According to a letter sent by Publicis Media to clients in late August and seen by the Wall Street Journal, the agency was told by Facebook that the previous counting method had overestimated the average viewing time by between 60% and 80%, a situation the agency called "unacceptable" in the note.
The Journal added, citing a person close to the issue, that Facebook had also notified GroupM of the discrepancy.
The metric Facebook gave advertisers for their average video view time counted only the people who watched the video for more than three seconds, the length of time Facebook equates to a "view". It didn't count anyone who didn't watch, or watched for a shorter time than three seconds, thereby inflating the average viewing time in its analytics.
Facebook said in a statement: "This error has been fixed, it did not impact billing, and we have notified our partners both through our product dashboards and via sales and publisher outreach. We also renamed the metric to make it clearer what we measure. This metric is one of many our partners use to assess their video campaigns."
As such metrics are a factor in media companies and publishers deciding what content to post and in marketers and media agencies allocating digital budgets, there has been widespread concern and a degree of cynicism about the new "Average Watch Time" which replaces the "Average Duration of Video Viewed" metric.
"Essentially, they're coming up with new names for what they were meant to measure in the first place," the Publicis memo said.
"This once again illuminates the absolute need to have third-party tagging and verification on Facebook's platform," it added. "Two years of reporting inflated performance numbers is unacceptable."
Data sourced from Wall Street Journal; additional content by Warc staff