A method is proposed which quantitatively clarifies the degree of improvement in power systems voltage control performance by different system information. The proposed method is expected to give knowledge useful in studying control/communication network in future power systems. The proposed method, in the first phase, calculates the trajectories of voltage control variables to given demand scenario which minimizes cost function represented with performance indices such as voltage deviation, operating cost of controllers, and reactive power flows. Secondly, so-called data driven technique is utilized to translate the obtained relation among system/control information into control rules. Here the choice of describing variables in these rules affects the final control performance, which can be considered as the improvement obtained by changing available information and its weighting factor. The validity of the proposed method is confirmed by numerical simulation.