The breakage ratio (selection function) and the breakage function are needed for population balance analysis, which is a common practice to evaluate the performance of comminution systems. In this paper, four materials of various sizes, impact velocities and number of impacts are tested in a new experimental apparatus (impact machine). The experimental results are compared to an existing empirical model for the breakage ratio and to a newly developed one that presents the breakage ratio as a function of the impact velocity and the number of impacts. The comparison to the new model shows a very good agreement. The same function is also used to describe the decrease of the median size. The breakage ratio is discussed comprehensively in the paper and the discussion is accompanied by experimental evidences that suggest that the main cause to the breakage ratio (selection function) is the particle strength distribution. (C) 2004 Elsevier B.V. All rights reserved.