  Third-party networks collect vast amounts of data about users via web sites
and mobile applications. Consolidations among tracker companies can
significantly increase their individual tracking capabilities, prompting
scrutiny by competition regulators. Traditional measures of market share, based
on revenue or sales, fail to represent the tracking capability of a tracker,
especially if it spans both web and mobile. This paper proposes a new approach
to measure the concentration of tracking capability, based on the reach of a
tracker on popular websites and apps. Our results reveal that tracker
prominence and parent-subsidiary relationships have significant impact on
accurately measuring concentration.
