Rating explained
Kills/Deaths (K/D) ratio is usually recognized by people as an indicator of how good a player or a team performance was, and it is a solid indicator in around 75% of situations. However, it is faulty and unfair in other situations, so with that in mind, we wanted a more solid indicator, so we introduced the Rating.
How is rating calculated?
Rating is calculated by comparing a player's average stats...
  • kills per round
  • survived rounds per round
  • a value based on number of rounds with multiple kills
...to the same averages for Counter Strike in general (which we calculated over a period of time).

Average number of kills per round is 0.679, so if a player had 22 kills in 18 rounds, or 1.22 kills per round, he would have a Kill-Rating of 1.80 (1.22/0.679), meaning he did 80% better than expected on average in the kills department.

In the same way a Survival-Rating is calculated, as well as a RoundsWithMultipleKills-Rating for which you can see the formula below. These three values are then added together, with Survival-Rating participating with a 0.7 factor (as surviving a round is less important that getting a kill) and then divided by 2.7 to give a 1.0 value for an average performance.

Note: Survived rounds are used instead of deaths, as deaths are reversely proportionate to other stats (having less is better, and more is worse).
What's wrong with K/D ratio?
K/D ratio has several downsides, mainly, it favors players who play fewer rounds and therefore have fewer deaths, giving them high K/D ratios. That is why it isn't really useful for comparing performances from different matches.

For example:

Player 1 had 13-6 (K/D = 2.2) in a match of 18 rounds and Player 2 had 37-17 (K/D = 2.2) in a match of 30 rounds.

K/D ratio would suggest they had a similar performance, while Player 1 actually had an average game with 13 kills in 18 rounds, with 5 rounds with 1 kill (1K rounds) and 4 rounds with 2 kills (2K rounds), and Player 2 had a great game with 37 kills in 30 rounds of a close match with nine 2K rounds, four 3K rounds a one 4K round. If we apply the Rating on the two performances, we get that Player 1 had 1.28, while Player2 had a much higher 1.91.

Another problem can be the fact that K/D ratio doesn't have an actual limit, since it can go from 0 to infinity. That makes it hard to compare performances from a single match as well.

For example:

Player 1 has 22-4 (K/D = 5.5) in 18 rounds, and in the same match Player 2 has 20-9 (K/D = 2.2). K/D ratio would suggest that Player 1 had a much better game than Player 2, while that wasn't actually the case, as he had only 2 kills more, and 5 deaths less. Rating shows Player 1 had 1.87 and Player 2 had 1.56, suggesting Player 1 had around 20% better performance.
Advantages of Rating
Rating disregards the amount of rounds played in a match, as it considers average values, so it is very useful for comparing performances. It has a limited range, as it can go from 0 to 3 (although rarely over 2, which is then an amazing performance). It also has a well spread range of values that should reflect properly on how many average, good and great performances there are.

So here is how Rating is calculated in detail:

(KillRating + 0.7*SurvivalRating + RoundsWithMultipleKillsRating)/2.7

KillRating = Kills/Rounds/AverageKPR
SurvivalRating = (Rounds-Deaths)/Rounds/AverageSPR
RoundsWithMultipleKillsRating = (1K + 4*2K + 9*3K + 16*4K + 25*5K)/Rounds/AverageRMK

AverageKPR = 0.679 (average kills per round)
AverageSPR = 0.317 (average survived rounds per round)
AverageRMK = 1.277 (average value calculated from rounds with multiple kills: (1K + 4*2K + 9*3K + 16*4K + 25*5K)/Rounds)

1K = Number of rounds with 1 kill
2K = Number of rounds with 2 kill
3K = Number of rounds with 3 kill
4K = Number of rounds with 4 kill
5K = Number of rounds with 5 kill