Multiple Factors
Kills : Career Length (CL)
Deaths : Career Length
Kills as a % of Total Team Kills (TTK)
Deaths as a % of Total Team Deaths
Total Points : CL
Assist Points : CL
Wins
Losses
((K/CL) + (K/TTK) + (P/CL) + (P/TTP) + (AP/CL) + (AP/TTP) + Wins)
-
((D/CL) + (D/TTD) + Losses)
= X
Then plug that number into a separate formula that will calculate their Standard Score, which compares them to the rest of the players (heres where you would only compare meds to meds and offense to offense, etc). Standard Score = Z score which is what they use on SATs etc to determine where you rank.
Z Score (Rating) = (X-Average of Players in class) / Standard Deviation of the population.
https://en.wikipedia.org/wiki/Standard_score