There exist many aspects involved in a video turning viral on YouTube. These include properties of the video such as the attractiveness of its title and thumbnail, the recommendation policy of YouTube, marketing and advertising policies and the influence that the video's creator or owner has in social networks. In this work, we study audience retention measures provided by YouTube to video creators which may provide valuable information for improving the videos and for better understanding the viewers' potential interests in them. We then study the question of when is a video too long and can gain from being shortened. We examine consistency between several existing audience retention measures. We end in a proposal for a new audience retention measure and identify its advantages.