Nielsen is reissuing total-day ratings from the beginning of the current TV season after it admitted today that it has misallocated some viewership results — possibly to the benefit of ABC, although it would not confirm that. “It’s a small error within statistical tolerance,” says Patricia McDonough, Nielsen SVP Planning Policy and Analysis. As much as 98% of programs won’t be affected by more than .05 of a ratings point. It’s not clear whether it might result in make-goods. Those are mostly based on the so-called C3 numbers — the amount of live and delayed viewing over three days. The first C3 numbers for the new season go out on Monday, and Nielsen says that they will not reflect the software error.

The problem was discovered October 6, and the software was updated last night, Nielsen Global President Steve Hasker says. The Media Rating Council will review what happened and issue an analysis.

hand-with-magnifying-glass_abcThe possibility of a problem surfaced at the beginning of the fall TV season as the number of viewers increased. It was particularly noticeable at ABC where — as my colleague Nellie Andreeva reported last week — the network’s fast nationals in adults 18-49 and total viewers were adjusted up every night for the first nine days. On eight of the nine nights, the entire ABC lineup went up in the finals. That meant 20 programs saw a lift without a single downward adjustment in 18-49. Over the same period, the other major broadcast networks saw 15 adjustments combined — nine up a tenth and six down a tenth. Nielsen declined to say whether ABC might have benefited from its glitch, saying that the network was entitled to review the numbers before they’re released on Monday.

Hasker says that, although the errors were small, Nielsen wanted to address it from a belief in “absolute transparency on ratings.”

The problem surfaced as Nielsen faces intensifying competition from Rentrak, following a major, multifaceted deal yesterday with ad giant WPP. In a call to discuss the ratings changes, Hasker blasted Rentrak for suggesting that its audience data are more accurate and complete. “Our friends at Rentrak never let the facts get into the way of a press release,” he says. The company has “never been transparent about [its] methodology” which is “not based on census data.” He added that Rentrak can only measure results from homes that its partners reach, which comes to about 40% of all U.S. households.

The arrangement with WPP is “a traditional set top box data deal with hyperbole thrown into the mix….It’s not currency data and doesn’t act as such.”