Audience measurement firm Taylor Nelson Sofres has rejected WPP Group’s £1 billion ($1.9 billion) offer as “opportunistic” and undervaluing the company, telegraph.co.uk reported. TNS has been trying to merge with German audience measurement company GfK in a deal valued at €100 million ($154 million). The tie-up would create the globe’s second-largest audience measurement firm behind Nielsen.

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WPP’s offer to purchase TNS contained a mix of cash and WPP shares at roughly 230 pence per share. Under the terms of the bid WPP would have paid 154 pence per share in cash and offered 0.1214 WPP shares for each TNS share. Last month, TNS bought Boston-based web traffic analytics provider Compete for $150 million.