Here are two videos from Tim Schmoyer at ReelSEO. In the first he looks at how viewing habits influence YouTube search rankings. He looks at

1) Watch time (the more watch time accumulated, the higher the search visibility).
2) Audience retention – the average point where a viewer abandons your video.
3) Viewing session – how your video influences other viewings.

If watch time is the most important statistic, in this video Tim shows you how you can increase it.