Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Sampling Errors”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,405 records · Page 78Linked to original sources
PubMed2003

A feasibility study on the predictive emission monitoring system applied to the Hsinta power plant of Taiwan Power Company.

The continuous emission monitoring system (CEMS) can monitor flue gas emissions continuously and instantaneously. However, it has the disadvantages of enormous cost, easily producing errors in sampling periods of bad weather, lagging response in variable ambient environments, and missing data in daily zero and span tests and maintenance. The concept of a predictive emission monitoring system (PEMS) is to use the operating parameters of combustion equipment through thermodynamic or statistical methods to construct a mathematic model that can predict emissions by a computer program. The goal of this study is to set up a PEMS in a gas-fired combined cycle power generation unit at the Hsinta station of Taiwan Power Co. The emissions to be monitored include nitrogen oxides (NOx) and oxygen (O2) in flue gas. The major variables of the predictive model were determined based on the combustion theory. The data of these variables then were analyzed to establish a regression model. From the regression results, the influences of these variables are discussed and the predicted values are compared with the CEMS data for accuracy. In addition, according to the cost information, the capital and operation and maintenance costs for a PEMS can be much lower than those for a CEMS.

Automation↗
PubMedDate not supplied

Performance of electronic flow rate meters used for calibration of air sampling pumps.

Electronic flow rate meters (EFRMs) have been used by industrial hygienists for more than 20 years and are useful because they reduce the time required to calibrate air sampling pumps. This study compared the variability of the traditional bubble burette meter with electronic flow meters and simultaneously compared several EFRMs under different calibration conditions. The flow rates of air sampling pumps were set using a standard bubble burette meter at flow rates of 2 or 50 mL/min using two pressure drops, 2 inches water and 10 inches water. Four airflow rate meters (Bios DryCal, A.P. Buck mini-Buck, MSA Accuflow, and Sensidyne Gilibrator) were concurrently compared at each of the pump flow rate and pressure drop combinations. Results indicated that the standard bubble burette method is more variable than the EFRMs and that the flow rates given by the EFRMs were significantly different (p<0.0001) at both the high and low flow rates. Although the calibrators gave significantly different flow rates, the difference was within the acceptable air sampling pump error of +/- 5%.

Air Pollution↗
PubMed1999

Measures of Inequality Are Not Equal.

Inequalities in reproductive success or resource acquisition are fundamental to evolution and population ecology. There is, however, no unique way to measure inequality. We review 21 measures used to quantify it and clarify the conceptual difference between inequality and skewness. In two very different families of distributions, all indices except three give higher values for more unequal distributions of resources, although some of them are poor at distinguishing between similar inequality values. When applied correctly by testing against a null hypothesis of no inequality among individuals, most indices can therefore be used to detect deviations from randomness, but with varying ease as most lack statistical tables and rely on resampling techniques instead. As an example to test the performance of the 21 indices, we used each index to analyze 71 data sets of unequal mating success in leks. In pairwise comparisons, 24% of the indices fail to show a positive intercorrelation. This reflects differences in how indices incorporate variation in the number of competitors and mean acquisition of the resource. All indices are sensitive to these aspects if inequality is measured in data arising from different distributions. These results illustrate the general conclusion that a unique "best" solution is not available; each measure presents its own definition of inequality. The choice of an inequality index requires specifying the null expectations and interpreting deviating values in relation to the biological question being addressed. This means, for example, considering individual male mating success in the context of lekking or relating the mass distribution of individual plants to alternative hypotheses about competition in plant population ecology. When sample sizes vary, testing robustness by using several measures is advisable.

leks↗
PubMed1992

Time estimation and reproduction is abnormal in Parkinson's disease.

We compared the performance of 44 patients with Parkinson's disease (PD), tested 12-24 h after withdrawal of dopaminergic medication, with 20 age-matched controls, on a verbal test for time estimation and in several time reproduction tasks. Patients with PD underestimated the duration of a time interval in the verbal time estimation task and showed overproduction of time intervals when required to reproduce a short time sample. Absolute errors were greater in the reproduction of longer time intervals in both control and PD patients, but especially in the latter. The presentation of time markers at faster rates had a detrimental effect on the performance of the patients but not of the controls. Patients with more severe PD performed worse on the time estimation and reproduction tasks compared with those with milder disease. Administration of levodopa-carbidopa (205/25 mg, p.o.) significantly reduced absolute errors in time estimation and reproduction in conditions where time markers were presented at the two faster rates of 5 Hz and 3.3 Hz. Performance in these two latter tests best discriminated patients and controls and had a positive significant association with simple reaction time and movement time. These results lead us to suggest that time estimation, i.e. the 'internal clock', is abnormally slow in PD.

