too many users are concerned with having hard and fast guidelines regarding what percentages are acceptable when there are so many variables involved that such guidelines really don't exist. as an extension on Goudo's example, let's say that movie "A" has a constant bitrate of 9800kbps (the max allowed for a dvd). compressing it to 50% will still have it looking great. on the other hand, let's say movie "B" has an average bitrate of 4000kbps. compressing this movie to 50% will look horrible.