This paper describes the p-diagram (parameter-diagram) and its application in planning a DoE (Design of Experiments). A case study describing an actual problem from industry is presented where the planning phase started to go wrong as difficulties in selecting the right variables for the DoE were discovered. Furthermore, running these experiments became prohibitively expensive, due to the large number of such experiments that would be needed, and though the exploitation of a p-diagram it was then possible to come up with a feasible DoE.