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A general view of atmosphere at the A+E Networks 2013 Upfront on May 8, 2013 in New York City. - 

May is the month of the television Upfronts, where networks show off their new programming to advertisers. But there's a growing issue in the TV world -- so called cord-cutters, the people who don't watch TV on TV. The Nielsen Company has traditionally been the authority on ratings, but not even they know how much TV the cord-cutters are watching online.

Bloomberg's Edmund Lee explains why Nielsen is loath to look at online numbers.

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