Comments on: A drop rate is a probability, not a guarantee

By: Cassie

Cassie — Wed, 18 Feb 2009 23:59:01 +0000

So when you are trying to figure out something and it says that the mob that drops it has a 3% drop rate, how do I calculate that. I am using the desktop calendar not a fancy scientific one. Not good at math. Think you could help me out?

By: Tal

Tal — Fri, 12 Sep 2008 10:36:58 +0000

I know, I’m really late to the party, still thought I’d share my 2c.

The math is mostly correct, except for the 65.13% you’ll get the item - that’s actually the chance you’ll get _at least_ 1 item. You could just as well get 20, if the RNG gods really like you.

Anyway, great blog. How come I never heard of it till now?

By: Ben

Ben — Thu, 22 May 2008 18:27:57 +0000

Just as an aside, I think its generally easier to work with expectations. IMHO, the question you are trying to answer here is “OH GOD, how many times do I have to do this?”. Unless you’ve got some crazy “a beautiful mind shit” going on, you probably can’t do the logs in your head. But, since the trials are iid, and since the expectation of a sum is the sum of the expectations:

E [sum_{i=1}^n x_i] = sum_{i=1}^n E [x_i] = n * E [x]

So, if I want to know how many times I should have to do something (n) on expectation to get some number of an item (k) with drop rate r, its just:

n = k / r.

By: Andrew Ferguson

Andrew Ferguson — Fri, 02 May 2008 17:35:02 +0000

Yeah it doesn’t seem to be doing the irritating pop-up thing anymore, but it was this one;

http://woweconomist.com/recommends/WarcraftRiches

I was using Firefox 2.0 on a Mac, Adblock enabled.

I agree, I’d rather see the gold making guides than gold-selling sites, I just have a reaction to the way their sites are presented. They’re structured identically to the bordering-on-illegal get-rich-quick scheme sites for real moneys.

By: Kevin

Kevin — Thu, 01 May 2008 21:02:04 +0000

We’ve gotta pay the server bill somehow, Andrew. I don’t think they are sketchy at all. I own both of the guides and they are great for new WoW economists. They go over many different gold making strategies, most of which are outside of playing the AH.

Which one had the pop-up? Neither of them had one when I left the page.

Our other option would be to add Google ads, which are usually cluttered with gold selling websites. I figured you guys would rather see us promoting legitimate gold making guides than illegal gold selling sites.

By: Andrew Ferguson

Andrew Ferguson — Thu, 01 May 2008 20:55:21 +0000

WoW Economist recommends some sketchy gold-making guides as advertisers -.-

One of the one ones on the side even had some BS popup ‘live agent’ when I tried to leave the page. Sketch & rude.

By: Eliah

Eliah — Thu, 01 May 2008 17:00:55 +0000

Traex, as the author of the WoW Insider post, I appreciate the link; I don’t feel like it was ripped off at all This is one of my favorite up-and-coming WoW blogs anyway.

Garf, thanks! I knew that 35% figure looked familiar; I must have seen it in other calculations before. 1/e, huh? Who would’ve guessed…

By: Rey

Rey — Thu, 01 May 2008 08:07:23 +0000

Whilst interesting from a math point of view, i was waiting for some insight to apply this to lining my coffers with gold.

I suppose its an education to people farming rare pets BoE items on what they should expect as a likely return.

Still nice to read something on the train to work that makes you think. Makes me wonder if there is room for an add-on that tracks published drop rates versus player loot and present you with a probability calc ingame. Hmmmm

By: Garf

Garf — Wed, 30 Apr 2008 23:16:08 +0000

You missed the “n attempts” vs “10 attempts”. So you’ll have to compare 0.9^10 to 0.999^1000 (which happens to be ~36.7%).

By: Maniac

Maniac — Wed, 30 Apr 2008 15:08:59 +0000

Garf, I don’t see how the percentage is pretty much the same for any item with 1/n? I’ll admit that I don’t really understand your point about convergence but surely,

1 in 10 drop rate, 10 tries = 0.9^10 = 0.3486 = 34.86% chance to miss
1 in 1000 drop rate, 10 tries = 0.999^10 = 0.9900 = 99.00% chance to miss

That seems to make sense to me, what am I missing?