Select arbitrarity a certain number of central observations which we omit from the analysis. (for n=30 , 8 central observations are omitted i.e. 1/3 of the observation are removed). The remaining n-c observations are divided into two sub-groups of equal size i.e. (n-c)/2, one sub-group includes small values of X and other sub-group includes the large values of X, as data set is arranged according to the magnitude of X.