Measurement uncertainty meaning

The uncertainty of measurement is defined as a parameter, associated with the result of a measurement, that characterises the dispersion of the values that could reasonably be attributed to the measurand . It is also known as measurement uncertainty. A simpler way of stating what measurement uncertainty means is to say that measurement uncertainty is the range of values within which the value of the quantity being measured is expected to lie with a stated level of confidence. It is not the same as an error, because to estimate an error the true value must be known. Error is the difference between a true value and the result. 

Where you devise original solutions to the measurement of characteristics the theory and development of the method should be documented and retained as evidence of the validity of the measurement method. Any new measurement methods should be proven by rigorous experiment to detect the measurement uncertainty and cumulative effect of the errors in each measurement process. The samples used for proving the method should also be retained so as to provide a means of repeating the measurements should it prove necessary. 

Example 57 The three files can be used to assess the risk structure for a given set of parameters and either four, five, or six repeat measurements that go into the mean. At the bottom, there is an indicator that shows whether the 95% confidence limits on the mean are both within the set limits ( YES ) or not ( NO ). Now, for an uncertainty in the drug/weight ratio of 1%, a weight variability of 2%, a measurement uncertainty of 0.4%, and fi 3.5% from the nearest specification limit, the ratio of OOS measurements associated with YES as opposed to those associated with NO was found to be 0 50 (n == 4), 11 39 (n = 5), respectively 24 26 (u = 6). This nicely illustrates that it is possible for a mean to be definitely inside some limit and to have individual measurements outside the same limit purely by chance. In a simulation on the basis of 1000 sets of n - 4 numbers e ND(0, 1), the Xmean. Sx, and CL(Xmean) were calculated, and the results were categorized according to the following criteria ... 

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. 

In case (b), although the measured value is less than the reference value, if the expanded uncertainty is taken into account it is possible that the actual concentration of X could exceed the reference value. In case (c), the measurement result equals the reference value. Although the measured value does not exceed the reference value, the expanded uncertainty means that the true value of the concentration of X could exceed the reference value. Finally, in case (d) the measurement value exceeds the reference value but if the expanded uncertainty is taken into account, it is possible that the true value of the concentration of X could be below the reference value. 

S. M. Sarge, W. Poe 3necker. Thelnfluence of Heat Resistances andHeat Transfers on the Uncertainty of Heat Capacity Measurements by Means of Differential Scanning Calorimetry. Thermochim. Acta 1999, 329, 17-21. 

The certified value is usually taken as the grand mean of the valid results. The organizer uses standard deviation as the basis for calculating the measurement uncertainty. Results from the laboratories will include their own estimates of measurement uncertainty and statements of the metrological traceability of the results. There is still discussion about the best way to incorporate different measurement uncertainties because there is not an obvious statistical model for the results. One approach is to combine the estimates of measurement uncertainty as a direct geometric average and then use this to calculate an uncertainty of the grand mean. Type A estimates will be divided by /n n is the number of laboratories), but other contributions to the uncertainty are unlikely to be so treated. 

Suppose a very large number of measurements could be made under conditions of measurement that allow all possible variation that could occur, including systematic effects from calibrations of balances, glassware, and so on. Also suppose that the material being measured was identical for all of these measurements, which were correctly applied with any identified systematic effects corrected for. The reality of measurement uncertainty is that these measurements would not be identical but would scatter around the value of the measurand. In the absence of any other information, this dispersion is assumed to be normally distributed, which can be described by two parameters, the mean (p) and the standard deviation (cr). It is not... 

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. 

The sketched lines represent the uncertainty of the modelled UV values ( 7.2% according to [6]). Diamonds included in the shaded area represent clear sky measurements. The mean percent difference between measured and computed irradiances is -1.5% at Rome and -1.4% at Ispra for clear sky conditions, respectively (the standard deviation is 3.9% for both sites). 

Any change in the international procedure or a shift to another measurement method means a new stated reference, usually giving new quantity values and measurement uncertainty as shown in the outcome of EQA. The international conventional reference measurement procedure is a part of the definition of the measurand. 

In Fig. 1, the essential elements of traceability are exemplified for the measurement of an element amount content using the internal standard method values linked by measurements to other values, thus constituting a chain (only a part of a chain is shown). The chain must ultimately end up in a value on a measurement scale (Fig. 2) [2] and therefore in a unit. All measurement results which are traceable to values on a common scale (or to the value of a unit) are comparable [3], meaning literally that they can be compared to each other. This does not imply that they must be concordant [3], i.e. result in the same value (within measurement uncertainty), but only that they can be validly compared for their magnitudes, even if these are (very) different. The measurement scale can be constructed from any sort of (internationally) agreed unit. If at all possible, this should be a base or derived SI unit because we have a solid international agreement on this. However, in chemistry, there are numerous measurement results which cannot be traced to (values of) SI... 

For this analyte, the assigned value was set from the inverse-weighted (for uncertainty) mean of two independent measures of component concentration ... 

In the IMEP programme1, Si-traceable values with a full measurement uncertainty according to the Guide to the expression of uncertainty in measurement (GUM) are disseminated by IRMM to field (and other) laboratories by means of appropriately prepared test samples. The uncertainties are the end-product of an evaluation process of all uncertainty sources which is as complete as... 

It is shown how reference values with a demonstrated traceability and demonstrated uncertainty (according to ISO-GUM) are disseminated by IRMM to field (and other) laboratories by means of appropriately prepared test samples. The reference values are established using internationally recognized measurement capabilities and are demonstrated to be equivalent at the international level. The uncertainties are the end-product of an exhaustive evaluation process, yet, the resulting combined and expanded uncertainty are sufficiently small for the intended use of the result (i.e. to be smaller than the expected interlaboratory spread of the participants measurement results). After having measured these samples, the participating field (and other) laboratories can compare their measurement results with these reference values, which are released after they have submitted their own results. Both the reference value and its measurement uncertainty as well as the participants values with their declared measurement uncertainty are displayed in simple, comprehensive pictures. 

In their regular day to day practice, field laboratories use commercial reagents or prepare in-house solutions for the calibration of instruments, and they rely on purity assessment of producers. For method validation and even measurement uncertainty, field labs regularly participate in proficiency testing schemes. In such inter-laboratory comparisons, the reference value is usually obtained as the arithmetic mean of results of participants. 

The ISO 5725 standard was used to interpret the data. Even if the main purpose of this standard is related to the validation of a method, it can be used to evaluate some components of the measurement uncertainty. The homogeneity of the population of results, in terms of mean and standard deviation was determined using statistical tests (Cochran and Grubbs). A few laboratories were rejected after the tests. Tables 3 and 4 present the comparison of overall performance of laboratories when working with usual and metrological calibrations solutions. 

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