important source of error in all steps of radiotherapy planning and their incidence should be investigated. As has been recognized by different authors and by the ICRU [4], the human error rate in the calculation of monitor units (MU) is relatively high. At our institute, we measured the human error rate in the calculation of MU by an independent check of the calculation. From September 1991 to June 1992 we identified and corrected 17 serious errors (deviation from the prescribed dose > or = 5%) over 890 controls (1.9%) (daily dose errors). We also found a serious global dose error rate (i.e. the errors induced on the total reference dose for the complete course of the treatment) of 1.3% (9/685) during the period November 1991-June 1992. These values suggest the importance of human errors in the calculation of MU and also confirm the validity of the independent check of MU calculation as one of the simplest ways of avoiding erroneous dose delivery by incorrect calculation of MU.

Humans↗
PubMed1985

Analysis of some errors in the measurement of oxygen extraction and oxygen consumption by the equilibrium inhalation method.

Some sources of error in the equilibrium inhalation method for the measurement of oxygen extraction fraction and CMRO2 by positron emission computed tomography scanning have been evaluated by computer simulation. Emphasis has been placed on errors that have not been thoroughly studied in past work. These include effects of random statistical errors, systematic errors in arterial blood radioactivity concentrations, and errors due to perturbations of the equilibrium state, to tissue inhomogeneity, and to subject motion.

Blood Volume↗
PubMed1998

Task analysis for the investigation of human error in safety-critical software design: a convergent methods approach.

An investigation was conducted into sources of error within a safety-critical software design task. A number of convergent methods of task- and error-analysis were systematically applied: hierarchical task analysis (HTA), error log audit, error observation, work sample and laboratory experiment. HTA, which provided the framework for the deployment of subsequent methods, revealed possible weaknesses in the areas of task automation and job organization. Application of other methods within this more circumscribed context focused on the impact of task and job design issues. The use of a convergent methods approach draws attention to the benefits and shortcomings of individual analysis methods, and illustrates the advantages of combining techniques to analyse complex problems. The features that these techniques should possess are highlighted.

Ergonomics↗
PubMed1993

Analytical goals developed from the inherent error of medical tests.

Biological variation is inherent in all medical tests and is a major source of error in clinical test interpretation. We use inherent test error due to biological variability as the touchstone by which to validate the clinical acceptability of analytical goals. Analytical goals should consist of sets of complementary limits for total error, bias, and imprecision and a medical usefulness criterion (MUC) to ensure that the total error goal is met with stated certainty. We propose analytical goals that are limits for the total error of a measurement system, the joint capability of the process control procedures and the measurement process. Thus, the goals are the allowable analytical error for proficiency evaluation, internal process control, and the total error budget for design of method performance characteristics. We define MUC as the allowable maximum proportion of test diagnostic efficiency lost due to analytical error near medical decision points. MUCs < 0.03 are not routinely clinically useful; MUCs > 0.08 allow method performance characteristics that are undesirable for general clinical use. Using MUC = 0.05 and recommendations that imprecision not exceed one-half of the biological CV, we propose that fixed bias should not exceed one-fourth to one-third of the intraindividual biological CV of the analyte and that total analytical error should not exceed 1.25 to 1.40 times the biological CV. Modification of these analytical goals may arise for reasons of documented clinical practice or process stability. Our model quantifies in medical utility terms the results of such modifications.

Bias↗
PubMed1991

A standardized technique for lower limb radiography. Practice, applications, and error analysis.

The normal standing radiograph, which provides a view of the knee only, is prone to errors of parallax and poor control of patient positioning. A standardized radiographic procedure was developed to control these sources of error. Anteroposterior and lateral views of the lower limb (hip and knee) are obtained without moving the patient from a standardized position; this includes control of ankle position and limb rotation. To correct for parallax error, radiopaque markers are positioned between the patient and the x-ray source. The locations of bone landmarks and reference markers on the radiographs are digitized, and a software package provides a display of key parameters. Error analysis of the method confirmed that most angles were sensitive to contrived positional variations, especially limb rotation and knee flexion. Load distribution between limbs was not critical. The greatest error was random. Most angles were reproducible within +/- 1.3 degrees or less at 95% confidence.

Aged↗
PubMed2002

Quantification of errors in volume measurements of the caudate nucleus using magnetic resonance imaging.

