When conducting workshops, people always ask me how they should calculate the ROI of their training. I tell them that demonstrating the monetary effect of training is complicated and that they should think twice before embarking on this endeavour. When calculating ROI, sooner or later you will encounter a few interesting fundamental measurement challenges that can't be solved with a simple algorithm. Over the years, we have developed a one-hour case study that helps to explain the traditional ROI methodology and provides some issues to think about.