A lesson in probability using World of Warcraft
Cal Henderson explains the maths behind his WoW addon, which calculates the “drop chances” of game items. The maths is not particularly complicated, but it’s a novel way of explaining some probability concepts. He uses the 0.01% drop rate of the game’s rarest pet, the Hyacinth Macaw to deliver some of his examples. Incidentally, the Macaw goes for an average price of about 7,000 gold pieces in the auction house, which is about US$50. Apparently, it’s been bought for as much as 75,000 gp (US$500). A real Hyacinth Macaw costs somewhere in the region of $10,000.
The thing which governs this is called Probabilistic Independence – the fact that whether one mob dropped the loot or not, this has no bearing on whether a second mob will drop the loot. By extension, having looted 1000 consecutive mobs which did not drop the loot has no effect on the next mob you loot. If the drop chance is 1 in 100, there will still be a 1 in 100 chance that the next mob you loot will drop the item.
But if you use BunnyHunter and loot 1000 mobs that drop the Azure Whelp [with a 1 in 1000 drop rate], it wont say 100%; it’ll say 63.2%. The reason we can come up with any number at all, is because we can derive the probability that a piece of loot will drop at least once in a given sequence of lootings.
From Waxy.
2:51pm 
