Acceptable measurement uncertainty

A method s limit is the point where the targets for acceptable measurement uncertainty or identification confidence can no longer be met. Given this definition, the empirical or top-down approach is preferable for describing method limits when... 

Maximal measurement uncertainty at volume measurements could be calculated as well. However volume measurements are generally only performed as second choice if weighing would pose too many drawbacks for the reliability of the preparation process (see Sect. 29.1.2). Therefore, the acceptable measurement uncertainty for volume measuring depends strongly on the nature of the process in question. Usually only inaccuracy is taken into consideration. It is limited by requiring a minimum filling of the device to be used, as is explained in Sect. 29.1.7. More exact calculations would be irrelevant. 

Some general quantitative characteristics (orders of magnitudes) of LDA systems are velocity measurement range 1 mm s -100 m s relative measurement uncertainty 0.1-1% rate of accepted data 0.1-10 kHz size of the optical probe 10 p.m-1 mm for each dimension measuring distance 0.1-1 m. 

Do take account of measurement uncertainty in determining the acceptability of product. 

The choice of method depends on the purpose for which the analysis is being performed. The customer requesting the analysis may specify the method to be used. Even in this situation, it is the responsibility of the laboratory to demonstrate that the method is capable of producing results that are reliable. When no method is specified the points to consider have already been identified in Section 4.2. The acceptable level of measurement uncertainty specified or implied will, to a certain extent, set the precision and bias levels. All of the topics covered in the following sections may be crucial, depending on the purpose of the analysis, and should appear on your list. 

Accuracy is often used to describe the overall doubt about a measurement result. It is made up of contributions from both bias and precision. There are a number of definitions in the Standards dealing with quality of measurements [3-5]. They are only different in the detail. The definition of accuracy in ISO 5725-1 1994, is The closeness of agreement between a test result and the accepted reference value . This means it is only appropriate to use this term when discussing a single result. The term accuracy , when applied to a set of observed values, describes the consequence of a combination of random variations and a common systematic error or bias component. It is preferable to express the quality of a result as its uncertainty, which is an estimate of the range of values within which, with a specified degree of confidence, the true value is estimated to lie. For example, the concentration of cadmium in river water is quoted as 83.2 2.2 nmol l-1 this indicates the interval bracketing the best estimate of the true value. Measurement uncertainty is discussed in detail in Chapter 6. 

The maximum acceptable measurement reliability (also known as measurement uncertainty) for each analytical result. 

A decision rule that describes how the measurement uncertainty wiii be taken into account with regard to accepting or rejecting a product according to its specification and the resuit of a measurement... 

Measurement uncertainty Since 2004 there has been considerable development in approaches to estimation of uncertainty and this chapter has been considerable revised and expanded in order to take into account new guidelines. Main difference is that several ways of estimating measurement uncertainty are know full acceptable and the analyst is free to choose approach dependent on scope and data availability. 

An important consideration for quality control in industry and commerce relates to the trend of developing faster analytical methods than those described in official sfandards the question in such cases is whether a proposed method is acceptable as replacement for the standard. This problem relates to the concepts fitness for purpose and measurement uncertainty, the latter serving for the estimation of the LOD and LOQ parameters of analytical quality. An example of this dilemma relating to the peroxide value is discussed in Section IV.B.5. 

Clinical trials have been reported, and these are not subject to the same levels of uncertainty. They have concentrated on bone mineral density, because this parameter is an acceptable measure of bone mass, is sensitive to the occurrence of osteoporosis and correlates well with the likelihood of bone fracture in patients affected by osteoporosis. Bone mineral density is known to increase in childhood and adolescence, to reach a maximum around the age of 40, then to decline [110,111]. In women in the years immediately following the menopause, it may sharply reduce, and if it reaches a level TA standard deviations below the young adult mean value, the condition is defined by the WHO as osteoporosis [110,111]. 

