Suzanne Vranica:

Facebook Inc. knew of problems in how it measured viewership of video ads on its platform for more than a year before it disclosed them in 2016, according to a complaint filed Tuesday by advertisers.

A group of small advertisers filed a lawsuit in California federal court in 2016, alleging the tech giant engaged in unfair business conduct by disseminating inaccurate metrics that significantly overestimated the amount of time users were spending watching video ads.

The plaintiffs later added a fraud claim, and in Tuesday’s court filing they alleged Facebook knew of irregularities in its video metrics by January 2015 and understood the nature of the miscalculation within a few months, but failed to disclose the information for over a year.

The filing followed the plaintiffs’ review of some 80,000 pages of internal Facebook records that they obtained as part of court proceedings.

The complaint, which cites the internal Facebook documents, also alleges that the scale of the miscalculation was far worse than understood.