I wish your comment was at the highest level. Everyone seems to be attempting to make the same point in the other subthreads. The example here makes it crystal clear.
http://www.adweek.com/news/technology/facebook-gave-ad-agenc...
> UPDATE: Facebook authored a blog post today to futher explain the metric-based miscue. David Fischer, vp of advertising and global operations, wrote: "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. The metric should have reflected the total time spent watching a video divided by the total number of people who played the video. But it didn't – it reflected the total time spent watching a video divided by only the number of 'views' of a video (that is, when the video was watched for three or more seconds). And so the miscalculation overstated this metric. While this is only one of the many metrics marketers look at, we take any mistake seriously."