Assessing Accuracy

The other favoured approach is the use of the statistic known as Pearson R. If there are n sets of data where x is the inferential and y the laboratory result. 

A better approach is to compare the standard deviation of the prediction error against the variation in the actual property (Tactual)- We show in Chapter 13 that benefit calculations are usually based on the assumption that the standard deviation of the actual 

A further limitation of the use of P is that, if there is a perfect relationship between inferential PVinferentiad and laboratory result PVlaboratory), the value of will also be 1 for 

for example, if ai had a value of 3 and oq a value of 0, then the inferential would be treble the laboratory result but, according the statistical test, be working perfectly The same would apply if a were negative - even though this reverses the sign of the process gain. 

To illustrate the difference between R and f , consider the data in Table 9.1. Column 1 is a series of laboratory results. Columns 2, 3 and 4 are the corresponding results fi om three inferentials derived using different values of aj and oq. 

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Calcium is titrated with EGTA (l,2-bis-(2-amminoethoxyethane N,N,N, N -tetracetic acid) in the presence of the zinc complex of zincon as indirect indicator for calcium. Theoretical titration curves are calculated by means of the computer program HALTAFALL in order to assess accuracy and precision. The method gives a relative precision of 0.00028 when applied to estuarine water of 0.05-0.35% salinity. 

In addition to the temporal correlation coefficient, the spatial correlation coefficient was calculated approximately for fixed values of time. Except for one of the mathematical models, all techniques showed a better temporal correlation than spatial correlation. The two correlation coefficients are cross plotted in Figure 5-6. Nappo stressed that correlation coefficients express fidelity in predicting tends, rather than accuracy in absolute concentration predictions. Another measure is used for assessing accuracy in predicting concentrations the ratio of predicted to observed concentration. Nappo averaged this ratio over space and over time and extracted the standard deviation of the data sample for each. The standard deviation expresses consistency of accuracy for each model. For example, a model might have a predicted observed ratio near unity,... 

Lion, one must be cautious of assessing accuracy by comparing results with consensus values because they are often computed Incorrectly in the literature. The quoted values may be based upon averages of all methods without regard to systematic bias that characterizes some methods of analysis. 

The selection of an appropriate reference material should be based upon the availability of a matrix that is similar to the anticipated routine unknowns. Similarity of chemical matrix and analyte concentrations is particularly important when attempting to assess accuracy of a method that requires destructive sample preparation. 

Collaborative Testing. A second approach to assessing accuracy, when no certified reference material is available, may be used in conjunction with analysis by independent methods and in-house materials. Sample exhanges with other laboratories can help establish the existence or absence of systematic errors in a method. Collaborative tests are most useful in this regard when some of the participating laboratories use different sample preparation and quantification. The utility of independent analysis methods and comparisons between destructive and non-destructive analysis is again emphasized here. 

The third method for assessing accuracy is to calculate an elemental composition for each LTA s corresponding oxidized ash, based on the reference mineral elemental compositions. Reasonably close agreement between the actual (obtained by ICP-AES) and calculated elemental compositions would substantiate (but not prove) the mineral analysis. The standard error of prediction (SEP) for... 

In the case where the deviation between the known and the experimental values is considered to be due to random errors, the method can be used to assess accuracy. If this assumption is not made, the deviation becomes a measure of the systematic error or bias. The approach to accuracy is limited to where test objects can be compared with reference materials, which is not always the case, for example, where... 

CRMs are often the best tools for assessing accuracy. The results obtained for FEP and PFA tubes digestion are in agreement with the certified contents for various types of materials, while with other digestion vessels, the lead content was higher than the certified value (Table 1.6). 

Quantitative SPECT regimes are well established and rely on fewer assumptions about, e.g., three-dimensional activity distribution than planar activity quantitation approaches. While phantom studies can be used to assess accuracy of activity quantitation, these may not entirely reflect the accuracy achieved in human subjects. Mass balance calculations, where the initial activity loaded into the delivery device is compared to the activity in the subject estimated by the imaging technique plus the residual activities in the device and exhalation filter, can provide a good... 

