Trueness and Accuracy

Accuracy is defined in the name of BIPM, IEC, IFCC, ISO, IUPAC, IUPAP, and OIML as the closeness of agreement between the result of a measurement and the true value of the measurand (ISO 3534-1 [1993] Fleming et al. [1996b]). Accuracy characterizes the absence of a relevant bias of a measured value. As can be seen from Fig. 4.1, accuracy is a measure that combines the effects of both random and systematic deviations. Therefore, a bias can only be detected if it exceeds the range of random error. 

The ISO recommendation [1993] should be followed and accuracy used only as a qualitative term. In case of quantitative characterization (by means of the bias), a problem may appear which is similar to that of precision, namely that a quality criterion is quantified by a measure that has a reverse attribute regarding the property which have to be characterized. If the basic idea of measures can be accepted, which is that a high quality becomes a high value and vice versa, bias is an unsuited measure of accuracy (and trueness). In this sense, accuracy could be defined by means of a measure proposed in the next paragraph. 

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For the characterization of the reliability of analytical measurements the terms precision, accuracy, and trueness have a definite meaning. 

Precision, accuracy and trueness are important performance characteristics in analytical chemistry. Each of them is well-defined in a positive sense ( closeness of agreement... ). However, their quantifying is done by means of unfavourable measures, namely by error quantities like, e.g., standard deviation and bias, respectively, which indeed do quantify imprecision and... 

Accuracy and trueness have been defined above and it was mentioned that these terms base on qualitative concepts (ISO 3534-1 [1993]). If it is necessary to have quantitative information, the bias, which is a measure of inaccuracy, should not be used to quantify accuracy and trueness, respectively. Instead of this, the following measures might be applied... 

When talking about quality of chemical measurements trueness, precision, accuracy and error are some of the more important keywords. Therefore a clear definition is necessary (see also chapter 11, slides 36 and 44). 

On the one hand, even if an in-house vahdated method shows good performance and reliable accuracy, such a method cannot be adopted as a standard method. In-house validated methods need to be compared between at least eight laboratories in a collaborative trial. On the other hand, a collaborative study should not be conducted with an unoptimized method [58]. Interlaboratory studies are restricted to precision and trueness while other important performance characteristics such as specificity and LOD are not addressed [105]. For these reasons, single-laboratory validation and interlaboratory validation studies do not exclude each other but must be seen as two necessary and complementary stages in a process, presented in Figure... 

Modern measurement has tried to get away from the traditional concepts of accuracy or trueness. These concepts are based on the false assumption that there is a single true value that lurks in the measurement system and that in principle can be accessed by a sufficient number of measurements done with sufficient attention to detail. In reality, the measurement defines to a large... 

As stated further above, under all circumstances trueness, accuracy and sensitivity of the assay should be demonstrated on a sufficient number of patient samples. In our view, however, the use of an additional, also easily realizable chromatographic dimension (online-SPE-LC-MS/MS) [67, 68] without a doubt represents the analytical state of the art in immunosuppressant TDM. Nowadays tandem MS instruments are used almost exclusively for the detection and quantification of analytes. The detection of analytes is generally performed in the selected reaction monitoring (SRM, synonym MRM) mode. Depending on which instrumentation is used, an analysis can be completed within two to four minutes. 

Meeting stringent performance characteristics for each analyte (in particular test accuracy, including both precision of the measurement and trueness of the measurement), and... 

ISO uses two terms, trueness and precision , to describe the accuracy of a measured value. Trueness refers to the closeness of agreement between the average value of a large number of test results and the true or accepted reference value. Precision refers to the closeness of agreement of test results, or in other words the variability between repeated tests. The standard deviation of the measured value obtained by repeated determinations under the same conditions is used as a measure of the precision of the measurement procedure. The repeatability limit r (an intra-laboratory parameter) and the reproducibility limit R (an inter-laboratory parameter) are calculated as measures of precision. Again, precision and trueness together describe the accuracy of an analytical method. 

Accuracy is often used instead of bias and trueness. It can be seen from Figure 2 that it involves bias and precision. 

Error is defined as the difference between the expected (true) value and the value obtained as a result of the determination. Thus, error can be calculated as a measured quantity value minus a reference value [3]. Measurement error is the consequence of the accuracy (as trueness and precision) of the analytical procedure applied for obtaining the measured quantity ... 

