Accuracy and Uncertainty

For example, if ever we want to understand whether global warming to a substantial degree is, or is not, caused by human activities, then we should first of all know and understand the performance of our models and the accuracy of data we put into these models. If ever we want to know whether all efforts to improve the quality of the sea for biota are effective, then we need to know the accuracy and uncertainty of all our data. If we do not want to take Quality Assurance into account, we will never be able to lay the solid scientific foundations for our research, political and legal actions etc. In other words we will remain only believers. 

Two technical papers recognized as significant early contributions in the discussion of the limitations of analytical accuracy and uncertainty include those by Horwitz of the U.S. FDA [1, 2], For this next series of articles we will be discussing both the topic and the approaches to this topic taken by the classic papers just referenced. The determination and understanding of analytical error is often approached using interlaboratory collaborative studies. In this book we have previously delved into that subject in Chapters 34-39. 

Answer (c) is incorrect as accuracy and uncertainty are not related in the way suggested. 

Methods for Parent Compound and Metabolites in Biological Materials. No information is available concerning the analysis of isophorone in biological materials. If information were available, it would allow both investigators and reviewers to assess the accuracy and uncertainty of the methods used. Furthermore, the ready availability of tested analytical methods would permit a standardized approach to the analysis of biological materials and allow a comparison of the levels of exposure with the possible health effects in humans. 

Many different methods for whole mixture assessment are available, and they vary substantially in level of accuracy and uncertainty. Figure 5.11 provides an... 

Powell, C. J., and Seah, M. P., Precision, accuracy, and uncertainty in quantitative surface analyses by Auger-electron spectroscopy and x-ray photoelectron spectroscopy, J. Vac. Sci. Technol., A8(2), 735 (1990). 

Method validation is the process of proving that the HX-MS method is acceptable for its intended purpose [22]. In addition, the investigators must document the experiments conducted during the validation exercise that assure the accuracy and uncertainty of the method [25]. Before beginning... 

The nuclear data libraries used in different countries result in different uncertainties in reactor parameters caused by nuclear data. But the magnitudes of these uncertainties are to some extent similar for most current nuclear data libraries because they are based on similar experimental information. So the comparison of target accuracies and uncertainties, provided by the ABBN data set, is of general interest. Some results of an analysis based on ABBN [4.21] are given in Table 4.1. 

Xie Y, Chen T, Lei M et al (2011) Spatial distribution of soil heavy metal pollution estimated by different interpolation methods accuracy and uncertainty analysis. Chemosphere 82 468-476... 

Accuracy and Interpretation of Measured pH Values. The acidity function which is the experimental basis for the assignment of pH, is reproducible within about 0.003 pH unit from 10 to 40°C. If the ionic strength is known, the assignment of numerical values to the activity coefficient of chloride ion does not add to the uncertainty. However, errors in the standard potential of the cell, in the composition of the buffer materials, and ia the preparatioa of the solutioas may raise the uacertaiaty to 0.005 pH unit. 

The presence of errors within the underlying database fudher degrades the accuracy and precision of the parameter e.stimate. If the database contains bias, this will translate into bias in the parameter estimates. In the flash example referenced above, including reasonable database uncertainty in the phase equilibria increases me 95 percent confidence interval to 14. As the database uncertainty increases, the uncertainty in the resultant parameter estimate increases as shown by the trend line represented in Fig. 30-24. Failure to account for the database uncertainty results in poor extrapolations to other operating conditions. 

AccuracyAJncertainty The lack of specific data on component failure characteristics, chemical and physical properties, and phenomena severely limit accuracy and can produce large uncertainties. 

Part of the planning should include the evaluation of test uncertainty. This evaluation can be limited to a common sense approach based on available instrumentation and the locations relative to the ideal. A more sophisticated study can be made in which instrumentation accuracy and the impact of any inaccuracy on the measured parameters is evaluated. This is a complex task with the need being based on the motivation for the test. If the test is being performed to settle a dispute, a formal understanding of the uncertainty should be developed. Methods for evaluation of test uncertainty are found in ANSI/ASME PTC 19.1 [11]. 

In ISO 9001 the terms accuracy and precision are used and these were explained previously. ISO/TS 16949 takes the study of measurement further and divides variation into five classes. Although, ISO/TS 16949 has not deleted the requirement on measurement uncertainty, compliance with the measurement systems analysis requirement achieves the same purpose. 

The serial numbers of the standards used to calibrate the device The environmental conditions prevailing at the time of calibration A statement of measurement uncertainty (accuracy and precision)... 

Thus curvature in an Arrhenius plot is sometimes ascribed to a nonzero value of ACp, the heat capacity of activation. As can be imagined, the experimental problem is very difficult, requiring rate constant measurements of high accuracy and precision. Figure 6-2 shows a curved Arrhenius plot for the neutral hydrolysis of methyl trifluoroacetate in aqueous dimethysulfoxide. The rate constants were measured by conductometry, their relative standard deviations being 0.014 to 0.076%. The value of ACp was estimated to be about — 200 J mol K, with an uncertainty of less than 10 J moE K. ... 

