According to a piece aired this morning on NPR, (listen or read here,) Nielsen tracks TV viewership in many many ways, but has difficulty tracking other ways people watch their favorite shows:

Nielsen tracks online viewers, but it does not aggregate that data with its regular TV ratings. Some Internet video sites, like Hulu, charge that Nielsen is vastly undercounting their viewers, as well.

Kate Sirkin, who directs global research at the media planning and buying firm Starcom MediaVest Group, says advertisers are starting to demand more comprehensive tracking of viewers across platforms.

“I’m not sure if that metric is the only metric we need going forward,” she says of traditional Nielsen television ratings. “We would love cross-media measurements.”

Sirkin says she would like measurements that aggregate regular TV