Method performance terms repeatability

Repeatability is a measure of the short-term variation in measurement results and is the precision that can be most easily determined. It is often used to establish compliance with method performance criteria. While repeatability is a... 

In addition to specified validation criteria, it is also useful during method development to assess column durability and column-to-column performance. Benchmarks have been established in a series of papers by Kele and Guiochon for short- and long-term repeatabilities and column-to-column... 

The precision of a determination is defined as the concordance of a series of measurements of the same quantity (V3). The definition of precision includes the terms repeatability and reproducibility (Hll). Repeatability is a measure of the deviation of test results from their mean value, all determinations being carried out by one operator and without change of apparatus, where apparatus can be significant (Hll). For reproducibility in clinical chemistry the definition provided by Hughes (Hll) is not entirely satisfactory, and an alternative is proposed as follows reproducibility of a method is a measure of deviation of test results from their mean value, the determinations being carried out over a period of weeks or months, perhaps by different operators, the operators in no case knowing the identity of the analytical specimens that are being used to assess the continued quality of performance of the technique. 

The method performance study focuses on the analytical method. In order to achieve a constructive work the organiser should assure that the method to be used by the participants has been investigated in detail prior to the start of the interlaboratory study. This preliminary investigation should be undertaken by a senior analyst. It should lead to a detailed draft analytical protocol, to a clear definition of the type and number of samples to be distributed and analysed, to indications on the repeatability of the method and its long-term reproducibility. In parallel, a study on the production of proper samples should be undertaken. Additional materials can also be prepared, e.g. calibration tools (pure calibrants or solutions), or blanks with a similar matrix than the test sample, spiked materials, etc. 

The term repeatability refers to the agreement between the results of a number of measurements of the same quantity performed by the same method, the same observer, and the same instrument in the presence of random fluctuations (i.e., errors). Here the random uncertainty, a relative or absolute plus/minus semirange of an interval around the mean value, is usually indicated. A measure is the standard deviation or a certain multiple of the standard deviation. A high repeatability means a small random uncertainty (i.e., small random errors). In most cases, the repeatability is higher than the accuracy because random errors can occur independent of systematic ones. Similar to the accuracy, the repeatability can vary from one process to another in a given instrument. Further details on the nature of the examined process must therefore be available in order to use the repeatability of an instrument as an efficient characteristic. 

Performance specifications of the method It is important to recall the previously-stated requirements of the method, which could be expressed in terms of accuracy, precision, repeatability, up-time, and/or other attributes. It s also quite possible that these specifications have changed since the method was first put into service. 

Remember a method may perform well in terms of repeatability and reproducibility of results on identical samples, but this does not necessarily mean that the results are accurate. 

Automated methods frequently exhibit remarkable performance not only in terms of sample throughput and cost, but in relation to the quality of results, especially in relation to repeatability and reproducibility. Several systems are currently available that enable simple automation of manual gestures these include automatic titra-tors, pH-meters with a circulating cell, etc., and involve approaches that are not fundamentally different to the corresponding manual method. These techniques are not, however, described in this chapter, nor are gas and liquid chromatography and capillary electrophoresis, automated techniques, that may be present but are not routinely used in oenological laboratories. 

Reproducibility is defined as the long-term variability of the measurement process, which may be determined for a method run, within a single laboratory, but on different days. Reproducibility also applies to a method, either run by different operators, different instruments, or a combination of the above. The reproducibility standard deviation is typically twofold to threefold larger than that for repeatability. Precision is often expressed relative to 1 day as intraday (within-day) precision or relative to a period of days, as interday (between days) precision. Reproducibility, in the sense of intralaboratory precision, is related to the procedure being performed at two or more laboratories as in, e.g., a collaborative study. 

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