This has been one of the big sticking points for TV Everywhere: Advertisers and programmers say they still can’t tell who’s watching when a show is streamed to online audiences. That could result in lots of lost ad revenues. It’s the opposite of what you might expect. Internet users give up gobs of information about themselves every time they click a keyboard or mouse, while ad rates for conventional TV depend on imperfect surveys. But Internet server measurement “systematically overstates audience because it cannot distinguish one person using multiple browsers, account for cookie deletion, or distinguish content served to non-human audiences (i.e. crawlers, bots),” Bernstein Research’s Todd Juenger says this morning in a report. He provides the clearest explanation I’ve seen so far of what advertisers do and don’t know about viewers from different platforms. Here (with his permission) is how he explains what an advertiser on Glee might learn about the show’s multiple audiences:

Conventional TV: A 30-second ad displayed to a viewer watching Glee on a traditional TV set is included in Nielsen’s C3 estimate for the applicable demographic.

Hulu: That same 30-second ad displayed to a viewer watching Glee on Hulu is reported by Hulu to the advertiser as an impression served, with no specific demographic information (Hulu will provide an estimate of the overall composition of site users).

YouTube: That same 30-second ad displayed to a viewer watching Glee on YouTube is likely served to the viewer’s browser based on specific behavioral characteristics bought by the advertiser, and reported by Google to the advertiser as part of that delivery.

Cable VOD: That same 30-second ad displayed to a viewer watching Glee on cable VOD may or may not be included in Nielsen’s C3 estimate (depending on the configuration of ads in the VOD version, and how long after original air-date the view takes place, among other things).

Mobile devices in home: That same 30-second ad displayed to a viewer watching Glee streaming to her iPad (in her home) isn’t captured at all.

Mobile devices away from home: That same 30-second ad displayed to a viewer watching Glee on a mobile device (outside her home) is reported by whatever service is delivering the video clip, with no specific demographic.