Boone (2008a) proposes a new competition measure based on Relative Profit Differences (RPD) with superior theoretical properties. However, the empirical applicability and robust-ness of the Boone-Indicator is still unknown. This paper aims to address that question. Using a rich, newly built, data set for German manufacturing enterprises, we test the empirical valid-ity of the Boone-Indicator using cartel cases. Our analysis reveals that the traditional regres-sion approach of the indicator fails to correctly indicate competition. A proposed augmented indicator based on RPDs performs better. The traditional Lerner-Index is still the only meas-ure that correctly indicates the expected competitive changes.