As a novice I kind of struggle with structuring the cost branch of my issue tree. If the interviewer has no prefered segmentation, Victor Cheng and the Preplounge tutorial recommend to structure costs into "costs/unit" and then into "fixed costs/unit" and "variable costs/unit". However, segmenting costs directly into "costs/unit" sounds kind of peculiar to me due to the counteracting forces of this equation: if from one year to the other, a company sells more "units" (assuming "variable costs/unit" stay the same, and total fixed costs did not change), the new "costs per units" will be less as "fixed costs/units" have gone down. Thus, its not really MECE as "units" affects the "costs/units" via "fixed costs/units".
I'm kind of confused by this cost segmentation and how it should help me to identify root causes of profitability problems. Can anyone recommend a good way to segment costs if nothing is given by the interviewer?