Quahty control samples at three concentrations (low, middle, and high) around the expected range can be used to assess accuracy by two to five injections. The acceptance criteria established for the SST evaluation must balance the need to ensure adequate performance with the practical reality of performing chemical analyses. This means that they must be sufficiently tight, but not so... 

This value, obtained by the factor-jump method of thermogravimetry, appears to be realistically precise, i.e., different runs agree well. There seem to be no other studies of comparable precision from which to assess accuracy. For the moment, we will assume the reported value is accurate as well as precise. 

Validation of bioanalytical methods requires experiments to assess accuracy, precision, limit of detection, limit of quantitation, range, linearity, selectivity, and sample stability (23). The sample preparation step has a great impact on all these parameters. The extraction efficiency must be experimentally determined by comparison of extracted samples to unextracted standards (24,25). The unextracted standards, because of interference, must be diluted in saline or the mobile phase rather than plasma. 

It is important to emphasise that the different manifold architectures originally designed for the application of SAM have also been used for spiking purposes to assess accuracy. This practice is however not recommended, as analyte addition for recovery experiments should always be done before the analytical procedure. 

Use of an independent QA/QC program is mandatory to evaluate laboratory quality. Use of blind standards and splits are essential in assessing accuracy and precision. Independent analyses outside the CLP may be necessary to obtain appropriate data for modeling and to more definitively evaluate selected areas. 

What are the main ways of assessing accuracy of a method ... 

A CRM can be used to assess accuracy. It must be emphasized that reproducible or accurate results, obtained respectively with reference materials (RMs) or CRMs, are not always sufficient in QA. It is necessary that the composition of the control materials is close to the composition of the unknown sample, this closeness involving matrix composition, possibly interfering major and minor substances, etc. 

Validation samples Biological matrix samples spiked with the analyte at predetermined concentrations it is used during prestudy validation to assess accuracy and precision. 

The design of the method also requires the analyst to consider how the method will be shown to be accurate and precise. This requires knowledge of how we assess accuracy and precision, discussed in Section 1.3. The analyst must evaluate interferences. Interference is anything that (1) gives a response other than the analyte itself or (2) that changes the response of the analyte. Interferences may be other compounds or elements present in... 

The present chapter reviews the factors that need to be considered when variational procedures are developed for complex atoms or ions where missing correlation is the main source of uncertainty in the accuracy of a calculation. The goal is to have a procedure for bound state calculations that can be applied to any element of the periodic table. Systematic methods for assessing accuracy and controlling the size of a calculation will be discussed. 

There are a number of issues which can have a negative impact on the accuracy of cognitive ability testing. Any issue which has a negative impact on the accuracy of the assessment will also have a negative impact on the accuracy of the decisions made based on the assessment information. While there are numerous textbooks devoted to psychological testing, the main issues which can influence assessment accuracy are relate to appropriate test selection, test validity and reliability, test administration, and test score interpretation. It is beyond the scope of this book to go into these issues in detail. However, the key point is that employees in an... 

While there are several possible measures of performance of a humanitarian relief supply chain, one approach, suggested by Fearon [37], is to compare an actual outcome with the counterfactual outcome. In such an approach, the question is whether the humanitarian intervention did in fact improve the system in terms of lives saved, diseases avoided, crop failure averted, market functionality maintained, and so on. But other suggestions focus on the success of the appeal coverage, lead time between donation and delivery of aid, financial efficiency and assessment accuracy. Each of these metrics focuses on the process of forecasting the aid required and garnering the resources and then efficiently dehvering the aid while respecting the planned humanitarian space. 

In the reported numerical results, the total energy per atom in electron-volts, this value can be used to assess accuracy of the final result the SCF is the iteration steps needed to reach self-consistency and the MVp counts the number of matrix-vector products. Clearly MVp is not the only factor that determines CPU time, the orthogonalization cost can also be a significant component. 

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