Chance to hit, crit, and miss

The original posts for this info are long gone, but they were blue text on the WoW forums. Enoy:

Hi all, this is motive, the QA lead tester for WoW and fellow rogue =). I have some information on how Rogue crit is calculated that will hopefully clear up some of this confusion.

The way WoW calculates crit rate is over ALL attacks. Crit rate is not based on hits only. In other words, if you have a 5% crit rate, that 5% chance includes misses.

All crit rate adjusting abilities, items, and talents add the flat % to the base % crit rate. So if I have a 5% base crit rate and then use an item or talent that increases that crit rate (let's use Improved Backstab talent for example - +30% crit), my new crit with backstab is 35%.

Regarding how defense decreases the rate of critical strikes, each point of defense that a target has over the attacker, the attacker loses 0.04% chance to crit. So, for example, if a level 60 Rogue is attacking a level 60 Warrior who has 25 defense, the rogue's crit rate will be decreased by 1%.

I hope that helps!

And a follow-up to that:

--------------------------------------------------------------------------------
Q u o t e:
Wait, motive can you verify some theoretical numbers for me? (assuming ideal distributions)
1) 1000 swings with a 50% chance to hit and a 50% chance to crit (against an equal level unarmored/no defense enemy) would result in
a) 500 crits, 500 misses (chance of regular hit = to hit - crit)
or
b) 500 crits, 250 hits, 250 misses (chance of regular hit calculated independently after crit roll)
or
c) 250 crits, 250 hits, 500 misses (chance of crit calculated independently after to-hit roll)

Sounds like you use option A as your calc, which is cool although not particularly intuitive.
--------------------------------------------------------------------------------

Option a) is correct. But keep in mind that 50% hit AND 50% crit is not a possibility since in your example the % chance to miss stays the same. The way this works is that when your chance to crit increases, your chance to hit is consumed by that increase. So if I initially have a 25% chance to miss and a 50% chance to crit, then I have a 25% chance to hit. If I increase my crit chance to 55%, then my chance to hit (not crit) becomes 20%.

--------------------------------------------------------------------------------
Q u o t e:
As for attack skill vs defense skill, am I correct to assume that the formula is crit adjustment = (ATTACK SKILL - DEFENSE SKILL) * 0.04?

So if rogue with 325 dagger skill and 50% crit attacks an enemy with 300 defense, the rogue's effective crit is 51%, and attacking a warrior with 425 defense the effective crit is 46%?
--------------------------------------------------------------------------------

Yes, this is correct, applying the formula to a rogue with 325 dagger skill vs. a target with 300 defense increases the rogue�s crit chance to 51%.

(325 - 300)*0.04 = +1%

Your second assumption is also correct, applying the formula to attacking a warrior that has 425 defense yields a -5% to crit rate, dropping this rogue�s crit chance vs. the warrior to 46%.

(325 - 425)*0.04 = -4%

--------------------------------------------------------------------------------
Q u o t e:
As for the original poster, Blizz you might want to reverify the (ATTACK SKILL - DEFENSE SKILL)*0.04 formula, as if that suddenly become (0 - DEFENSE SKILL) * 0.04 while tweaking defense, it would explain the ~12% drop in crit percentage.
--------------------------------------------------------------------------------

I can't make any assumptions about the original poster's data because the weapon skill and target defense ratings were not posted. As many people have stated above also though, probability distribution is not always perfect and there is still a chance that once can be "unlucky". It's not impossible to toss tails 50 times in a row, it's not impossible to only crit 40% of the time when the chance to do so is 50%.

Some of the language in the above article can be confusing. When they say "chance to hit" remember that what they actually mean is "chance to get a normal hit" (as opposed to your chance to crit.)