Estimation of Accuracy

Accuracy is the ability of any assay to provide the correct result. Ideally, the assay should detect all of the analyte (100% recovery) and nothing else (no interference or cross-reactivity). To estimate the method accuracy, a comparison of method results with tme sample concentrations must be completed. A straightforward procedure involves the use of a standard reference material, in which the analyte concentration is known with high accuracy and precision. Standard reference materials are not generally available for biochemical analytes, however. When a reference material is not available, accuracy can be established by comparison with alternative previously validated analytical techniques, or currently accepted methods. Intralaboratory tests of matrix effects and interferences are also conducted in order to establish the accuracy of a new method. 

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A priori estimates obtained in Chapter 6, Section 2 provides the sufficient background for a- uniform estimate of accuracy in the norm of the grid space C such as... 

Define the terms precision and accuracy as they are used in analytical chemistry. Indicate how they may be estimated quantitatively. To what extent is the estimate of accuracy dependent upon precision ... 

The estimation of accuracy of quantitative predictions and the degree of uncertainty of the assessment findings should be attempted as well. 

Estimation of accuracy is based upon knowledge of analytical recoveries and instrumental accuracy. Standardization of the glc/ms/comp system was achieved by injecting three sets of standard mixtures at varying relative concentrations to the... 

A quality assurance component must be integral to the study to verify the accuracy of its measurements and to estimate a precision for each one. This component should include co-located sampling, replicate analysis, station audits, data validation, interlaboratory comparisons and a full set of standard operating procedures. The quality of the data set generated should be discussed and should include the results of validation, analysis of outliers and overall estimates of accuracy and precision. 

There has been much discussion of the probable limits of error of atomic coordinates in crystal structures, the upshot of which is the conclusion that earlier estimates of accuracy were too optimistic. Cruickshank (1949) showed that the relation between the standard deviation a x) of a coordinate of an atom and the differences between... 

An extensive compilation of RKR potential curves for NO+ has recently been reported by Albritton, et. al.87 Their studies include all of the experimentally known electronic states of NO+ below 24 eV. Potential energy curves and spectroscopic constants are tabulated and estimates of accuracy in the location of the excited states are reported. 

Another estimation of accuracy may be obtained from Figures 4.18 and 4.19 that compare Katz s Kvs( values with Kvst values obtained from the methods of... 

Based on the work of Almy and Gillette kn/kn = 0.176 0.044. The estimate of accuracy is based on that given by Almy and Gillette coupled with the work of Porter and of Backstrom and Sandros that all molecules which do not emit from the singlet state (fraction = 0.0025) cross over to the triplet state. Process (72) has such a high activation energy that it may be neglected at room temperature.One other process... 

Explore the effect of variability in study quality and other characteristics (setting, type of population, disease spectrum, etc.) on estimates of accuracy, using subgroup analysis or modeling. 

A pure scientist makes a series of measurements and upon the basis of these makes the best estimates of accuracy and precision, regardless of the number of measurements. It is readily admitted that future studies may prove such estimates to be in error. All that is claimed that they are as good as any reasonable scientist could make upon the basis of the data available at the time. 

The fact that the values of Vl/X do not differ from a smooth curve law by more than the indicated errors of measurement shows that our estimates of accuracy are substantially correct. 

The determination of enzyme activity for diagnostic purposes presents a different problem. No estimates of accuracy can be made except where pure preparations of enzymes are available, so that at present the accuracy of most clinical enzyme assays can be expressed only in terms of other methods which are believed, on theoretical grounds, to provide a valid basis for determination of activity. 

In this section, three categories of experimental design are considered for method validation experiments. An important quality of the design to be used is balance. Balance occurs when the levels each factor (either a fixed effects factor or a random effects variance component) are assigned the same number of experimental trials. Lack of balance can lead to erroneous statistical estimates of accuracy, precision, and linearity. Balance of design is one of the most important considerations in setting up the experimental trials. From a heuristic view this makes sense, we want an equivalent amount of information from each level of the factors. 