Adult↗
PubMed1987

Attributable risk ratio estimation from matched-pairs case-control data.

Explicit formulas are provided for estimating the attributable risk ratio among the exposed and the entire target population utilizing matched-pairs data. Large-sample standard errors and corresponding confidence intervals are provided. These estimates can be obtained from the cross-classification frequencies of matched pairs by disease and exposure status in the usual 2 X 2 table. The key to the development of these formulas lies in recognizing that attributable risk among the exposed is a direct function of the odds ratio, and population attributable risk is a direct function of the odds ratio and exposure prevalence among only the cases (assuming a rare disease). The formulas presented in this paper require only a calculator for computation. The methodology is illustrated with data from a matched-pairs case-control study of oral conjugated estrogens and endometrial cancer.

Contraceptives, Oral↗
PubMed2000

Fast algorithm for finding the eigenvalue distribution of very large matrices

A theoretical analysis is given of the equation of motion method, due to Alben et al. [Phys. Rev. B 12, 4090 (1975)], to compute the eigenvalue distribution (density of states) of very large matrices. The salient feature of this method is that for matrices of the kind encountered in quantum physics the memory and CPU requirements of this method scale linearly with the dimension of the matrix. We derive a rigorous estimate of the statistical error, supporting earlier observations that the computational efficiency of this approach increases with the matrix size. We use this method and an imaginary-time version of it to compute the energy and specific heat of three different, exactly solvable, spin-1/2 models, and compare with the exact results to study the dependence of the statistical errors on sample and matrix size.

Journal Article↗
PubMed1990

Quantitative estimation of hair growth. I. androgenetic alopecia in women: effect of minoxidil.

Quantitative growth of hair over a 40-week period is reported for eight women with androgenetic alopecia. Using a random, double-blind protocol, the women were given either a 2% minoxidil solution or a placebo of vehicle only. Hair in a permanently marked site on the fronto-parietal scalp was pulled through a 1-cm-square clear plastic template, and the outline of the template was drawn on the scalp. The hair was carefully hand clipped and collected at five eight-week intervals (one untreated and four treated), using great care to collect only hairs within the marked area. Subsequent measurements included the total weight of hair grown in the marked area, the total number of hairs, and, on a randomized 50-hair subsample, the weight, lengths, and optical diameters. Calculated quantities included average weight per hair, average length, and average optical width. The average total hair weight of minoxidil-treated subjects increased over the 32-week test period by 42.5%, compared to 1.9% for the placebo-treated subjects (average p = 0.018). Changes for the average number count were 29.9% and -2.6%, respectively (average p = 0.022). These increases, observed using an unusually small number of subjects, clearly distinguished the treated subjects from the untreated. During the same test period, the averaged quantities of weight, diameter, and length from the 50-hair subsample showed insignificant change (p usually greater than 0.5). In addition to showing a larger percentage increase than did the total number, the total weight is not only easier to obtain, but less prone to error during sampling and measurement. Therefore, we recommend that total weight from a defined area be considered as the primary quantitative estimator for hair growth.

Adolescent↗
PubMed1999

Treatment comparisons based on two-dimensional safety and efficacy alternatives in oncology trials.

In addition to their desired anticancer effects, most cancer treatments may also cause transient toxicity, permanent organ damage, or death. A critical question in comparing an experimental treatment to a standard is how much increase in an adverse event rate is an acceptable trade-off for achieving a targeted improvement in efficacy, or vice versa. We consider settings where one may characterize patient outcome as a bivariate (efficacy, safety) variable and quantify treatment effect as a corresponding two-dimensional parameter. A set of target parameters, each representing a clinically meaningful improvement over the standard, are elicited from the physician. Each target is a two-dimensional generalization of the usual one-dimensional shift parameter. We define the alternative hypothesis in the two-dimensional effect space as the convex hull of the sets of parameters that are at least as desirable as each target point. The rejection region is obtained by shifting the alternative toward (0,0) to achieve a given type I error, with sample size computed to achieve a given power at the targets. The method is illustrated by application to two cancer chemotherapy trials.

Antineoplastic Agents, Alkylating↗
PubMed2006

On error prediction in circular systematic sampling.

An extended covariogram model is discussed for estimating the precision of circular systematic sampling. The extension is motivated by recent developments in shape analysis of featureless planar objects. Preliminary simulation results indicate that it is important to consider the extended covariogram model.

