(B) Vs and Zs to get rid of the effects of X to facilitate introduction, the author assumes that X values??only in two levels: first, Y from not 'handle' (Treatment) of the control group (Control Group); the second is to Since the treatment group received treatment (Treatment Group) (for example, accelerated depreciation method to test the theory, the control group may be defined as using the straight-line depreciation of the company, handling group may be defined using accelerated depreciation for the company). In order to simply reasons, the authors assumed that only the pre-impact factors V Y, whether the object is from the control group or treatment group, V of Y have the same effect. Moreover, assuming no influence over the same period factors Zs Y, Y of the model is changed to: