Repeatability of results

Analysis precision, n - a statistical measure of the expected repeatability of results for an unchanging sample, produced by an analytical method or instrument for samples whose spectra represent an interpolation of a multivariate calibration. The reader is cautioned to refer to specific definitions for precision and repeatability based on the context of use. 

Repeatability Repeatability of Results cf Ivteasurements Repeatability of a Ivteasuring Instrument... 

Figures 3.1 through 3.6 show the repeatability of results in our ARCO unit when North Sea atmospheric residue is used as feed. The time between the first test and the repeatability check is about 2 years. As can be seen in Figures 3.1 through 3.6, the two tests give almost the same results in the ARCO pilot unit even with a North Sea atmospheric residue feed. This shows that the ARCO unit is just as suitable for a residue feed as for a vacuum gas oil feed.

Feinberg 1, Fein G, Floyd TC Period and amplitude analysis of NREM EEG in sleep repeatability of results in young adults. Electroencephalogr Clin Neurophysiol 48 212-221, 1980... 

There is often confusion concerning the distinction between accuracy and precision. The term precision is frequently used to describe the reproducibility or repeatability of results(85> (see previous section). Accuracy denotes the nearness of a measurement to its accepted value and should be expressed in terms of error (see below). Thus, accuracy involves a comparison with respect to a true or accepted value, and precision compares a result with the best value of several measurements made in the same way. An alternative (but less carefully worded) distinction is to describe precision as the number of digits employed in a calculation (which may not all be correct) and accuracy as the number of digits to which the result of that calculation is correct. 

Repeatability (of results of measurements) is the closeness of agreement between the results of successive measurements of the same measurand on identical test portions carried out under defined conditions. Conditions will... 

Precision, which quantifies the variation between replicated measurements on test portions from the same sample material, is also an important consideration in determining when a residue in a sample should be considered to exceed a MRL or other regulatory action limit. Precision of a method is usually expressed in terms of the within-laboratory variation (repeatability) and the between-laboratory variability (reproducibility) when the method has been subjected to a multi-laboratory trial. For a single-laboratory method validation, precision should be determined from experiments conducted on different days, using a minimum of six different tissue pools, different reagent batches, preferably different equipment, and so on, and preferably by different analysts Repeatability of results when determined within a single laboratory but based on results from multiple analysts is termed intermediate precision Precision of a method is usually expressed as the standard deviation. Another useful term is relative standard deviation, or coefficient of variation (the standard deviation divided by the absolute value of the arithmetic mean result, multiplied by 100 and expressed as a percentage). 

Based on our own experience, solid phase extraction with 500 mg C18 SEP-Pak disposable cartridges works satisfactory in terms of extraction yield and repeatability of results. We use methanol/formic acid = 9/1 (v/v) for anthocyanin extraction, followed by evaporation to dryness and reconstitution in water/formic acid = 9/1 (v/v). This solution is brought onto cartridges equilibrated with water/formic acid = 9/1 (v/v). The samples are then washed with the same solvent and eluted with methanol/formic acid =9/1 (v/v) and again evaporated to dryness. After reconstitution in an appropriate solvent, the samples are ready for use. On average, the recovery of anthocyanins is better than 95 %. Repeated extraction (n=15) of a reference standard (en-3-O-gle) gave a scattering of less than 7 % (relative standard deviation). 

As measures of workability of the fresh mix, several different tests are proposed, but each of them gives a certain indication which is not comparable with the others. The slump test (Abrams cone) and the Vebe test give only single values by which the mix is characterized, in millimeters and seconds, respectively. Therefore, nothing more complicated than the Newton model is used and the test results do not characterize the mix without ambiguity. However, these and other similar one-point tests are used frequently, mainly for their simplicity and, in most cases, acceptable repeatability of results. The required workability is expressed as minimum slump, maximum Vebe time, or any other measure, and in many cases such a simple result is sufficient however, it is necessary to understand its limitations. In certain compositions the results are ambiguous or even impossible to execute very stiff mixes cannot be tested simply and compared with valid results the same concerns very fluid mixes with superplasticizers. 

Application of glass electrodes for pH-measurements in non-aqueous media caused some controversies in the past " and in more recent literature. All the above remarks, related to accuracy and repeatability of results in non-aqueous and mixed-solvent media testify well on account on its applicability in such media. Some troubles and the non-compatibility of the results obtained with the expected ones may arise when the organic solvent (e.g. acetonitrile) is aggressive towards the electrode" or a solute tested. 

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