“About a month ago, we found an error in the way we calculate one of the video metrics on our dashboard — average duration of video viewed,” wrote David Fischer, a vice president of video and marketing partnerships for Facebook, in a post on Friday. “While this is only one of the many metrics marketers look at, we take any mistake seriously,” he wrote. “Our clients’ trust and belief in our metrics is essential to us, and we have to earn that trust.”

Facebook acknowledged in an update to its advertiser support page last month that it had been excluding videos watched for fewer than three seconds from the average viewing times. That greatly inflated the figures presented to partners. (Facebook publicly counts video views after three seconds. By comparison, YouTube counts a view after 30 seconds; Vine, where videos are limited to six seconds, counts after a full video loop.)

Advertisers, however, buy Facebook video according to different metrics: either by “impressions,” which are measured as soon as a user sees a video in the news feed, or in 10-second views.

“Average duration of video viewed” is neither a public metric nor does it determine how much advertisers pay for video placement. But the figure is displayed prominently in partners’ dashboards, and it has helped assuage discomfort about the platform’s generous view-counting methods.

While Facebook’s update last month largely passed under the radar, Mr. Fischer’s post on Friday was prominent and contrite. “We sincerely apologize for the issues this has created for our clients,” he wrote.