WPP chief Sir Martin Sorrell has said he is "puzzled" by some of the debate circulating around transparency in the programmatic ad trading market. Speaking at the Ad Tech conference in London, he described WPP's trading desk Xaxis as an “agnostic” proposition and said when it moved into the space it gave existing clients an opt-in choice rather than opt-out. "We decided to do this on an opt-in basis. We ripped up 2,500 media contracts. We explained to clients what was happening and told them they could opt in."