This report presents the results of a statistical study of the sensitivity of European (SAE equivalent, J1470 OCT84) and U.S. (SAE J986 FEB87) passby noise test procedures to critical parameters of the test methodology. The critical test parameters for J1470 are start speed, start location and end location, and for J986, start speed and start location. Utilizing central composite design of experiments, test parameter tolerances are determined for both procedures using eight light vehicles representative of current and future U.S.-designs. Definition of parameter tolerances leads directly to improved data repeatability and reliability for passby noise tests. Further, the technical knowledge of the European procedure gained through this investigation provides the background information necessary to better promote the concept of international harmonization of noise test procedures.