Journal Article↗
PubMed2001

Sampling considerations for intensity modulated radiotherapy verification using electronic portal imaging.

A model has been developed to describe the sampling process that occurs when intensity modulated radiotherapy treatments (delivered with a multileaf collimator) are imaged with an electronic portal imaging device that acquires a set of frames with a finite dead-time between them. The effects of the imaging duty cycle and frame rate on the accuracy of dosimetric verification have been studied. A frame interval of 1 s with 25%, 50% and 75% duty cycle, and a 50% duty cycle with frame intervals of 1, 2, 4, 8, and 16 s have been studied for a smoothly varying hemispherical intensity profile, and a 50% duty cycle with frame intervals of 1, 2, 4, and 8 s for a pixellated distribution. In addition an intensity modulated beam for breast radiotherapy has been modeled and imaged for 0.33 s frame time and 1, 2, and 3 s frame separation. The results show that under sparse temporal sampling conditions, errors of the order of 10% may ensue and occur with an oscillatory pattern. For the beams studied, imaging with a 1 or 2 s frame interval resulted in small errors at the 1%-2% level, for all duty cycles shown.

Breast↗
PubMed2006

Emergence of CXCR4-using human immunodeficiency virus type 1 (HIV-1) variants in a minority of HIV-1-infected patients following treatment with the CCR5 antagonist maraviroc is from a pretreatment CXCR4-using virus reservoir.

Antagonists of the human immunodeficiency virus type 1 (HIV-1) coreceptor, CCR5, are being developed as the first anti-HIV agents acting on a host cell target. We monitored the coreceptor tropism of circulating virus, screened at baseline for coreceptor tropism, in 64 HIV-1-infected patients who received maraviroc (MVC, UK-427,857) as monotherapy for 10 days. Sixty-two patients harbored CCR5-tropic virus at baseline and had a posttreatment phenotype result. Circulating virus remained CCR5 tropic in 60/62 patients, 51 of whom experienced an HIV RNA reduction from baseline of >1 log(10) copies/ml, indicating that CXCR4-using variants were not rapidly selected despite CCR5-specific drug pressure. In two patients, viral load declined during treatment and CXCR4-using virus was detected at day 11. No pretreatment factor predicted the emergence of CXCR4-tropic virus during maraviroc therapy in these two patients. Phylogenetic analysis of envelope (Env) clones from pre- and posttreatment time points indicated that the CXCR4-using variants probably emerged by outgrowth of a pretreatment CXCR4-using reservoir, rather than via coreceptor switch of a CCR5-tropic clone under selection pressure from maraviroc. Phylogenetic analysis was also performed on Env clones from a third patient harboring CXCR4-using virus prior to treatment. This patient was enrolled due to a sample labeling error. Although this patient experienced no overall reduction in viral load in response to treatment, the CCR5-tropic components of the circulating virus did appear to be suppressed while receiving maraviroc as monotherapy. Importantly, in all three patients, circulating virus reverted to predominantly CCR5 tropic following cessation of maraviroc.

Anti-HIV Agents↗
PubMed1995

The AAPM/RSNA physics tutorial for residents. Introduction to emission CT.

The article explores the fundamentals of emission computed tomography (CT) from a nonmathematical approach. Tomographic images reveal the internal distributions of radioactivity in three-dimensional objects, and thus allows anatomic localization and improves contrast. Tomography requires a stable distribution of radionuclides, uniform detector response, an accurate center of rotation, and a complete set of projections. In emission CT, a large number of measurements, called projections, are collected at various angles about the patient during the examination. This information is organized by the angles of acquisition into a stack, called a sinogram. Each projection is modified by applying a reconstruction filter (eg, ramp or windowed reconstruction filters). These modified projections are backprojected to form the transverse tomographic images. The quality of tomographic images generated from filtered backprojection depends on the underlying assumptions about the projections. Typical artifacts that result from violations of these assumptions include motion, uniformity, and attenuation artifacts. In addition, an inaccurate center of rotation, insufficient angular sampling, and errors in selection of pixel size can result in poor-quality reconstructed images.

Artifacts↗
PubMed1999

Differential expression of serotonin 5-HT2 receptors during rat embryogenesis.

