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PubMed2002

Quantification of variability and uncertainty in air pollutant emission inventories: method and case study for utility NOx emissions.

The quality of stationary source emission factors is typically described using data quality ratings, which provide no quantification of the precision of the emission factor for an average source, nor of the variability from one source to another within a category. Variability refers to actual differences caused by differences in feedstock composition, design, maintenance, and operation. Uncertainty refers to lack of knowledge regarding the true emissions. A general methodology for the quantification of variability and uncertainty in emission factors, activity factors, and emission inventories (EIs) is described, featuring the use of bootstrap simulation and related techniques. The methodology is demonstrated via a case study for a selected example of NOx emissions from coal-fired power plants. A prototype software tool was developed to implement the methodology. The range of interunit variability in selected activity and emission factors was shown to be as much as a factor of 4, and the range of uncertainty in mean emissions is shown to depend on the interunit variability and sample size. The uncertainty in the total inventory of -16 to +19% was attributed primarily to one technology group, suggesting priorities for collecting data and improving the inventory. The implications for decision-making are discussed.

Air Pollutants↗
PubMedDate not supplied

Uncertainties in doses and risks from internal radiation.

Many uncertainties exist in current estimates of radiation doses and risks: larger uncertainties exist with internal radiation. These arise mainly from the many steps used to derive doses, and partly from lack of statistical precision in deriving risks from epidemiology studies. The size of these uncertainties has been estimated by a number of expert dosimetrists: for some nuclides these are very large. The recent report by the CERRIE committee recommended that uncertainties should be acknowledged and dealt with by the government. Its parent committee, COMARE, backed these findings. A number of practical recommendations are suggested for government action in the light of the CERRIE report.

Bystander Effect↗
PubMed2001

A review of the effects of multiple stressors on aquatic organisms and analysis of uncertainty factors for use in risk assessment.

Risk assessment procedures use toxicity tests in which organisms are subjected to chemicals under otherwise constant and favorable experimental conditions. Because variable and suboptimal environmental conditions are common aspects of natural ecosystems, the hazard of underestimation of risk arises. Therefore, an uncertainty factor is used in the extrapolation of results of standard toxicity tests to field situations. The choice for these uncertainty factors is based on little ecological evidence. This review discusses studies on the toxicity of various chemicals to aquatic organisms, modified by temperature, nutritional state and salinity, excluding papers on changes in bioavailability of compounds. Collected data were analyzed quantitatively to evaluate the validity of toxicity data obtained from standard toxicity tests in the laboratory under field conditions. Generally, organisms living under conditions close to their environmental tolerance limits appeared to be more vulnerable to additional chemical stress. Usually, increasing temperature and decreasing food or nutrient level raised toxicity. The influence of salinity was less clear; metal toxicity increased with decreasing salinity, toxicity of organophosphate insecticides increased with higher salinity, while for other chemicals no clear relationship between toxicity and salinity was observed. The interactions can be explained by several physical and physiological processes, acting on factors such as bioavailability, toxicokinetics, and sensitivity of organisms. Quantitative analysis of data indicated that an uncertainty factor for the laboratory to field extrapolation should be smaller than one for an ecosystem in a temperate region, while a factor greater than one would be appropriate for systems nearby discharge points of cooling water. The factor should be greater than one when varying nutritional state is concerned, but smaller than one with respect to salinity. Dependent on the effect parameter used, the differences in toxicity between laboratory and relevant field situations ranged from a factor of 2.6 to 130 and 1.7 to 15 for the two temperature conditions and 1.2 to 10 for nutritional state. A salinity increase from freshwater to marine water decreased toxicity by a factor of 2.1. However, as less extreme salinity changes are more relevant under field conditions, the change in toxicity is probably much smaller. To obtain uncertainty factors that sufficiently protect natural systems without being overprotective, additional research is required.

Animal Nutritional Physiological Phenomena↗
PubMed2000

Tissue resistivity estimation in the presence of positional and geometrical uncertainties.

