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

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;