From a general viewpoint, the credibility and comparability of environmental analytical data can be assessed through two major parameters, i.e., trueness and precision, which combine into accuracy and express the closeness between an experimental measurement with its uncertainty and the true (or supposedly so) value of a given quantity (40). Such concepts were developed long ago, but are still being refined to accommodate the progress made so far in this sector. Definitions of these and other quantities relevant to this field, for which consensus has been achieved by the scientific community, are listed in Table 1.3. 

Method validation makes use of a series of tests to determine its performance characteristics and to establish the method s acceptance for general use. The following are a list of criteria associated with the validation of a method selectivity and specificity, linearity and calibration, accuracy or trueness, range, precision, limit of detection, limit of quantification, ruggedness and application. 

As already discussed in Chapter 4, CRMs can be used to assess the precision and trueness of a method. Precision has already been largely studied and improved by using proper internal validation procedures and can be maintained through appropriate control charts. For all those activities, simple RMs are sufllcient. To assess trueness, the analyst has to look for external help. One simple way is to purchase a CRM. This CRM will help him to solve his accuracy problems if two conditions are fulfilled. First, he must choose an adapted CRM — representative of the daily routine samples secondly, it must be properly certified — the certified value must be a good estimate of the true value [1] ... 

Accuracy is a concept that encompasses getting the answer right (sometimes known as trueness) with acceptable uncertainty (i.e., with good precision). The relationship between accuracy and precision is shown in figure 1.6 where high precision is represented by the closeness of the cluster of hits on a target and high accuracy is... 

Information on MU may be made available by inclusion in the method SOP or in a memorandum to the residue program manager. Measurement uncertainty includes two major elements the closeness of the analytical result to the true value trueness or accuracy) and the variability associated with the measurement (precision). There... 

The accuracy of an analysis is determined by its precision and trueness. Precision corresponds to the fraction of random errors and trueness to that of systematic errors (bias). In Figure 4.3, the two error types for measurements of a signal, y, are illustrated. 

Measurement accuracy is the closeness of agreement between the measured quantity value and the true quantity value of a measurand. VIMS regards accuracy and measurement error as idealized quantities that we may not know exactly [14]. VIMS terminology uses measurement trueness, which is the closeness of agreement between the average of an infinite number of replicate measured quantity values and a reference quantity value, as measured using a CRM [13]. In the absence of a CRM that shares sufficient similarity to the protein of interest, any locally prepared material can only provide insight into precision, not trueness. 

Figure 4.1 Diagrams depicting relationships of trueness, accuracy, and precision, (a) Connection map among the error, performance characteristics during measurements, and the quantitative performance characteristics. The measurement is more accurate when it presents a smaller measurement error, more true when the bias is small, and more precise when the random error is small [22]. Adapted from Ref I23J, with kind permission from Springer Science+Business Media, (b) Bull s-eye target representations of shooting patterns manifesting good and poor trueness, accuracy, and precision...

The corabination of an inductively coupled plasma ion source and a magnetic sector-based mass spectrometer equipped with a multi-collector (MC) array [multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS)] offers precise and reliable isotope ratio data for many solid elements. In fact, MC-ICP-MS provides data, the trueness (accuracy) and precision of which is similar to, or, in some cases, even superior to, that achieved by thermal ionization mass spectrometry (TIMS), considered the benchmark technique for isotope ratio measurements of most solid elements [1], The basic strength of ICP-MS lies in the ion source, which achieves extremely high ionization efficiency for almost all elements [2, 3]. Consequently, MC-ICP-MS is likely to become the method of choice for many geochemists, because it is a versatile, user-friendly, and efficient method for the isotopic analysis of trace elements [4-8], The ICP ion source also accepts dry sample aerosols generated by laser ablation [9-16], The combination of laser ablation (LA) with ICP-MS is now widely accepted as a sensitive analytical tool for the elemental and isotopic analysis of solid samples. 

Description The term accuracy combines precision and trueness (i.e., the effects of random and systematic factors, respectively). Suppose the results produced by the application of a method show zero or very low bias (i.e., are true ), their accuracy becomes equivalent to their precision [27]. In general, accuracy is not a quantity and is not given a numerical value. Due to the worldwide concept of measurement uncertainty as expressed in ISO [33] and EURACHEM [34] documents, it is recommended to use the clearer and less ambiguous term uncertainty instead of accuracy. ... 

Appraising trueness and accuracy has to proceed with an evaluation of precision in order to finally ensure and prove the traceability of analytical results to known standards. Thus, in the examples given herein, the focus is on calibration data and characteristics in order to derive an accurate estimate of the precision and trueness of the method. 

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