Generally speaking, Damage Contribution is computed by dividing a player's damage dealt to champions to the sum of his team's damage dealt to champions. While this can be useful in evaluating the playstyle of teams and of players, a problematic issue with this measure is that it does not take into account of the champions being played. Simply put, a pokey and/or AOE heavy champion will do far more damage than a single target, bursty champion (aka assassin), skewing the numbers depending on the champion played.
To quickly illustrate this effect, I retrieved 150k Platinum+ NA solo queue games from Patch 5.12 and computed the Damage Contribution of 20 most popular mid laners and Kog'Maw for won games.
|Champion||Average Damage Contribution|
Below is a graphical representation of the comparison of Damage Contribution for Zed, Viktor, and Kog'Maw.
Therefore, we see that on average, mid lane Ezreal is about 8% higher Damage Contribution than Fizz on average, seemingly due to the fact that they are different champions and they deal damage differently: Ezreal is a poke champion, Fizz is an all-in assassin. While this is data from solo queue and not competitive, it's not hard to see that the similar effect can be observed for competitive (but we can never verify the effect due extremely small number of competitive games per patch). With this in mind, it is interesting to ponder if it is meaningful to compare Damage Contribution for different players who may have been playing different champions.