If you’ve been following the television ratings this season, you may have noticed that almost all of the ABC shows have been adjusted upward — from the fast affiliate numbers (which are released in the morning) to the final nationals (which are released in the afternoon). No other network was seeing the same number of upward adjustments. It’s been very strange to watch but now, we may have a reason.

Nielsen Research, the company that puts together the TV show viewing data, says that they’ve found a system glitch that has been affecting the ratings for a small number of primetime network shows (not cable) since last March. The glitch wasn’t noticed until now because of viewing in the summer months can often be quite low.

Nielsen says they’ll go back and recalculate all of the primetime network ratings since the start of the 2014-15 season. They plan to release those numbers by late next week. The individual ratings pages on this site will be updated with the new data as it becomes available. It seems likely that ABC shows will see the most adjustments.

Though Nielsen didn’t address the long run of ABC upward adjustments, today’s final ratings didn’t change the ABC fast affiliate numbers at all — a first for this season.

Here’s their press release:

NIELSEN STATEMENT REGARDING NETWORK RATINGS ATTRIBUTION

In response to recent ratings irregularities, Nielsen conducted an extensive internal investigation of our systems and processes. On Oct. 6, 2014, we uncovered a technical error that impacts national network television ratings over several months.

The technical error was introduced on March 2, 2014, and was generally imperceptible until we saw high viewing levels associated with fall season premiere week. As a result, small amounts of viewing for some national broadcast networks and syndicators were misattributed. Cable networks and local TV ratings were not affected by this error.

A software fix to correct the problem was deployed on Oct. 9, 2014, meaning that all data being released today and going forward is correct.

In addition,

– All of the commercial data—including C3—for the current TV season, which will begin releasing this weekend, will be correct.

– All previously released data since September 22nd will be reprocessed and reissued by Oct. 17, 2014.

– We will also reprocess all of the impacted data going back to Aug. 18, 2014, when the first new season broadcast network program aired. This data will be reissued by Oct. 31, 2014.

– Nielsen is also conducting an impact analysis to determine whether additional weeks should be reprocessed. We will work closely with our clients and the industry to provide updates as soon as possible.

– This issue has to do with difficult-to-attribute content called “all other tuning with code” (AOT with code). This data represents between 0.1% and 0.25% of all viewing minutes that we credit nationally. In the vast majority of cases, the impact is small; in a handful of cases, the impact is more significant.

As part of our investigation, we have also determined that there are no issues with the National People Meter, our data collection process, our panel, our TV audience measurement methodology or the total TV viewership data produced during this affected period.

We are working closely with our clients to manage this situation and will continue to be transparent with the industry and the media about our plans. In addition, we will undertake an exhaustive post-mortem—internally and with our clients—and we are asking Ernst & Young and the MRC to join us in these efforts.

Nielsen is committed to upholding the highest standards of television audience measurement and data processing, in order to provide the most effective audience measurement solutions to meet client needs.

What do you think? Which shows do you think have benefited from this glitch? The ABC shows? Others as well?