Using factorial design, the important parameters affecting the DI is identified. The factorial design is a powerful tool for determining how a system responds to controlled changes in some inputs or factors, by the observation of the corresponding changes in the responses or outputs. It is considered an appropriated research tool to deal with problems with many interacting variables and it is successfully used to identify and quantify the principal system variables and their effects. The full factorial design uses all possible combinations of levels and factors, therefore all possible variable interactions are included.