Activist investor Elliott Management has taken a large stake in Nielsen Holdings as it plans to push the TV-ratings company to sell itself, The Wall Street Journal reports, citing sources. Elliott has accumulated an 8% stake, which could be worth at least $640M, in Nielsen, according to people familiar with the matter, who add that Elliott could unveil a new campaign as soon as Monday. “Multiple” private equity firms have shown interest in Nielsen, the sources say. Elliott believes that Nielsen’s “buy” segment, which measures retail and consumer behavior, has failed to keep up with competitors like market research company IRI.