Bloomberg: ComScore, the Internet researcher, has agreed to buy Rentrak in an all-stock merger valued at more than $800 million, combining two media-monitoring companies to create a more formidable rival to Nielsen. The merger represents a new challenge to Nielsen, which produces the TV audience ratings that underpin advertising rates and does a range of research for media and consumer-products companies. The deal combines two of its competitors — ComScore tracks web activity while Rentrak reports on TV viewing, DVD sales and the movie box office, as well as some online activity.