Although there is a great deal of interest in expert systems, there is still much uncertainty regarding tangible benefits. In a survey of 74 companies in the US in 1993 [28] the main benefits identified were improved productivity, more consistent decision making, increased accuracy, and improved competitiveness. However, it is more pertinent to discuss those benefits found by users of systems in pharmaceutical formulation. These include ... 

Only a direct matrix match of sample and CRM, and the CRM s use as a direct calibrant will allow the user to demonstrate accuracy and subsequently traceability close to the uncertainties established during the CRM certification ( note matrixmatching may not be necessary with matrix-independent techniques). This reality places a significant burden on the CRM producers, since large uncertainties in the certified values may degrade the perceived value of the CRM. 

In their broadest application, CRMs are used as controls to verify in a direct comparison the accuracy of the results of a particular measurement parallel with this verification, traceability may be demonstrated. Under conditions demonstrated to be equal for sample and CRM, agreement of results, e.g. as defined above, is proof. Since such possibilities for a direct comparison between samples and a CRM are rare, the user s claims for accuracy and traceability have to be made by inference. Naturally, the use of several CRMs of similar matrix but different analyte content will strengthen the user s inference. Even so, the user stiU has to assess and account for all uncertainties in this comparison of results. These imcertainty calculations must include beyond the common analytical uncertainty budget (i) a component that reflects material matrix effects, (2) a component that reflects differences in the amount of substance determined, (3) the uncertainty of the certified or reference value(s) used, and 4) the uncertainty of the comparison itself AU this information certainly supports the assertion of accuracy in relation to the CRM. However, the requirement of the imbroken chain of comparisons wiU not be formally fulfilled. 

If users are to benefit from the implementation and/or verification of traceability in analytical chemistry the unbroken pathway of references must be kept short. The uncertainty of the references (CRMs) used may significantly widen the uncertainty a user must attach to the result of his measiuement when addressing accuracy and traceability through comparison with a CRM. These comparisons should be only considered in a first or second level step as to keep the uncertainties of the results within limits fit for the purpose. The producers of CRMs must keep their uncertainties sufficiently small to allow introduction of the CRM at different points in the analytical pathway, without limiting the usefiilness of results through unduly expanded uncertainties. 

After a simple Fourier inversion of a set of magnetic structure factors MbU, one can retrieve the magnetisation density. A much better result, e.g. the most probable density map, can be obtained using the Maximum Entropy (MaxEnt) method. It takes into account the lack and the uncertainty of the information not all the Bragg reflections are accessible on the instrument, and all the values contained in the error bars are satisfactory and have to be considered. However, as this method extracts all the information contained in the data, it is important to keep in mind that it may show spurious small details associated to a low accuracy and/or a specific lack of information located in (/-space. 

A mass of evidence seems to confirm that the mixing rate of radiocarbon in the atmosphere is rapid, and that with respect to its radiocarbon content the atmosphere can be considered as a homogeneous entirety. The contamination of samples with matter from an extraneous source can nevertheless invalidate this assumption. Two types of contamination can be differentiated physicochemical contamination and mechanical intrusion. There are two forms of physicochemical contamination. One is due to the dilution of the concentration of radiocarbon in the atmosphere by very old carbon, practically depleted of radiocarbon, released by the combustion of fossil fuel, such as coal and oil. The other is by the contamination with radiocarbon produced by nuclear bomb tests during the 1950s and later in the twentieth century. The uncertainties introduced by these forms of contamination complicate the interpretation of data obtained by the radiocarbon dating method and restrict its accuracy and the effective time range of dating. 

As a measuring science, analytical chemistry has to guarantee the quality of its results. Each kind of measurement is objectively affected by uncertainties which can be composed of random scattering and systematic deviations. Therefore, the measured results have to be characterized with regard to their quality, namely both the precision and accuracy and - if relevant - their information content (see Sect. 9.1). Also analytical procedures need characteristics that express their potential power regarding precision, accuracy, sensitivity, selectivity, specificity, robustness, and detection limit. 

First of all, we should make a clear distinction between accuracy and precision. Accuracy is a measure of how close a given value is to the true value, whereas precision is a measure of the uncertainty in the value or how reproducible the value is. For example, if we were to measure the width of a standard piece of paper using a ruler, we might find that it is 21.5 cm, give or take 0.1 cm. The give or take (i.e., the uncertainty) value of 0.1 cm is the precision of the measurement, which is determined by how close we are able to reproduce the measurement with the ruler. However, it is possible that when the ruler is compared with a standard unit of measure it is found to be in error by, say, 0.2 cm. Thus the accuracy of the ruler is limited, which contributes to the uncertainty of the measurement, although we may not know what this limitation is unless we can compare our instrument to one we know to be true. 

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