Geometrical uncertainties (organ boundary variation and electrode position uncertainties) are the biggest sources of error in estimating electrical resistivity of tissues from body surface measurements. In this study, in order to decrease estimation errors, the statistically constrained minimum mean squared error estimation algorithm (MiMSEE) is constrained with a priori knowledge of the geometrical uncertainties in addition to the constraints based on geometry, resistivity range, linearization and instrumentation errors. The MiMSEE calculates an optimum inverse matrix, which maps the surface measurements to the unknown resistivity distribution. The required data are obtained from four-electrode impedance measurements, similar to injected-current electrical impedance tomography (EIT). In this study, the surface measurements are simulated by using a numerical thorax model. The data are perturbed with additive instrumentation noise. Simulated surface measurements are then used to estimate the tissue resistivities by using the proposed algorithm. The results are compared with the results of conventional least squares error estimator (LSEE). Depending on the region, the MiMSEE yields an estimation error between 0.42% and 31.3% compared with 7.12% to 2010% for the LSEE. It is shown that the MiMSEE is quite robust even in the case of geometrical uncertainties.

Algorithms↗
PubMed2001

The effect of measurement location on tibia lead XRF measurement results and uncertainty.

The aims of this study were to determine whether the location on the tibia measured by 109Cd-based K-shell x-ray fluorescence (XRF) affected the measurement result and its uncertainty, and whether higher tibia lead levels at the extremities of the tibia and/or inhomogeneity in the distribution of lead in the tibia could be inferred therefrom. Replicate XRF measurements were performed at multiple locations on ten adult cadaver intact legs and on nine bare tibiae dissected from them. Mean lead levels in the bare tibiae ranged from 16 to 48 microg Pb per g of bone mineral. Bare tibia measurements showed that both the XRF result and its uncertainty increased towards the proximal and distal ends of the tibia. The XRF result decreased away from the medial-lateral mid-point of the tibia, but XRF uncertainty was not significantly affected. Intact leg measurements showed no effect of proximal distal location on XRF result but did show an effect on XRF uncertainty. We conclude that the XRF method used can determine the differences in bone lead level resulting from the more trabecular composition at the ends of the tibia, and we present limited evidence for localized regions of low tibia lead level.

Bone and Bones↗
PubMed2004

Radiation-induced statistical uncertainty in the threshold voltage measurement of MOSFET dosimeters.

The results of a recent study on the limiting uncertainties in the measurement of photon radiation dose with MOSFET dosimeters are reported. The statistical uncertainty in dose measurement from a single device has been measured before and after irradiation. The resulting increase in 1/f noise with radiation dose has been investigated via various analytical models. The limit of uncertainty in the ubiquitous linear trend of threshold voltage with dose has been measured and compared to two nonlinear models. Inter-device uncertainty has been investigated in a group of 40 devices, and preliminary evidence for kurtosis and skewness in the distributions for devices without external bias has been observed.

Algorithms↗
PubMed2006

Simulation and visualization of dose uncertainties due to interfractional organ motion.

In this paper, we deal with the effects of interfractional organ motion during radiation therapy. We consider two problems: first, treatment plan evaluation in the presence of motion, and second, the incorporation of organ motion into IMRT optimization. Concerning treatment plan evaluation, we face the problem that the delivered dose cannot be predicted with certainty at the time of treatment planning but is associated with uncertainties. We present a method to simulate stochastic properties of the dose distribution. This provides the treatment planner with information about motion-related risks of different plans and may support the decision for or against a treatment plan. This information includes the display of probabilities of individual voxels to receive doses from a therapeutical interval or above critical levels, as well as a diagram that shows the variability of the dose volume histogram. Concerning the incorporation of organ motion into IMRT planning, we further analyse the approach of inverse planning based on probability distributions of possible patient geometries. We consider three different sources of uncertainty, namely uncertainty about the amplitude of motion, a systematic error and a random error. We analyse the impact of these sources of uncertainty on the optimized treatment plans for prostate cancer.

Artifacts↗
PubMedDate not supplied

Uncertainty in quantitative thin-layer chromatography.