Serotonin (5-HT) is a neurotransmitter that also functions as a hormone and a growth factor. 5-HT is involved in numerous physiological actions and displays complex pharmacological properties. As a growth factor, 5-HT plays a role in cell proliferation and differentiation of neuronal and nonneuronal tissue and it transduces its signals through more than fourteen subtypes of 5-HT receptors. Since determination of the expression and distribution is important for understanding the role of the 5-HT receptors, we have developed an RNase protection assay (RPA) that allows the simultaneous analysis of 5-HT2AR, 5-HT2BR, and 5-HT2CR per sample of RNA. This multiprobe set also comprises probes for two house-keeping genes, L32 and GAPDH, which control for sample-loading errors. Using this RPA probe set, we have examined the relative expression of 5-HT2AR, 5-HT2BR, and 5-HT2CR in rat embryos inclusively from embryonic day (ED) 9 to 21 of development. Our data indicate that 5-HT2AR levels gradually increased from ED11 to ED21. The expression of 5-HT2BR was decreased between ED9 to ED11 then remained relatively constant through ED21. 5-HT2CR was initially expressed at residual levels between ED9 and ED12 but dramatically increased to a peak level at ED13, then decreased by ED17. Expression of the 5-HT2 receptors in these tissues was confirmed independently by RT-PCR indicating that there is developmental regulation in the expression of these receptors. The 5-HT2R multiprobe assay will be useful for detecting relative changes in the expression of these receptors in developmental, normal and pathological tissues as well as for monitoring relative changes in expression resulting from the use of pharmaceutical agents.

Animals↗
PubMed1980

Outlying values in the National Quality Control Scheme.

A modified statistical method for the detection of outlying values has been applied to five fortnightly sets of data from the National Quality Control Scheme. The overall outlier rate was 2.4%, not varying greatly between analytes. Many of the outliers appear to be due to sample identification errors.

Chemistry, Clinical↗
PubMed1998

Multianalyte serum analysis using mid-infrared spectroscopy.

This study assesses the potential for using mid-infrared (mid-IR) spectroscopy of dried serum films as the basis for the simultaneous quantitation of eight serum analytes: total protein, albumin, triglycerides, cholesterol, glucose, urea, creatinine and uric acid. Infrared transmission spectra were acquired for 300 serum samples, each analysed independently using accepted reference clinical chemical methods. Quantitation methods were based upon the infrared spectra and reference analyses for 200 specimens, and the models validated using the remaining 100 samples. Standard errors in the IR-predicted analyte levels (Sy/x) were 2.8 g/L (total protein), 2.2 g/L (albumin), 0.23 mmol/L (triglycerides), 0.28 mmol/L (cholesterol), 0.41 mmol/L (glucose) and 1.1 mmol/L for urea, with correlation coefficients (IR vs reference analyses) of 0.95 or better. The IR method emerged to be less suited for creatinine (Sy/x = mumol/L) and uric acid (Sy/x = 140 mumol/L) due to the relatively low concentrations typical of these analytes.

Blood Chemical Analysis↗
PubMed1994

Cognitive distortions and adolescent affective disorder. Validity of the CNCEQ in an inpatient sample. Children's Negative Cognitive Error Questionnaire.

Despite a proliferation of recent research examining childhood and adolescent depression, the area still lags behind the adult depression field, particularly in the investigation of cognitive correlates of affective psychopathology. To advance cognitive research with youth, the Children's Negative Cognitive Error Questionnaire (CNCEQ) was developed to provide a measure of cognitive distortions or errors in children and adolescents. Yet, few studies have employed the CNCEQ and no evidence exists supporting the validity of its four component cognitive error scales. The purpose of the present study was to examine the construct validity of the CNCEQ and its constituent scales through the use of factor analysis and criterion-group comparisons. Groups of adolescent psychiatric inpatients, diagnosed as affective or disruptive disordered, completed the CNCEQ following admission. Results failed to support the implicit four-factor structure of the CNCEQ, instead suggesting the appropriateness of a single-factor solution labeled "negative thinking." Despite no diagnostic group differences on the CNCEQ total or other scale scores, affective disordered patients evinced more cognitive errors on the Overgeneralizing scale. Findings suggest that the CNCEQ in its current stage of development holds promise, yet requires refinement to produce a valid measure of cognitive functioning in youth.

Adolescent↗
PubMed1997

The proper use of self-report questions in effective measurement of health outcomes.

Outcomes research often relies on the use of general population survey data. The recent efforts to employ cognitive science theory in the development of data collection instruments has led to the establishment of new questionnaire design research methods. Intensive cognitive techniques are used in laboratory interviews to develop and pretest self-report survey questions with the goal of reducing non-sampling response error. This article presents the theoretical and applied approach used in the cognitive laboratory to study the ways people respond to surveys, and offers guidelines for improving self-report survey questions.

Cognition↗