Individual Performance in Rating
Rating change is first and foremost decided by win/loss (and by how many rounds). You will always lose rating on a loss and win rating on a win. That is the way the ranked system was created and how most online competitive ranked ladders are formulated.
Individual performance is only a large factor in your rating changes at the start of your ranked “career” (approximately the first 15 games). Afterwards, rating is almost exclusively decided by win/loss and your team score round difference (the effect of performance is higher in low ELO and cuts off around Diamond). This is set up to be able to quickly allow players to find their individually performing level at first, then prove their rank based on the much more consistent metric of win/loss ratio.
This is also set up to avoid some (arguably) large issues with KDA being a poor indicator of total "ranking".
Some quick examples:
1) Some agents (Phoenix, Reyna) statistically have higher KDA's on average than others (Sage, Cypher). This is independent of rank and individual player quality.
2) 'KDA padding' becomes an issue when it is significant part of the reward function for rank increase. This includes...
a) baiting people on entry to preserve your death toll
b) playing site exit on a bomb explosion (exit frags) without trying to push site
c) playing only flank all game to receive a statistically safer KDA (generally less deaths, more consistent single kills per round)
Here is a comment by Riot EvrMoar, a ranked developer:
“Performance is less about your performance compared to your teammates and more of a performance based on what we expect from you (The amount of points you get for this is very small as well, and more of a “good job” call out). So if you pop-off in a match where the system expected you to go even, you will get slightly more ranked rating.
You have two MMR's, one for skill and one based on wins. In lower ranks skill MMR matters more, in determining your true hidden MMR. As you climb winning becomes more important.
When you get into a match, we know every players hidden MMR and used that to put you in that match. The match maker has a level of confidence in how it thinks you will perform. If you are one of the higher MMR players in the game, you probably should perform better then others in that match. But if you have the lowest MMR, you probably should be lower in performance.
If you exceed the expectation we reward you a few extra ranked rating points. Eventually I'd like to tell you when you had a stellar performance.
Also the system looks at a ton, more so then I think players realize. Things like healing, ability usage, 1v1's, being an entry fragger, setting up teammates, etc. So it's not only about topping the leaderboard, it's about using your agent effectively to help your team win.”
And finally, the followup changes in patch 2.02 on this topic:
“Players Iron through Diamond who perform exceptionally well in a match (weighed against their own average performance) will gain bonus Rank Rating
We want to reward players that “pop-off” in a match, performing above expectations.
This will help highlight those matches where you “outclassed” your average performance. In turn, you will rank up just a little faster, and be rewarded for games where you kill it. Remember, this is you doing better than your average, not you doing better than your teammates or opponents.
Consider this another system that helps combat the smurfs who do real well in a game. They will now more quickly rank up so that their rank better reflects their skill (along with getting put into higher skilled matches).”