There is an equation in this standard for a primary outdoor air fraction: Zp=Voz/Vpz (eqn 6-5) which is pretty much a percentage of outside air where Voz is the outside airflow and Vpz is the primary airflow. But then it shows in table 6-3 in the same standard that the higher the Zp value, the lower the ventilation efficiency (Ev). I would guess that the higher Zp (percentage outside air), the higher efficiency it should be. This is a big mystery for me. Am I seeing things or interpreting it wrong? Anybody who has experience with this standard and can help would be greatly appreciated.