In many cases and circumstances of the daily quality control of analytical work RMs and CRMs are helpful tools. Very often RMs are sufficient, in particular for statistical control actions. Where a rough estimate of accuracy or even precision is sufficient, a simple RM or calibration material is also largely adequate. However, for the establishment of the accuracy in the procedure of method development and validation, for revalidation of modified methods or whenever the analyst needs to demonstrate accuracy, e.g. measurements for court cases, CRMs should be employed as they have the advantage of being certified. It will be up to the operator and the laboratory s quality management to determine when, where, and how RM or preferably CRMs shall be used. 

The results themselves have a subtlety associated with their interpretation owing to the presence of the volume-ratio parameter and, optionally, the initial density parameter. The Burnett equations have more flexibility to fit Burnett data than only a density series to PVT data. The statistical uncertainties reflect the quality of the experimental data relative to the particular model used to describe the experiment. The estimation of accuracy for Burnett results is necessarily somewhat subjective since the effect of systematic errors on parameter values is not explicit in nonlinear equations, such as the Burnett equations. Accuracy, however, can be estimated from a study of the effects of systematic errors in computer model calculations and from the magnitude of the change in the volume-ratio value determined with nonideal and nearly ideal gases. For these reasons, we include such information along with our virial coefficient results for ethylene. 

The purpose of monitoring measurement and test systems is to determine whether the measurement system is stable, where stability is the absence of drift or other changes over time. This wiU ensure that the estimates of accuracy and precision remain the same over time. In many cases, when the measurement system is consistently stable over a long period of time, a reduced calibration frequency can be implemented. 

In Chapter 14, Roux and Temprado have provided a detailed survey of the many facets of thermochemistry, including the history of the subject, methods of measurement of thermodynamic quantities, calibration of instruments, estimation of accuracy and the application of correction factors. Reference materials are discussed and data bases of thermodynamic properties are described, including an introduction to computation thermochemistry. The chapter concludes with some examples of the solving of thermochemical problems. 

Table I. Estimation of Accuracy and Sources of Data for the Various Regions...

The reader is reminded that no experimental data are available in regions II and III and that the estimations of accuracy given for these two regions are based on the reliability of the values taken from [ ] and the Hirschfelder equation [ ]. 

For the checking measurements conducted by the customer, reflectors were positioned in such away that an optimal view of the dikes was possible from instrument locations. Subsequently the measurements were collected with reference to element-coordinates. Estimates of accuracy concluded that a standard deviation for element-coordinates between 10 and 15mm needed to be adhered to. Due to the incessant motion of the element induced by tides and currents all measurements were carried out on the floating element. 

Further implications of differences between incurred bioanalytical samples and calibrators and QC samples prepared in accord with accepted practice (FDA 2001) have been discussed (Gallicano 2006). It was emphasized that if the imprecision of data obtained for incurred samples differs from that of spiked samples used in the original validation, this will lead to questioning the estimates of accuracy, LLOQ and stability for the incurred samples. If the incurred sample data are of low precision it will be necessary to analyze more samples to investigate these parameters. It was proposed (Gallicano 2006) that an LLOQ higher than the lowest standard concentration (LSC) should be considered if the incurred samples show more imprecision than spiked samples, because the main criteria for selection of the LSC is its precision on the basis of spiked samples . 

Monte Carlo Continuous energy possible Estimate of accuracy given Follows common particles well Large computer time requirements 1 / fi /2 convergence... 

For more precise estimates of accuracy for particular polyatomic gases, it is necessary to examine the deviation plots given by Boushehri et aZ. (1987) and Bzowski et al. (1990). 

One piece of information that is often missing from research papers is a statement of the uncertainty in the measurements. Even if the author gives an estimate of accuracy, the process of critical evaluation implies an independent assessment of the reliability of the data. This process should be conducted by scientists who are familiar with the type of data in question and who have had experience in the measurement techniques that produced the data. There are usually some subjective elements of the evaluation process. For example, the evaluator will generally have a feeling for the accuracy of each measurement technique and for the pitfalls that can lead to unsuspected errors. The reputation of the researcher or laboratory from which the data came is also a factor, since some groups are known to take greater care in their work than others. 

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