The error affected a Facebook metric called “average duration of video viewed”, which was supposed to tell publishers for how long, on average, people had watched a video. However, the metric did not include viewers who had watched for less than three seconds in the count. Discounting the shorter views – including people who had ignored a video in their news feed – inflated the average viewing times for each video.
Facebook’s previous “average duration of video viewed” metric was inflated by upwards of 60% to 80% because of the three-second cutoff, according to a letter ad agency Publicis sent to clients that was obtained by the Wall Street Journal.
Ad buying agency Publicis Media, was notified that Facebook had overestimated average time spent watching videos by between 60% and 80%, after Facebook reportedly told its advertisers that it had accidentally been excluding videos that were watched for less than 3 seconds from its "Average duration of video watched" measurement.
Facebook is also introducing a new measurement called "Average watch time" that will include all the videos watched irrespective of the duration it has been watched for, Business Insider reported.