Arbitron shares are up 25% in pre-market trading after Nielsen Holdings said it would buy the rival ratings company best known for its measurement of radio audiences. Nielsen said it will pay $48 a share, a 26% premium over yesterday’s closing price, for a total of $1.26B. ”Arbitron will help Nielsen better solve for unmeasured areas of media consumption, including streaming audio and out-of-home,” Nielsen CEO David Calhoun says.