Hey,
With the old method of a strait average, it was possible, for example, for a group of DBZ fans to collaborate and put all 10's on a mediocre video to make it to the top of the list instantly. Should the video be on the #1 spot? No, not really. The new system helps take care of that. I'll try to explain how this works.
downwithpants has posted a mathematical
technical definition of what is going on with the calculations, but I'll try to explain further. Here's the formulas:
old score = avg(compscore + overall + review)
new score = (votes ÷ (votes + min_votes)) × old_score + (min_votes ÷ (votes + min_votes)) × avg(all videos)
The new score formula has two parts which are added together:
(votes ÷ (votes + min_votes)) × old_score
and
(min_votes ÷ (votes + min_votes)) × avg(all videos).
Each part is a ratio multiplied by either the old_score or the average of all videos. With few opinions (ex. 1 opinion), the overall average has the greatest weight:
(1/(1+7))*old_score = 1/8 * old_score
while
(7/1+7)*overall_avg = 7/8 * overall_avg
If you'll notice that since we are adding the two parts together, 1/8 + 7/8 = 1, which means that we are getting a whole score.
As far as the average goes, this means that, mathematically, any new video is assumed be be an average video until proven otherwise. As the video gets more opinions (ex. 50 opinions), the average of the video has more weight than the overall average:
(50/(50+7))*old_score = 50/57 * old_score
while
(7/50+7)*overall_avg = 7/57 * overall_avg
By this time, the actual average of the video is dominating the value of the new score. If your average is above average, your video will march up the rankings. If your average is below average, your video will march down the rankings.
If we go back to the buttmunch DBZ fanboy example who are trying to artificially push a video to the top of the list, here's what happens at 7 opinions of all 10s:
current overall average = 7.406224807350
(7/(7+7))*10 + (7/(7+7))*7.406224807350 = 5.00000 + 3.70311 = 8.70311 = 8.70
So instead of the video taking the #1 spot, the video takes a much lower position. Say the video reaches 30 opinions and the average is a little more sane at 8:
(30/(30+7))*8 + (7/(30+7))*7.406224807350 = 6.48649 + 1.401178 = 7.887668 = 7.89
At 50 opinions and the average still being 8, the new score looks like this:
(50/(50+7))*8 + (7/(50+7))*7.406224807350 = 7.01754 + 0.909534 = 7.927074 = 7.93
This method has now prevented an artifical score inflation by a bunch of fanboys, but then yields a score closer to the actual average as the number of opinions grows. The more opinions a video has, the more valid the score becomes.
I hope this explanation helps.
Phade.