The ratings company made this eagerly awaited announcement as ad executives converge on New York for Advertising Week, a dizzying collection of meetings and seminars for the industry. Nielsen’s new service is sure to create some buzz. Many TV programmers have been reluctant to put their shows online because they couldn’t demonstrate to advertisers how many people were viewing on mobile devices including tablets and smartphones. The ratings company says it now can offer that information through its new Nielsen Cross-Platform Campaign Ratings, available beginning today. Tests from March through August — with companies including ESPN, Facebook, GroupM, Hulu and Unilever — showed that Nielsen could offer “comparable metrics across TV and digital, measuring unique audience on each, along with overlapping audience and total combined unique audience,” the company says. Nielsen figures that more than half of Americans watch video online. In a separate report out today, Pew Research Center’s Project for Excellence in Journalism says that 12% of tablet owners watch videos daily, and 38% watch at least once a week.