The effectiveness of the player is how much he decreases the party effectiveness with only him having 1 less mod….
No it is not. That is the party effectiveness, not the individual players effectiveness.
Is a paralyzed player still effective because the party wins 99% of the fights without him? I mean the party effectiveness didn't change, so I guess paralyzation does not change the player effectiveness.
If your position is that having a low stat player does not have a big impact on the party effectiveness I agree with you, but I do not agree that it is not a significant impact on the PC themself.
Changing the rest of the party nullifies that test. You are no longer testing player effectiveness but party effectiveness. That’s what you are doing with your tests.
The rest of the players are not a constant, they are a variable. If they are not part of a test or part of a control but they are influencing the outcome in a non-stochastic fashion then they are a random variable. In any test you want to isolate random variables to the maximum extent possible.
By changing all the players I am evaluating a party of low stat players against a party of non-low stat players. This makes all the players in the party as part of the test group and part of the control group respectively. If you insist on using multiple players and using overall party effectiveness as your MOP that is the only valid way to do it. Otherwise your results are trivial and driven primarily by on the number of additional players (i.e. random variables), not the effectiveness of the player you are trying to evaluate.
You can evaluate a single player in a group of other non-similar players but you need different MOPs, you would need to compare individual party members to each other, not the overall party performance of one party vs another.
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