Maurice Levy, chief executive officer of Publicis Groupe SA, called the Association of National Advertisers’ pending study on transparency in the ad business “unfair and an unwarranted attack on the entire industry.”

The probe is expected to reveal that there is a widespread lack of transparency in the ad buying business and that ad companies are collecting rebates from media vendors in the U.S. on behalf of their clients as a reward for buying a certain amount of ad time, The Wall Street Journal reported last week. The report, which the trade group says will be released imminently, isn’t expected to call out specific companies or individuals that allegedly have received rebates, according to people familiar with the matter.

Read the full Wall Street Journal article here.