This paper considers the usage of Taguchi's contribution ratio as well as an adjusted Pareto diagram for determining the size of influential design factors in experimental design on output values in order to determine the best combination of input factors, as well as factors that can determine output. Two case studies that cover extreme examples are presented in that aim. The first case study examines one input workspace design distributed on tree experimental designs defining space coordinates. Every design has several output values that were measured. The second case study presents two experiments regarding injection of plastic molding process, with same input factors at parts which are different in material and dimensions with geometric deformations as output. It was shown that different experiments lead to different results, of which one is acceptable, while other is useless for further examinations. Accordingly, this paper gives guidelines how to use Taguchi's contribution ratio and Pareto diagram effectively in determination of influential factors in experiments.