OOPS: Advertisers Allege Facebook Failed to Disclose Key Metric Error for More Than a Year. “Facebook knew of problems with how it measured viewership of video ads for more than a year before it revealed them in 2016, according to a complaint filed by advertisers.”

Damping marketers’ concerns is important for Facebook as it seeks a bigger piece of U.S. spending on online video ads, which is projected to grow 30% this year to $27.8 billion. Facebook is expected to account for almost 25% of U.S. video ad spending, eMarketer estimates.

For two years, Facebook had counted only video views that lasted more than three seconds when calculating its “average duration of video viewed” metric. Video views of under three seconds weren’t factored in, thereby inflating the average length of a view.

Facebook replaced the metric with “average watch time,” which reflects video views of any duration.

After disclosing the issue in 2016, Facebook said in a statement that it had “recently discovered” the error.

Facebook told some advertisers that it likely overestimated average time spent watching videos by 60% to 80%. The plaintiffs alleged in Tuesday’s complaint that the error was much larger and that the average viewership metrics had been inflated by some 150% to 900%.

Only Facebook knows Facebook’s actual numbers, so who knows how much worse the problem might have been — or might still be.