WASHINGTON — Facebook has apologized to advertisers after disclosing that for two years they have been overestimating the amount of time people spend watching video ads.

Facebook posted on its Advertiser Help Center several weeks ago that its metric for figuring out that average time only factored in video views of more than three seconds, The Wall Street Journal reports. Ads that were clicked through, or off, sooner than that weren’t counted, leading to an inflation of the average time by 60 to 80 percent.

“While this is only one of the many metrics marketers look at, we take any mistake seriously,” said David Fischer, Facebook’s vice president of business and marketing partnerships, in a post.

Facebook said in an earlier post that the mistake “did not impact billing” and has told advertising partners about the miscalculation and said that it has renamed the metric from Average Duration of Video Viewed to Average Watch Time.

The Journal says that while that may be true, the incorrect data may have affected decisions about buying ads on Facebook as opposed to other video services such as YouTube or Twitter, or what kind of content to post.

The snafu could also feed concerns about Facebook’s strict limits on third-party monitoring of its performance. Right now, assessments of that performance come from Facebook itself, which Keith Weed, chief marketing officer of Unilever, told The Journal last year was equivalent to “letting them mark their own homework.”

The ad-buying agency Publicis, which spends about $77 billion a year on behalf of clients for online ads, told clients in a note that The Journal acquired that “In an effort to distance themselves from the incorrect metrics, Facebook is deprecating [the old metrics] and introducing ‘new’ metrics in September. …

“Two years of reporting inflated performance numbers in unacceptable.”