1. A design of experiments (DOE) method for predicting a wafer fabrication outcome comprising: establishing a plurality of first correlation-suppressing relations between each of a plurality of design variables and each of a plurality of first outputs, establishing a plurality of second correlation-suppressing relations between each of a plurality of variances of each of the plurality of design variables and each of the plurality of first outputs, and extracting a correlation-suppressing plurality of second outputs from the plurality of first outputs, by using a 3-sigma analysis; and establishing a plurality of first causality relations between each of the plurality of wafer design variables and each of the plurality of second outputs, and establishing a plurality of second causality relations between each of the plurality of variances of each of the plurality of wafer design variables and each of the plurality of second outputs; wherein both the plurality of first outputs and the plurality of second outputs are utilized for indicating wafer fabrication outcomes of a DOE system, and both the plurality of first causality relations and the plurality of second causality relations are utilized for predicting a wafer fabrication outcome.