Passive defense activation explanation

The concept isn't hard, but you need to know a few key concepts and how to calculate odds and probability. Here's the equation in words anyway: Just going to use melee PD, but any of the 3 would do. I'll use the numbers from the original page: 20%, 8% and 5%

Probability of pinging while wearing 3 equips with PD= 1 - [ (1 - probability of trans melee PD working) x (1 - probability of armor PD working) x (1 - probability of shield PD working) ]

The (1-probability of ____ PD working) is the probability that the PD of ____ won't activate (trans, armor or shield in this case). You use that equation for every source of melee PD you have (you've got 3 in this case - trans, armor and shield). When you multiply those 3 values together, you get the probability that none of the PDs on the 3 items will activate. The final 1 - [things in these square brackets] will give you the probability of at least 1 of the PDs of those 3 items activating when you get hit.