But as usual, I have still few question to better understand your definition of impurity.
pi_imp = PID_{e} > X / pi_all
I can understand about the nominator, that is reconstructed true pion, which is tested MC true PID matching. 

I'm wondering about the PID_{e} > X. 
Is it "selected true pion after doing MC PID match and requiring PID probability" 
or 
"any kind of tracks just passed given probability condition"? 
I am not clear for this which one have to be applied to see whatever impurity or misID.