Globalization forces analysts to demand extended control of variability in analytical measurements. A calculation procedure named the "error budget model" following recommendations proposed more than 20 years ago by the Bureau International des Poids et Mesures is established as a rule for evaluating and expressing the measurement uncertainty across a broad spectrum of measurements. This metrological approach common in physical measurement is not applicable in separation techniques and cannot quantitate measurement uncertainty. Our experiments show that it can be used as a planning tool in the validation of thin-layer chromatographic (TLC) methods. A computer program that quantitates uncertainty components associated with potential sources of uncertainty in quantitative TLC is prepared and tested with experimental data. TLC plates with different qualities of stationary phases (TLC and high-performance TLC) spotted with different types of samples are measured. Application is performed manually and automatically. Plates are scanned with UV-vis scanners and a video documentation system in remission and transmission mode and fluorescence. Although the calculated values are close to the values obtained with validation procedures, the error budget approach cannot substitute validation. Calculated results can predict critical points in real quantitative TLC, but they cannot confirm the validity of a selected chromatographic procedure.

Journal Article↗
PubMed2001

Uncertainties in estimated body burdens of caesium-137 by whole-body counting.

It is very important to evaluate the uncertainties in individual monitoring for internal exposure of workers. The uncertainties in estimated body burdens of 137Cs with the JAERI whole-body counter were investigated using Monte Carlo simulation and measurements. It was found that the uncertainties of estimated body burdens with the whole-body counter are strongly dependent on various sources of uncertainty, such as radioactivity distribution within the body and counting statistics and that the 137Cs body burden assessed from the result of the whole-body count can be within +/- 60% in error.

Adolescent↗
PubMed2001

Reliability of the ICRP's dose coefficients for members of the public. 1. Sources of uncertainty in the biokinetic models.

During the decade following the Chernobyl accident, the International Commission on Radiological Protection (ICRP) developed dose coefficients (doses per unit intake) for ingestion or inhalation of radionuclides by members of the public. The level of uncertainty in those coefficients varies considerably from one radionuclide to another, due largely to differences in the level of understanding of the biological behaviour of different elements in the human body. This paper is the first in a series that examines the sources and extent of uncertainties in the ICRP's biokinetic and dosimetric models for members of the public and the dose coefficients derived from those models. The present paper describes the different types of information generally used to develop biokinetic models for radionuclides, the main sources of uncertainty associated with each type of information, and the approach used in subsequent papers in this series to quantify the uncertainties in biokinetic and dosimetric estimates.

Age Factors↗
PubMed2004

Uncertainty evaluation of radon measurements with LR115 detector and spark counter.

To evaluate the uncertainties for nuclear track detectors used in radon measurements, a full understanding is required of the physical phenomena involved and the behaviour of the instruments utilised in the measuring process. As it concerns the LR115 nuclear track detector, an overall evaluation of uncertainty was given. It was assessed taking into account different contributions and determining their relative weights. Since such detectors are often read by a spark counter device, a model to describe its behaviour was developed and a saturation factor was estimated. Its expression and its associated uncertainty are given. Hence, it has been possible to draw a calibration curve, in which all the uncertainty sources have been considered.

Air Pollutants, Radioactive↗
PubMed2005

Bias and uncertainty of penetrating photon dose measured by film dosemeters in an epidemiological study of US nuclear workers.

A retrospective exposure assessment of 1269 study subjects was completed for use in a multi-site case-control study of the relationship between protracted workplace external radiation exposure and leukaemia mortality. The majority of exposure data result from film badge monitoring programmes at the four US weapons production facilities and a US Naval shipyard. Bias and uncertainty in reported exposures among study facilities and across time were as result of differences in incident photon energy, exposure geometry, dosemeter type and dosimetry methods. These sources of measurement uncertainty were examined by facility and time to derive bias factors (B) for normalising exposures. In conjunction with facility reported results, the bias factors provide a means to estimate the equivalent dose, penetrating to a depth of 10 mm [H(p)(10)] and the equivalent dose to the active bone marrow for use in the epidemiological study. Uncertainty was expressed as the constructed 95% confidence interval (i.e. the 2.5th-97.5th% range) of the estimated parameter. The bias factors indicate that recorded exposures provide a reasonable estimate of H(p)(10) (bias factor near unity) and overestimate equivalent dose to active bone marrow (H(T)) by a factor between 1.2 and 1.7. On average, dosemeter-response uncertainties estimated using Monte Carlo simulation were approximately +/-19 and +/-33% for H(p)(10) and H(T), respectively.

