The market research firm has been quietly exploring a take-private, sources tell CNBC. : Potential bidders could be Hellman & Friedman with Blackstone, or Goldman Sachs Private Equity with Advent International, the sources say. The market research firm, which had its initial public offering in 2011, has been quietly pursuing a take-private deal and a round of bids was due in January, the sources said. Any deal would follow an $11 billion buyout announced earlier this week for Ultimate Software Group, which agreed to be acquired by Hellman & Friedman. Ultimate is a maker of cloud-based human resources management software.