PURPOSE: To quantify the various sources of error in measuring the volume of the caudate nucleus and to understand these errors would lead to the standardization of the MRI protocol and would make the utility of data from around the world more viable in a global database. MATERIALS AND METHODS: We collected data at four different sites all using a Siemens 1.5T Vision MR Scanner. In all cases the same 3D gradient-echo scans were used on a single volunteer and analyzed by a set of five observers. RESULTS: The errors estimated were: system calibration (a random variation of up to 1.2%), partial volume error (a bias of up to 1.5% using isotropic resolution of 1 x 1 x 1 mm(3)), geometric distortion (a potential bias of 1%), intra-observer error (a random variation of up to 3%), effects of ringing (area biases of up to 7% when a zoom of 4 was used) and inter-observer error (with a bias of usually 5- 10% but sometimes as large as 16% among our five observers). Individual mean variations from one system to another differed by less than 5% (except for two observers at one site), consistent with a maximum error of 7% coming from the area bias due to limitations in the images themselves. We also measured the effect of variable resolution on the volume estimates and found that the measured volumes were consistent over a broad range of signal-to-noise-ratios (SNRs). CONCLUSION: Given the observed dependence of the caudate volumes on SNR and resolution, if isotropic resolution is required because a complicated structure is being imaged, then the lower SNR suffered by collecting 1 x 1 x 1 mm(3) data at 1.5T still appears to be sufficient to make accurate volume measurements as long as the contrast-to-noise ratio (CNR) is on the order of 4:1. Based on our results, predictions are made as to what the best approach would be to improve the data acquisition scheme to keep individual errors under 2% and biases under 3.5%. We conclude that if users can be trained to identify the structure of interest in the same way, the inter-observer error could be reduced to that of intra-observer day-to-day error.

Algorithms↗
PubMed2004

More learned irrelevance than perseveration errors in rule shifting in healthy subjects.

The present experiments examined the extent to which two possible sources of error affect healthy subjects' performance in a rule-shift task. All 115 participants first received a discrimination learning task, in which a pair of different visual stimuli was presented on each trial, one of which had to be identified as 'correct.' Each stimulus varied in two dimensions: a task-relevant and a task-irrelevant dimension. Feedback on correctness was given after each choice. After eight successive correct choices, the nature of the task-relevant dimension changed: the post-shift learning phase. Two types of error can occur in this phase: continued responding to the former relevant, but now irrelevant, dimension, a perseverative error, and non-responding to the former irrelevant, but now relevant, dimension, an error due to learned irrelevance. Different groups received a post-shift task in which none, one, or both of these two types of error could affect performance. The number of incorrect choices in the post-shift phase was significantly affected by learned-irrelevance errors but not by perseverative errors. An associative-learning model incorporating feedback-induced changes in both associative strength and saliency of the elements comprising the stimuli can explain these results.

Adolescent↗
PubMed2003

Reconstruction of the isotopic history of animal diets by hair segmental analysis.

Carbon and nitrogen isotope signatures (delta(13)C and delta(15)N) of animal tissues provide information about the diet and, hence, the environment in which the animals are living. Hair is particularly useful as it provides a stable archive of temporal (e.g. seasonal) fluctuations in diet isotope composition. It can be sampled easily and with minimal disturbance from living subjects. However, derivation of the temporal record along the hair length may be subject to errors and uncertainties. This study investigates (and suggests means to minimize) several sources of error, including (a) incomplete sampling, (b) sampling during the quiescent (telogen) phase, (c) non-representative sub-sampling, (d) ignorance of hair growth rate, i.e. time-position relationship of isotope signatures, and (e) non-optimal compromise between analytical/procedural precision and effort/cost. Cattle tail switch hair was collected from animals of different breed, sex and age. Hair was washed, sectioned, and 5- or 10-mm-long sections were analyzed for C and N isotope composition. Signatures along paired hairs were similar (r(2) approximately 0.8) and distances between isotopic minima and maxima nearly identical, indicating that a single hair constituted a representative sample and (except for telogen hair) hair growth rate was the same for paired hairs. However, cutting hair, instead of plucking, caused a variable loss of recently grown hair and information. Telogen hair was identified and data loss due to cutting error reduced when more than one hair from the same animal and sampling region was compared to spot and delimit common and missing regions. Similarly, comparison of isotopic profiles from hair collected at different times identified the segment produced during the respective interval and allowed calculation of average hair growth rate, which varied between animals (0.69-1.06 mm d(-1)). Analysis of alternate 10-mm-long sections for two hairs per animal provided a good compromise between precision/resolution and effort. The method should be applicable to other mammalian species including man.

Aging↗
PubMed1977

Sweat testing for cystic fibrosis: errors associated with the in-situ sweat test using chloride ion selective electrodes.

The in-situ sweat test is prone to errors from various sources. This paper examines errors due to evaporation, absorption of water into the skin, and pressure of the electrode on the skin. Only evaporation caused serious errors. The accuracy and precision when measuring small chloride/sweat samples on the skin are not significantly worse than when measuring bulk solutions.

Chlorides↗