Testing for the linearity of the calibration is an important feature in the development of an analytical method. The correlation coefficient is often used in this context but is open to misinterpretation by the unwary and is to be discouraged. When data are collected in well designed calibration experiments, tests for non-linearity can be based on the analysis of the residual variance from a regression into parts owing to ‘lack-of-fit’ and ‘pure error’. The calculations can be executed by means of a few simple commands in many statistical software packages.