Facebook drastically misled advertisers and media publishers by overestimating average video viewing times across its platform for a total of two years, according to an explosive report in The Wall Street Journal. The company did so by not factoring into its calculations any videos that were viewed for less than three seconds — thereby greatly exaggerating overall averages.

Facebook disclosed the inflation and said it had remedied the problem several weeks ago in its Advertiser Help Center, according to the Journal. However, major ad agencies — including Publicis and GroupM — eventually pushed for more details, whereupon Facebook disclosed that average watch time metrics had been overestimated by a staggering 60% to 80%.

On Friday, Facebook’s VP of business and marketing partnerships, David Fischer, offered an apology: “While this is only one of the many metrics marketers look at, we take any mistake seriously,” he wrote in a Facebook post. “We know we can’t have true partnerships with our clients unless we are upfront and honest with them, including when we make mistakes like this one.” Going forward, average watch time will be calculated using views of any duration.

The Journal notes that the fact that marketers and media companies had grossly inaccurate data about ad performance for two years likely impacted crucial decisions about how they allotted ad dollars across competing platforms like YouTube, Twitter, and television. It is an especially critical blow as Facebook has moved in recent years to tout its influence within and stated emphasis on the increasingly lucrative online video industry.

In a memo to clients obtained by the Journal, Publicis — which spent a reported $77 billion on ads on behalf of its clients in 2015 — said, “This once again illuminates the absolute need to have third party tagging and verification on Facebook’s platform.”

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