First, please note that the rating system may be updated at any time and without warning. The folowing is user driven research from August. Please understand the following information could be out of date. It's only meant to help us all understand the rating system better.
There are two important things to understand about the rating system. One is that ratings age (or you can say decay). The older your creature gets, the less each vote counts. There are reasons for this, but that's another topic. Here's the rate of decay as of 12 August 2008:
0 days old (just shared) 100%
14 days old 70%
26 days old 55%
38 days old 44%
46 days old 40%
54 days old 36%
(note: Percentages are rounded to the nearest whole number. The actual percentages go out many decimal places. I'm only showing a few intervals. Each day has decay. No, I don't know the bottom limit.)
The second important thing to understand is that a creation's displayed rating (faces or numerical rating with Sporescope - http://www.gamingsteve.com/blab/index.php?topic=13592.0
) often does not update until someone casts a new vote. This means a creature with a blue face will sometimes stay blue for months if no one votes. Don't think of this as a good thing. The true rating is still aging. It only takes one vote (including a thumb up) to reveal the true score and it may result in a yellow face. More on this later, but first let me cover some basic elements.
At the beginning of a calculation, each vote counts as 1. A thumb up is +1. A thumb down is -1. The score is simply thumbs up minus thumbs down. Whether the decay is factored in before the subtraction or after doesn't matter ( yay algebra!
Here's an example (old one sorry):
Castor and Pollux
User Rating: 7.89
That information is stored by EA. Sporescope reveals it to us. Since I voted a thumbs up today (forcing the true score to show), we can calculate this:
46 days old.
With each vote counting about 40%, that's 20 votes up.
This is not 20 thumbs up, it is 20 more thumbs up than thumbs down.
It could be 21 up and 1 down or it could be 563 up and 542 down.
Sorry, no information on total votes that I have access to.
So how can a thumb up cause a blue face to become a yellow face? Here's a hypothetical example (old one sorry):
July 3rd, 14 days old, 70%
User Rating: 5.6, blue face
8 votes up
No votes for seven weeks.
August 12th, 54 days old, 36%
User Rating: 5.6, blue face until...
...someone thumbs up.
User Rating: 3.24, yellow face
9 votes up
I tested these numbers on creations from many players and they've been 100% accurate thus far, but there can easily be variables in the rating system I am unaware of.
edit: So in conclusion, it's still important to rate creations even if it appears you're hurting their score. You're not. The only way to maintain a good score is to keep receiving thumbs up.