Method performance terms relative standard deviation

Option (Valid) presents a graph of relative standard deviation (c.o.v.) versus concentration, with the relative residuals superimposed. This gives a clear overview of the performance to be expected from a linear calibration Signal = A + B Concentration, both in terms of (relative) precision and of accuracy, because only a well-behaved analytical method will show most of the residuals to be inside a narrow trumpet -like curve this trumpet is wide at low concentrations and should narrow down to c.o.v. = 5% and rel. CL = 10%, or thereabouts, at medium to high concentrations. Residuals that are not randomly distributed about the horizontal axis point either to the presence of outliers, nonlinearity, or errors in the preparation of standards. 

An assessment method is useful for establishing a stable FIA-ECD system with satisfactory precision to obtain highly reproducible and sensitive results. The measurement precision is described in terms of the standard deviation (SD) and/or relative standard deviation (RSD) measurements [8]. For example, when repetitive measurements (n < 30) are performed, measured data sets are obtained as x, Xi,. .., x . In this case, the equation of SD is... 

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. 

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. 

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