The selection of control locations and feedback signals is a key step in designing wide-area power system stabilizers（WPSSs）.The residue method，which is the most commonly used method to select locations and feedback signals，suffers a scaling problem when comparing different signals.The joint observability/controllability geometric index is used to select the most effective stabilizing signals and control locations.The observability of different types of signals such as tieline active power flows，bus voltage angle differences，center-of-inertia（COI）difference between areas，generator output powers and generator rotor speeds are considered in this method.The tie-line active powers are found to be the most effective input signals，considering the requirement of measurement devices and communication channels in actual power systems.Taking a2-area power system as case study，the joint geometric index based WPSS design procedure is illustrated.The importance and feasibility of the method are also discussed in the same study.The simulation results demonstrate the advantages of the proposed WPSS through improved system damping.