Kaizen costing focuses on continuous reductions of costs, which should be realized for existing products in a company. For planning and control purposes, comprehensive and efficient tools for measuring performance are required. For this purpose we suggest the so-called ”half-life model”. It is based on the practical experience, that any defect level decreases at a constant rate over a certain time period. This paper gives the mathematical formulation for this model and illustrates its practical use with different examples. pdf 1998