How is the completion rate calculated?

Completion Rate: The completion rate is calculated by taking the total number of Player Completed Views divided by the total number of Player Starts. This is then multiplied by 100 to be displayed as a percentage. 

Player Starts vs Impressions:  An impression is counted as soon as the ad is loaded on the page and the player starts when the video is actually played. A player could start/play the video multiple times and it still counts as one impression.