Look out Nielsen: Audience measurement power comScore just said that it has agreed to acquire Rentrak in a stock swap.

ComScore shares are up more than 7% in post-market trading, and Rentrak is up 12.9% following the announcement. Rentrak, which had a market value of $702 million before the news, will become a wholly owned subsidiary of comScore. Each Rentrak share will be worth 1.15 shares of comScore. When the deal closes, Rentrak shareholders will own about 33.5% of comScore.

ComScore’s CEO Serge Matta will run the combined company, and Magid Abraham will be Executive Chairman. Rentrak chief Bill Livek will become President.

The companies say the union will enable them to “introduce a more comprehensive and precise set of solutions for measuring media consumption and advertising across platforms, setting the standard for the next generation of cross-platform measurement solutions.”

ComScore specializes in measuring digital audiences. Rentrak is strong in census-based worldwide movie and video-on-demand measurement, as well as TV. But its stock — which appreciated nearly 91% in 2014 — has fallen more than 40% so far in 2015 as its financial performance failed to live up to Wall Street expectations.

Says Matta, “Together we have an even more powerful ability to deliver what our clients and the media industry have long been asking for: a comprehensive cross-platform measurement currency that accounts for all the ways in which content is consumed, whether that happens on a desktop, mobile device, live or time-shifted TV, video on demand or through over-the-top devices.”

He’s especially enthusiastic about the companies’ blended ability to count viewers who watch TV on digital devices. Many network owners have said that they don’t get credit, or enough ad dollars, for digital viewers because they aren’t in the Nielsen ratings. The measurement giant says that the networks’ own rules governing ratings bar it from adding digital viewers.

In regards to the new deal, Nielsen says that it “has the only Total Audience measurement, comparable across all screens. All of our data is fully representative of the U.S. population, and we deliver truly independent measurement. There are myriad analytics options for the media industry, but Nielsen’s focus is on delivering the actual currency ratings data used for trading billions of dollars in advertising. This requires superior quality, industrial-strength delivery, and gold-standard audited processes and methods.”

(In June Nielsen hired banker Jordan Edmiston to explore a sale of its National Research Group. It’s one of the largest companies that provides film tracking data to movie studios.)

Last year ad giant WPP raised its ownership stake in Rentrak to 16.7%; it also owns about 20% of comScore. The company will own 16% of comScore after the merger with Rentrak, and can buy as much as 20%. WPP chief Martin Sorrell lately has called for the companies to collaborate.

Today’s merger announcement is sure to dominate the chatter at Advertising Week, an array of industry-related gatherings in New York.

“Rentrak’s expertise in precisely measuring TV and movies, and comScore’s industry-leading digital measurement capabilities, are natural complements,” Livek says. “Combined, our expertise and information assets will enable us to provide the industry with the most granular measurement solutions that reflect the ever-changing way that people are consuming content across platforms. ”

ComScore expects the merger to take place early next year following a federal antitrust review. It enlisted J.P. Morgan Securities for financial advice and Wilson Sonsini Goodrich & Rosati to offer legal counsel. Rentrak used Goldman, Sachs & Co. and law firm Perkins Coie.