precision
Precision (also obsolete repeatability ) is a criterion for assessing the quality of a measurement or a measurement process . Precision and correctness are components of accuracy . The precision characterizes the spread of the values obtained.
The precision of a method is quantified by repeating the measurement many times under the same circumstances (also: under repetition conditions) with the same measuring device or measuring system and reducing the individual series of results according to standardized algorithms of error and compensation calculation . A very precise procedure delivers almost the same series of results for the same task. The precision does not make any statements about how far the individual measured values are from the true value , but rather describes the stability of the method. The precision can numerically z. B. can be specified by a spread or a standard deviation .
In order to delimit the terms accuracy and precision from one another in a meaningful way, it should first be noted that high precision in technical contexts is a necessary but not sufficient requirement for high accuracy. This relationship, which is a logical consequence of the abovementioned brief definitions, is better expressed by the outdated wording “repeatability”: Precision is a component of accuracy that is limited to the aspect of reproducibility. Technical processes can still run imprecisely with high precision, but conversely, imprecise technology cannot produce precise results; see also the definition of precision according to DIN.
definition
The meaning of the term precision is below 55350 DIN by pointing to the importance of equality to formerly -13 repeatability defined feature mentioned.
- Definition according to DIN
- The precision describes how small the maximum deviations of mutually independent determination results become, which were obtained by the examiner using a specified determination procedure several times under specified conditions. The formulation implies that high precision is expressed by low absolute and relative values. The DIN also explicitly points out that an investigation procedure works more precisely, the smaller the "random result deviations" of the procedure.
The term "repeatability" should no longer be used in the field of standardization . However, not only in older specialist literature, but also in new texts, the repeatability is often quantified as a quality feature of measuring devices and production machines.
application
To determine the repeatability, the measuring range is run through several times under the same conditions as possible. The resulting measurement diagrams are now compared with one another. In order to clearly differentiate such “comparison measurements with oneself” from absolute calibrations, the term “repeatability” - in the sense of precision - is indispensable, especially in the field of measuring systems of the highest precision and absolute accuracy. The repeatability can be better than the absolute accuracy. But the absolute accuracy can be immeasurable at the time of the quality assessment within the scope of the technical development status. The main reason for the difference is that, for measuring devices that function very well in principle, the individual measurement results are mostly symmetrically scattered around the actual physical value as Gaussian distributions . In special cases, however, systematic errors of a measuring device can concentrate the absolute measuring errors in partial areas of the entire measuring range, while the remaining areas measure with much higher accuracy. If there are systematic errors of this kind that result from the limits of technical feasibility, as is typical with measuring devices for calibration, then two successive series of measurements can provide almost identical diagrams, but which, viewed in absolute terms, have typical errors in the same spectral ranges.
If there is no way at all to detect measurement errors in a calibration device using a slightly better measuring process , then the repeatability is initially the only indication for at least a rough estimate of how good the measuring process or the measurement method could be. In this case, the repeatability is the dominant component of the measurement uncertainty .
The aspects of precision that are relevant to calibration standards can be seen particularly well on measuring machines of the PTB : An additional check of all determination results of a calibration device by comparison with the corresponding determination results of at least one non-identical precision machine for the same area of application is of great importance.
Web links
literature
- German Institute for Standardization : Quality Management and Statistics . DIN-Taschenbuch 223.Beuth Verlag, Berlin / Vienna / Zurich 2001, ISBN 3-410-15136-2
Individual evidence
- ↑ First international practical test for the hydrodynamic test field , research news from Department 1, PTB. December 22, 2004.