ComScore Inc., the Internet researcher, agreed to buy Rentrak Corp. in an all-stock merger valued at more than $800 million, combining two media-monitoring companies to create a more formidable rival to Nielsen.

Investors will receive 1.15 shares of ComScore for each Rentrak share they own, according to a statement Tuesday from the companies. ComScore gained in extended trading after the announcement, rising as high as $45, suggesting the deal could be worth as much as $51.75 a share to stockholders of Rentrak, which also advanced.

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