The purpose of an analytical method is the deliverance of a qualitative and/or quantitative result with an acceptable uncertainty level. Therefore, theoretically, validation boils down to measuring uncertainty . In practice, method validation is done by evaluating a series of method performance characteristics, such as precision, trueness, selectivity/specificity, linearity, operating range, recovery, LOD, limit of quantification (LOQ), sensitivity, ruggedness/robustness, and applicability. Calibration and traceability have been mentioned also as performance characteristics of a method [2, 4]. To these performance parameters, MU can be added, although MU is a key indicator for both fitness for purpose of a method and constant reliability of analytical results achieved in a laboratory (IQC). MU is a comprehensive parameter covering all sources of error and thus more than method validation alone. 

Measurement results that are traceable to the same reference (often but not necessarily an SI unit), can only be compared in relation to their uncertainties. Figure 7.3 shows the comparison of two measurement results. Because it is unlikely that two measurements made of the same material will give identical results, it is only by evaluating the measurement uncertainties that the client can decide whether he or she will accept that the two materials are equivalent, in terms of the quantity measured. 

If a method must be developed from scratch, or if an established method is changed radically from its original published form, then before the method is validated, the main task is simply to get the method to work. This means that the analyst is sure that the method can be used to yield results with acceptable trueness and measurement uncertainty (accuracy). When the analyst is satisfied that the method does work, then the essentials of method validation will also have been done, and now just need to be documented. If there is an aspect of the method that does not meet requirements, then further development will needed. Discovering and documenting that the method now does satisfy all requirements is the culmination of method validation. 

For laboratory accreditation, based on ISO guide 25 [27] and the EN 45001 standard, as well as for certification, based on the ISO 9000 series of standards [1], it is required that measurement and test results be traceable to international, defined, and accepted physical and physicochemical standards [28], This requirement includes the use of conventionally expressed quantities and units in conformity with the SI [29], It also includes the proper use of the concept of measurement uncertainty. All these are necessary conditions for reliance on the measurement results of another laboratory. Accreditation is granted when a laboratory has demonstrated that it is competent and capable of working in the above-mentioned sense. Technical trade barriers then fall away, and the needs and requests from industrialists, traders, and the general public can be met in the interest of open and fair trade, health, safety, and the environment. 

So, today the measurement assurance approach to calibration is widely used in the United States and in many other places around the world. Increasingly, people appreciate the fact that traceability to national standards is not rigorous unless measurement uncertainty is quantified. While other laboratories and individual metrologists outside NBS also advanced the concept, NBS/NIST is proud of its contributions to putting measurement assurance on a solid technical foundation, and to gaining widespread acceptance for it in the international metrology community. 

In presentation and interpretation of results, NARL aims for objectivity, clear presentation, and statistical data treatment that is transparent to participants, internationally accepted and metrologically sound. Sources of chemical standards, statements concerning traceability and estimates of measurement uncertainty are included in the study report. 

The prerequisite for the mutual acceptance of analytical data such as pH is comparability. Comparability requires the complete evaluation of the measurement uncertainties which in turn are based on traceability to recognised references. The need for traceable pH measurements and the confusion resulting from the ambiguous IUPAC recommendation led to various international initiatives being taken. 

An important precursor of valid measurement, and the establishment of measurement traceability, is an adequate description of what is to be measured (the measurand), which includes the measurement units and consideration of the acceptable level of measurement uncertainty (MU). Clearly, if different characteristics are measured, or different measurement units are employed, then different measurement results can be expected. Clarity on this issue can be vital to subsequent decision making. For example, in environmental studies, it may be more important to know the amount of extractable pollutant in a geological material, rather than the total amount of the pollutant. Thus, although self evident when we think about it, it is important to remember that, in addition to making traceable measurements, it is also important to make the right type of measurement. 

Setting Criteria for Acceptable Reported Measurement Uncertainties in IMEP... 

Measurement results and standards are internationally accepted via their demonstrated comparability and known quality. The test results of a food testing laboratory that is accredited to the ISO/IEC 17025 Standard should be recognized in other countries where laboratory performance is also assessed in accordance with this Standard. This is an aspect of the ILAC MRA [31]. The ISO/IEC 17025 Standard requires traceability to internationally accepted stated references together with their stated measurement uncertainties [22]. 

Uncertainties are a prerequisite for the comparability of analytical measurement results. The ISO/IEC 17025 standard requires traceability to internationally accepted stated references together with their stated measurement uncertainties. 

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