Bias↗
PubMed1998

Information needs, uncertainty, and anxiety in women who had a breast biopsy with benign outcome.

A retrospective, descriptive study to determine the information needs of women who underwent a breast biopsy with a benign outcome and to ascertain the levels of uncertainty and anxiety they experienced was conducted in two community health care sites in Winnipeg, Manitoba. A sample of 70 women completed a four-part survey after learning the benign breast biopsy diagnosis. The survey consisted of an Information Needs Questionnaire, Mishel Uncertainty in Illness Scale--Community Form, State-Trait Anxiety Inventory, and a Demographic Questionnaire. Before the study, nine information needs were identified in a focus group composed of women (n = 9) who had a benign breast biopsy. The nine information needs were arranged in 36 pairs in the Information Needs Questionnaire. Profiles of information needs were developed through Statistical Analysis Systems analysis using Thurstone's Law of Comparative Judgement--Case V. The most important information need of women who underwent a benign breast biopsy was knowing when they would learn the diagnosis. The next four information needs were categorized as information about the risks of developing breast cancer. Information about follow-up and diagnostic tests were less important than information about the threat of breast cancer. Women experienced heightened uncertainty and anxiety levels related to the benign breast biopsy experience. Profiles of information needs were developed for women experiencing various levels of uncertainty and anxiety, women of different age groups, and women who experienced with a loved one with breast cancer.

Adolescent↗
PubMed2000

The influence of uncertainty in illness, stress appraisal, and hope on coping in survivors of breast cancer.

On the basis of Mishel's uncertainty in illness theory, Lazarus and Folkman's stress on appraisal and coping, and Herth's perspective of hope, this cross-sectional, correlational study was designed to examine the influence of uncertainty, stress appraisal, and hope on coping in 71 survivors of breast cancer. Uncertainty was measured by the Mishel Uncertainty in Illness: Community Scale, stress appraisal by the Stress Appraisal Index, hope by the Herth Hope Index, and coping by the Ways of Coping Questionnaires (WCQ).

Adaptation, Psychological↗
PubMed1993

Statistical uncertainties associated with multiplexed sampling with a continuous radon monitor.

One common type of continuous radon monitor is a flow-through device that counts alpha emissions from a gas sample as it passes through a scintillation cell. To improve the ability of this type of continuous radon monitor to track time-varying radon concentrations, a forward-marching data analysis procedure that considers the production, deposition, and decay of radon progeny as a function of time had been previously developed. This paper examines some of the statistical issues associated with using such a monitor and applying the forward-marching analysis procedure to the problem of multiplexed radon concentration samples. It uses Monte Carlo simulations, together with limited experimental data, to perform: 1) an examination of the potential bias of the forward-marching analysis technique; 2) an examination of a simplified methodology for evaluating concentration-measurement uncertainty for various radon concentration histories; and 3) an analysis of an example application of the simplified uncertainty analysis to multiplexing of soil-gas samples. The results of these analyses indicate: 1) that the forward-marching analysis technique is relatively unbiased, at least at concentrations > 148 Bq m-3 for time steps longer than 120 s; 2) that a simplified uncertainty analysis procedure based upon Poisson statistics and no error correlation between intervals provides generally acceptable estimates of measurement uncertainty; and 3) that such a monitor and analysis technique can be successfully employed to measure multiplexed soil-gas samples, but is not appropriate for multiplexing of most building concentrations.

Air Pollution, Indoor↗
PubMed1999

Probabilistic approach to ranking sources of uncertainty in ELF magnetic field exposure limits.

