What makes a Mode Stirrer efficient in a reverberation chamber, and how can one find the parameters of major importance? This is a problem that might be very complicated analyse theoretically or to model. An alternative interesting approach, that can be performed quite easily, is to carry out a Design of Experiment (DOE). The focus has been to investigate traditional rotational mode-stirrers. To perform a general factorial design, an investigator selects a fixed number of "levels" (or "versions") for each of a number of variables (factors) and then runs the experiments with all possible combinations. The test criterion selected in this case study has been the specific lowest frequency for each stirrer chamber combination, which corresponds to at least 200 uncorrelated stirrer positions. The outcome of the factorial design experiment shows that the effect of changing the diameter of the stirrer is much greater than changing the height. This seems to be more pronounced at 50 uncorrelated stirrer steps than at 200. The effect of changing the chamber volume is rather small. The latter is illustrated by the fact that the lowest frequency for 200 uncorrelated stirrer steps improves about 60 % if it is use in the small 1 m3 chamber compared to the 210 m3.