Calculation of PD activation

I don't understand the formula...

Well it'd be easier to explain if you've taken a data managment(at least gr. 12) course. The probability of event "A" occurring is: p(A)=n(A)/n(s), where n(A) is the total number of possibilities of event A occurring, and n(s) is the total number of possibilities. Also, the probability of event A occurring + probability of event A not occurring = 1. In the example given on the page, the probabilities are 20% from trans pd, 8% from armor and 5% from shield. This means that the probabilities of each pd not activating are 0.8, 0.92 and 0.95 respectively (1-0.2, 1-0.8 and 1-0.5). Multiply those 3 numbers together and you get 0.6992, which is the probability that none of the events will occur. Then subtract that probability from 1 and you uave the probability of pinging, which is 0.3008 or 30.08%.

I still don't understand. Also, what's p in that equation?

To put it simply, each item is calculated individually. Should a person get hit and one of the items actually activate the passive, it'll activate, regardless if the other passives didn't land in that chance of activation.

I still don't get it...

p is the probability of an event happening.