Résumé : The note presents a simpler alternative to the Modified Herfindahl-Hirschman Index to measure the risks of market concentration in the presence of common owners such as institutional investors owning shares in multiple firms expected to compete in the same market. This new measure, the Amplified Herfindahl-Hirschman Index, delivers the same insights as the MHHI but is less data intensive and less sensitive to outliers. It thus offers a more “user friendly” and more precise alternative to competition and regulatory agencies as a decision trigger for more detailed investigations of market power risks associated with the growing presence of common ownership.