Conventional true value

The accuracy of an analytical procedure expresses the closeness of agreement between the value, which is accepted either as a conventional true value or an accepted reference value and the value found. This Is sometimes termed trueness . 

Bias (systematic deviation) of an individual test value x (or y) from the (conventional) true value xtrui, (or ytrue)... 

Xtrue (Conventional) true value of a (certified) reference sample ... 

Comparison of test values with a conventional true value ( reference value ) of a (certified) reference material (RM, CRM). In method development and validation of analytical procedures, the comparison of experimental results with standards of diverse kind (laboratory standards, certified reference materials, primary standards) plays an essential role. The decision as to whether an experimental result hits the reference value depends not only from the result itself but also from its uncertainty interval. 

Fig. 9.5. Relationship of a uniform a priori expectation range (between xmin and max)> normal-distributed (t-distributed) measured values, and the (conventional) true value xtrue... 

Since the true vaiue is principally unknown, a conventional true value is used to estimate the bias... 

A result obtained by using several independent methods in several expert laboratories on one measurand Is regarded as conventional true value of a quantity... 

A conventional true value is in general, regarded as sufficiently dose to the true value... 

Conventional true value is defined as a value attributed to a particular quantity and accepted, sometimes by convention, as having an uncertainty appropriate for a given purpose . ... 

A conventional true value that all analysts are aware of is the number of atoms in a mole - 6.022 x 10. However, it is known accurately and if required 6.0221367(36) can be used. 

The accuracy of an analytical method is the degree of agreement of results generated by the method to the tme value or a conventional true value. Accuracy can be assessed by applying the analytical method to samples or mixtures of sample matrix components to which known amounts of the analyte have been added, above and below the normal levels expected in the samples. Method accuracy is the agreement between the difference in the measured analyte concentrations of the fortified (spiked) and unfortified samples and the known amount of analyte added to the fortified sample. 

Total Error or Measurement Error The measurement error of an analytical procedure expresses the closeness of agreement between the value measured and the value that is accepted either as a conventional true value or an accepted reference value. This is also the definition of accuracy in ICH Q2R. This closeness of agreement represents the sum of the systematic and random errors, that is, the total error associated with the observed result. Consequently, the measurement error is the expression of the sum of the trueness and precision, that is, the total error [19,22]. 

An analytical procedure is in statistical control when the variation among the observed results can be attributed to a constant system of chance causes. It is assumed that an analytical method is in a state of statistical control when the distribution of results can be approximated by the normal distribution around the conventional true value ( Xr) with a standard deviation ( t). When a measurement system is in statistical control the data have statistical predictability and several statistical calculations can be performed for its evaluation and documentation. 

FIG. 2. RMSe values and the upper tolerance limit vs. the conventional true value of lead in blood. 

The use of Shewhart control charts with multiple decision rules provide a more sensitive and reliable quality control than Shewhart control with simple decision rules. However, it should be noticed that the multiple decision rules are only valid if the conventional true value pt and the standard deviation oy are known. In practice this means that (jl-t is traceable to a certified reference material (CRM) or established using a definitive method by a validated laboratory [5], and 0-7 is established by method evaluation with a sufficiently high number of method evaluation samples. 

It may be necessary to use weighted least squares regression calculations in the method evaluation to obtain the Oji value, corresponding to the standard deviation at the conventional true value pti having unit weight. A valid estimate of aj,- at the concentration p-x,- of the control sample can be computed using the equation [12] ... 

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