Private equity firm Hellman & Friedman has agreed to buy a majority stake in Information Resources Inc., a market-analytics business backed by Vestar Capital Partners and New Mountain Capital, with plans to merge it with portfolio company NPD Group.

The deal values the combined entity at more than $8 billion, including debt, according to a person familiar with the matter, who asked to not be identified because the terms are private. Bloomberg News reported in March that NPD was in talks to buy IRI for more than $5 billion, including debt. Vestar and New Mountain will remain investors, according to a statement Thursday.

Hellman & Friedman agreed to acquire NPD last year. NPD is the eighth-largest research company in the world, covering more than 20 industries, according to a statement at the time of the deal.

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