A little experiment: on a limited sample of random videos, it seems like around 1 of 30 people watching will use the like/dislike button. Seeing that a video with 3.6 mil views that has only 9.5k likes would give you a rough estimation of how many people disliked it. Is there a real study on the subject? It seems like it could be used as some sort of heuristic if there is nothing better.