There is a need to improve the biological data, dosimetry, and risk assessment methodology used for setting guidelines for occupational exposures to extreme-low-frequency magnetic fields. This paper illustrates how a probabilistic approach can be used to determine priorities for future research based upon the analysis of biological and dosimetric variables that affect stimulation of the heart by magnetically-induced currents. A model was constructed to predict a level of whole-body magnetic-field exposure below which no cardiac stimulation is expected (Bncs). For each iteration of the model, a value was selected from cardiac stimulation threshold, shape factor, and conductivity distributions by Latin Hypercube sampling, and a value for Bncs was computed. The very wide range of simulated Bncs magnetic-field values obtained indicates that there is considerable uncertainty about what constitutes a "safe" level of exposure. The results show that the major occupational-exposure guidelines are very conservative with respect to risks of cardiac stimulation. The minimum Bncs value computed (0.01 T) by either a circular-loop model or an ellipsoid dosimetric model is ten times the highest recommended workday exposure value in a guideline. The lowest 5% Bncs value calculated for a circular-loop model is about 50-times the ICNIRP occupational exposure limit; the lowest 5% Bncs value calculated for an ellipsoid model is more than 100 times the ACGIH occupational exposure limit. A new finding is that the method specified by several guidelines for determining guideline compliance when exposures occur at multiple frequencies (additive weighting of harmonics) leads to substantially lower Bncs estimates relative to a probably more valid rms-weighting method. Probabilistic sensitivity analyses indicate that lack of knowledge of the threshold for cardiac stimulation is the greatest source of uncertainty as to what is a "safe" level of exposure to extreme-low-frequency magnetic fields. Additional research to address this source of uncertainty may be expected to have the greatest potential impact to reduce overall uncertainty about safe magnetic field exposures.

Animals↗
PubMed2000

The uncertainty in the activity estimate from a lung count due to the variability in chest wall thickness profile.

Calibration of a lung counter requires the use of a realistic torso phantom. The depth profile of both torso phantoms' (LLNL and JAERI) chest plate covers is fixed and assumed to be equivalent to a person's chest wall; however, ultrasound measurements of humans have shown this to be an approximation. When the depth profile of a calibration phantom is different from that of a subject, then a systematic uncertainty will be introduced into the activity estimate. Monte Carlo simulation has shown that changes in the depth profile of the chest wall thickness affect the counting efficiency. Ultrasound measurements have suggested that the coefficient of variation in the depth profile of the chest wall thickness lies between 13% and 26% for male workers; therefore, the added uncertainty to an activity estimate will be an over or underestimate of about a factor of 1.07 resulting from the different depth profile. The factor will be somewhat higher for females, probably about 1.2 at the extreme. These additional uncertainties resulting from depth profile differences are small compared with other uncertainties commonly encountered in lung counting: detector positioning, deposition patterns of the activity, measurement of the chest wall thickness, etc.

Computer Simulation↗
PubMed2000

Evaluation of uncertainties in estimates of fetal doses from 59Fe kinetic studies at Vanderbilt University.

In a 1997 paper, Stabin et al. published estimates of the fetal radiation doses for women who received oral administrations of 59Fe at Vanderbilt University in the 1940's. These authors concluded that there was "considerable uncertainty... in the amount of radioactive material administered to these subjects." In an effort to quantify this uncertainty, the underlying factors in the input data used in the Stabin et al. dose estimates have been examined in detail. Such factors include (a) an absence of detailed information on, and discrepancies in, the amounts of 59Fe reported to have been administered; (b) the probability that the radioactive iron included 55Fe as well as 59Fe; (c) uncertainties as to the period of time that elapsed between the administration of the radioiron and the taking of the maternal blood samples, and the accompanying impacts of radioactive decay; (d) possible losses of 59Fe in the procedures used in preparing the blood samples; and (e) questions as to the reported efficiency of the counting equipment. Our principal conclusion is that, due to the significant uncertainties and the lack of key information, it is not possible to estimate the doses accurately. An ancillary conclusion, however, is that the doses were probably significantly higher than previously estimated. This latter possibility should be carefully considered by any investigators who subsequently seek to use these estimates to quantify the relationship between the doses to the fetus and the resulting health effects.

Beta Particles↗