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On the misuse of the correlation coefficient to assess linearity of calibration curves.

By J Van Loco, M Elskens, Christophe Croux and H Beernaert

Abstract

The correlation coefficient is commonly used to evaluate the degree of linear association between two variables. However, it can be shown that a correlation coefficient very close to one might also be obtained for a clearly curved relationship. Other statistical tests, like the Lack-of-fit and Mandel's fitting test appear therefore more suitable for the validation of the linear calibration model. A number of cadmium calibration curves from atomic absorption spectroscopy were assessed for their linearity. All the investigated calibration curves were characterised by a high correlation coefficient (r > 0.997) and low quality coefficient (QCEfficiency; Distribution; Variable; Value; Expected; Yield;

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