Fidelity of measurement results

Fidelity in the context of method validation is better known as reproducibility or repeatability. However, each term possesses a precise meaning in terms of quality and fidelity and repeatability should not be confused. The definitions can vary from one field to another and it is important to describe fidelity and repeatability in the documentation of an analytical procedure and detail the conditions of their measure number of samples analyzed, time lapses between samples, and other factors. The definitions below were used by the European pharmaceutical industry at the time of the writing of this chapter. The criteria linked to the fidelity of measures are generally expressed as coefficients of variation (CVs). 

Fidelity —The fidelity of an analytical procedure expresses the narrowness of the accord (measure of dispersion) between the measures obtained from several portions of a single homogeneous sample. 

Repeatability— This characteristic is fidelity estimated when the results of independent trials are obtained with the same method on identical portions of a sample in the same laboratory by the same operator, using the same equipment during a short lapse of time. Repeatability is generally estimated over a day and called intra-day repeatability in some of the literature. 

Robustness— This is a measure of the ability of an analytical procedure to not be affected by minor modifications of factors associated with the procedure. The estimation of the robustness of a method is a subjective process, depending on the context and choice of parameters to be studied determined by the person in charge of the validation. The legitimacy of the choice must be justified during the procedure. One can, for example, study the influence of the pH of the matrix (e.g., urine) during the preparation of the sample on the extraction yield of the analyte. The aim is to verify that a minor modification of pH does not significantly alter the extraction yield. If the room where the samples are prepared is not air conditioned, one can also check that a modification in temperature does not alter the dosage results. 

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