Glossary of Terms
The following terms are used here and in literature elsewhere to discuss reference materials.
Accuracy – the closeness of agreement between the true value and the measured value.
Analytical Outlier – a single result or entire set of results deviating in either accuracy or precision from others in the set or from other sets, respectively, due to errors associated with measurement.
Assay Standard – synonymous with reference material.
Bias – how far the measured value lies from the true value it is estimating (see Accuracy).
Coefficient of Variation – measures the spread of a set of results as a proportion to its mean (SD/mean).
Confidence Interval – a range of values within which the Recommended Value is expected to lie. The magnitude of the confidence interval is inversely proportional to the number of participating laboratories and inter-laboratory agreement. It is a measure of the reliability of the recommended value; the narrower the confidence interval the greater the certainty in the recommended value.
95% Confidence Interval – an interval for which we can assign a 95% probability that the Recommended Value lies within; alternatively, we can say that 95% of all confidence intervals determined in a like manner (from different samples of the population) will include the Recommended Value (it is important to note that this is not equivalent to two standard deviations of the Recommended Value).
Consensus – the quality of agreement in measurements of the same parameter in the same RM between different laboratories in a certification program.
Control Limit – a window of acceptability for results obtained by a laboratory for a reference material and generally calculated from multiples of the standard deviation of the certification data.
CRM – certified reference material (as defined in ISO Guide 35) for which one or more parameters have been certified by a technically valid and recognised procedure and for which a certificate or other valid documentation has been issued by a certifying body.
IRM – internal reference material (generally an in-house SRM).
Measurement Error – error attributed to the method of measurement of a particular parameter at a particular laboratory.
OREAS – the range of CRMs produced by Ore Research & Exploration Pty Ltd (OREAS is an acronym for Ore Research & Exploration Assay Standards).
Performance Gate – synonymous with Control Limit .
Physical Outlier – a single result or entire set of results deviating in either accuracy or precision from others in the set or from other sets, respectively, due to inhomogeneity of the reference material.
Pigeon Pair, Paired Offsets, Bracketed Standards, Variance Standards – terms used to describe a pair of standards of almost identical matrix that differ in concentration of one or more certified elements by an amount approximating the typical measurement error of methods in common use at commercial analytical laboratories.
Precision – the closeness of agreement between results obtained by measurement of the parameter numerous times under prescribed conditions.
Recommended Value – a statistically robust estimate of the true value of a reference material parameter (equivalent to the Certified Value for a CRM).
Repeatability – the closeness of agreement between results for the same parameter carried out by the same method, by the same operator, with the same instrument, in the same laboratory, over a short interval of time.
Reproducibility – the closeness of agreement between results for the same parameter carried out by a different method, or different operator, or different instrument, or in a different laboratory, or over an interval of time quite long in comparison to the duration of a single measurement.
RM – reference material for which one or more parameters are sufficiently well characterised to be used for the calibration of instruments, the assessment of analytical methods, or for assigning values to unknown materials.
SRM – standard reference material, synonymous with reference material (note: in the past this acronym has also been used for “Secondary Reference Materials” – materials characterised to a lower level than and generally not conforming to all ISO criteria for a CRM ).
Standard – literally this refers to a recognised and approved method or procedure (such as the standards published by ISO) but its use to describe reference materials is also well entrenched in the mineral and chemical industries.
Standard Deviation (SD, sd or s) – a measure of the spread or dispersion of a set of results and calculated from the square root of the variance.
Statistical Outlier – a single result or entire set of results deviating in either accuracy or precision from others in the set or from other sets, respectively, to a degree greater than can be justified by statistical fluctuations associated with a given frequency distribution.
Tolerance Interval – a measure of homogeneity of one or more parameters in a reference material in which there is a fixed probability the specified interval will contain at least a specified proportion r of the population from which the sample is taken.
95% Tolerance Interval – (where r = 0.95 & 1- a = 0.99) may be defined as meaning that 99% of the time at least 95% of samples will have values lying between the specified interval. Put more precisely, this means that if the same number of samples were taken and analysed in the same manner repeatedly, 99% of the tolerance intervals so constructed would cover at least 95% of the total population, and 1% of the tolerance intervals would cover less than 95% of the total population.
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