The technique of developing curves of how the operation and product of a mill responds to a change in one of the prime operating variables is presented. This has been investigated to the great benefit of research into the comparison of mill performance when an operating parameter of interest is changed. These 'grindcurves' can be used to select the correct conditions for meaningful comparative tests. It is shown that the conclusions drawn from pilot work can be totally changed by shifting the mill to operate under optimal conditions for the comparison. The use of the technique in quantifying SAG mill performance is demonstrated. (C) 2006 Elsevier Ltd. All rights reserved.