Youtube : Users will give rating to videos they like very much or hate very much

Youtube 在官方blog上公布了一个图，显示1到5分的用户评分分布，这个图显示，绝大多数的用户评分都是5分，第二多的评分是1分，而其他2，3，4分非常少。这说明，用户只有在极端喜欢一个视频或者极端不喜欢一个视频的时候才会使用评分系统。

这个例子让我想起了推荐系统，youtube所说的这个现象似乎在imdb和netflix中都没有发现。我觉得这是因为，在imdb中，用户并不能在网站上看到视频，他们都是在看完某个电影后才登陆imdb，所以一个用户如果登陆imdb，评分可能就是他的一个任务。

但是在youtube上却不是这样，用户上youtube的第一任务不是评分，而是看视频，所以只要视频能够满足他们的要求，他们就会不停的看下去，不会想到要评分，只有当某个视频刺激到他，而他觉得有必要表达一下自己的意愿的时候，才会评分。

Youtube publish an article in its blog and said that most of users in youtube tend to give 5 starts and 1 star to videos they watch. They think, this is because most of users only rate videos they like very much or they hate very much.

This result is different from ratings in IMDB and Netflix. I think, the first task users want to do in imdb is to rate a movie, because they can not watch movies in IMDB. However, in youtube, the most important task is watching videos. Therefore, if all videos are OK, users will not stop watching and rate videos. They will only rate a video if the video is so special and they think they must express their views.