With a time component, the actual formula is actually (this is not including the drop change with your level => http://rode-r.doddle...om/viewpage/d/3 ) :
1-((1-probability)^occurrence) = chances of having the item
For example, if you kill approximately 10 andre/minute, and the drop chance of the card is 0,01% (0,0001)
1-((1-0,0001)^10)) = 0.000999550119979002519790012 chances of having the item after 1 minute-> 0,1%
Now after an hour : you killed about 600 andres :
1- ((1-0,0001)^600) = 0.058238291893480576241755707966 chances of having the card after 60 minutes -> 5,82%
More explanation with a poker example :
You have 9 and J of hearts. On the flop there is a K of spade, a 5 and a 7 of heart.
If you want to chase the flush and don't count the cards in the hand of the other player, you have 1/4 of obtaining it on the turn and 1/4 of obtaining it on the river.
NOW, the total chance is calculated by the formula...
1-((1-probability)^occurrence) = 1-((1-0,25)^2) = 0,4375 = 43,75% of obtaining your flush.
NOTE THAT THIS IS IF YOU DON'T COUNT THE CARDS OF THE OTHER PLAYERS!!!
Hope it helps somehow!
Edited by Drak231, 02 November 2010 - 06:32 PM.