Originally Posted by
Astator
It is actually based on 3 metrics:
Enemy team MMR
Personal CR
Personal hidden MMR
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Your personal hidden MMR will be impacted by the game outcome (win or loss) and the enemy team MMR. (Enemy CR doesn't matter.)
The magnitude of the impact on your personal hidden MMR is based on the "likeliness" of such game outcome. The impact being much greater if the less likely outcome happened. The "likeliness" of game outcome is determined based on the difference between: your personal CR and the enemy team MMR. (Note that "likeliness" does not exactly correspond to the probability (percentage chance) of your team winning the game since it focuses on your personal CR and not your team MMR.)