Nielsen has uncovered a software error that rendered its recent national network television ratings inaccurate, the data company said on Friday.
"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," Nielsen said. "As a result, small amounts of viewing for some national broadcast networks and syndicators were misattributed."
In an unusual development, the firm said in a statement that it will reissue all of the affected data going back to Aug. 18, before the start of the critical fall season premiere week.
It is not known what the effect will be on ratings for networks that just started unveiling their new fall shows.
Nielsen said that in most cases the effect is small, but more significant in a handful of instances.
The company said it fixed the problem Thursday and that all data since the fix is correct.
Nielsen will redo its more recent data first, starting next week. On Oct. 17, Nielsen will reissue the faulty numbers going back to Sept. 22. The rest of the data since Aug. 18 will come out on Oct. 31.
"Nielsen is also conducting an impact analysis to determine whether additional weeks should be reprocessed," the company said in its statement. "We will work closely with our clients and the industry to provide updates as soon as possible."
Ratings for cable networks and local TV are not affected.