Uncertainty uncertainty

Uncertainty ) Uncertainty s Uncertainty ) Uncertainty Uncertainty ) Uncertainty ) Uncertainty Uncertainty... 

As we have seen in previous sections, the result of a measurement is not complete unless an estimate of the uncertainty associated with the result is available. In any measurement procedure, there will be a number of aspects of the procedure that will contribute to the uncertainty. Uncertainty arises due to the presence of both random and systematic errors. To obtain an estimate of the uncertainty in a result, we need to identify the possible sources of uncertainty, obtain an estimate of their magnitude and combine them to obtain a single value which encompasses the effect of all the significant sources of error. This section introduces a systematic approach to evaluating uncertainty. 

Converting Data to Standard Uncertainties Uncertainty estimates can be obtained from a number of sources. However, they may not be expressed as a standard uncertainty and there are a number of rules for converting data. 

Standard uncertainty Uncertainty of a measurement expressed as a standard deviation. 

Apostolakis (1994, 1999) Aleatory uncertainty Epistemic uncertainty Uncertainty... 

Hoffman and Hammonds (1994) Type A uncertainty Type B uncertainty Uncertainty... 

Propagation of systematic uncertainty Uncertainty in mass of n identical atoms = n x (uncertainty In atomic mass). 

According to the international vocabulary [3], traceability is the property of the result of a measurement or the value of a standard whereby it can be related to stated references, usually national or international standards, through an unbroken chain of comparisons all having stated uncertainties. Uncertainty is a parameter associated with the result of a measurement, which characterises the dispersion of the values that could reasonably be attributed to the measurand/analyte [3, 4]. 

Scenario uncertainty Uncertainty in specifying the exposure scenario that is consistent with the scope and purpose of the assessment. 

Model uncertainty Uncertainty due to gaps in scientific knowledge that hamper an adequate capture of the correct causal relations between exposure factors. 

Parameter uncertainty Uncertainty involved in the specification of numerical values (be they point values or distributions of values) for the factors that determine the exposure. 

The median estimate of the 97.5th percentile exposure is x, but this is affected by a number of uncertainties. Uncertainties A, B, C. .. were analysed probabilistically, resulting in a p% confidence interval of y-z. This included reasonable worst-case assumptions for uncertainties D, E, F. .., which were not treated probabilistically. Evaluation of further, unquantified uncertainties suggests that the true 97.5th percentile exposure could be higher/is unlikely to be higher, because. .. 

This highlights the inadequacy of sensitivity analysis of nominal optima as a basis for choosing a control scheme in the face of uncertainty. Uncertainty gives substantial economic penalty with this scheme, which incorporates no feedback. 

IMEP Results According to Reported Uncertainties Uncertainty is a tool for harmonization, QC, and validation of a method. Nonetheless, it is evident... 

Laboratory simulations of aqueous-phase chemical systems are necessary to 1) verify reaction mechanisms and 2) assign a value and an uncertainty to transformation rates. A dynamic cloud chemistry simulation chamber has been characterized to obtain these rates and their uncertainties. Initial experimental results exhibited large uncertainties, with a 26% variability in cloud liquid water as the major contributor to measurement uncertainty. Uncertainties in transformation rates were as high as factor of ten. Standard operating procedures and computer control of the simulation chamber decreased the variability in the observed liquid water content, experiment duration and final temperature from 0.65 to 0.10 g nr3, 180 to 5.3 s and 1.73 to 0.27°C respectively. The consequences of this improved control over the experimental variables with respect to cloud chemistry were tested for the aqueous transformation of SO2 using a cloud-physics and chemistry model of this system. These results were compared to measurements made prior to the institution of standard operating procedures and computer control to quantify the reduction in reaction rate uncertainty resulting from those controls. 

See absolute uncertainty and relative uncertainty. Uncertainties are always present the experimenter s job is to keep them as small as required for a useful result. We recognize two kinds of uncertainties indeterminate and determinate. Indeterminate uncertainties are those whose size and sign are unknown, and are sometimes (misleadingly) called random. Determinate uncertainties are those of definite sign, often referring to uncertainties due to instrument miscalibration, bias in reading scales, or some unknown influence on the measurement. 

Method vaiidation Standard uncertainty Uncertainty component... 

Expected uncertainty Uncertainty varies depending upon the model chemistry chosen, but it can be as large as 60 percent. 

Since risk is "the chance of Injury or loss" risk assessment necessarily must deal with uncertainty. Uncertainty may enter through exposure processes, through effects processes or through both. This uncertainty may take at least three forms ... 

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