Quote:

**BoogiepopPhantom**:

Man, I'm a bit tired of everyone always saying armor pen without any thought or explanation behind it. Do some math guys, it ain't hard.

Look, Armor has diminishing returns, the more you get, the less effective it is to get more. With me so far? Therefore armor penetration is the exact opposite. The more you have the more effective it is to get more. Also the less armor the enemy has the more effective armor penetration is.

Armor penetration is therefore best early game when low armor is common, and much less effective late game when high armor is common. It will give you a high percent damage increase early when your damage is low, and a low percent damage increase late game when your damage is high. You can get people below zero armor.

Critical chance is a percent increase in damage. This is therefore not as good early game when your damage is low, and is much better late game when your damage is high.

Critical damage is also a percent increase in damage, so it is better late game. It also suffers if you don't get critical chance items. If you don't plan on getting some crit chance items then don't use these runes. If you plan to build for high crit chance, then crit damage runes give the best damage output per rune, but only after you get a good crit chance item set.

(Shortened quote.)

And I'm tired of people thinking Crit Dmg is better late game. Here's why you shouldn't get crit dmg:

http://www.leagueoflegends.com/board/showthread.php?t=70173
You said "Do some math guys, it ain't hard," but you didn't do any math to justify your claim that crit dmg/crit chance are any good.

True. Armor penetration has a decreased effect the more armor they have. It still beats out crit dmg until around 110 armor, and then if you get last whisper armor penetration beats it out all the way up to around 180 armor. Infinity Edge also increases the point to which armor penetration is better as well.

**AND THIS IS ALL ON A CRIT. THIS ISN'T INCLUDING ALL THE NON-CRIT ATTACKS!**
Crit chance runes give you approximately 1% crit chance for each 2 armor pen you would otherwise have. I don't have as nearly a big problem with people taking them over armor pen. At the very least, they're great on Tryndamere (of course, he's quite UP at the moment.) Or you can make the argument that you want burst damage and like the crit chance (crit damage doesn't make the cut even for burst damage.) If you're going for DPS, then:

Avg Dmg = Base * ((1 - crit chance) + (crit chance * crit dmg)) * 100 / (100 + armor - armor pen)

So if you have high crit chance already, you get reduced relative damage output from crit chance. If they have high armor, then you get less relative output from armor pen. Makes sense. So a great reasonable case for crit chance:

(14% crit chance vs. 29 armor pen)

IE, lethality, sunder, base crit chance of ~4%, high armor (say, 140 after Last Whisper if you have one.)

Base * ((1- .38) + (.38 * 2.6)) * 100 / (100 + 140 - 6) = 68.7% base damage on average

Base * ((1- .24) + (.24 * 2.6)) * 100 / (100 + 140 - 35) = 67.5% base damage on average

Ratio: 101.79% damage in favor of crit chance.

For armor pen:

No IE, 8% crit from item, lethality, sunder, base crit of ~4%, medium armor (say 60)

Base * ((1- .26) + (.26 * 2.1)) * 100 / (100 + 60 - 6) = 83.5% base damage on average

Base * ((1- .12) + (.12 * 2.1)) * 100 / (100 + 60 - 35) = 90.56% base damage on average

Ratio: 108.45% damage in favor of armor pen.

So you're giving up around 8.5% damage early-mid for around 1.8% damage late. Your choice. I'd go with

**armor pen**.

tl;dr:

**ARMOR PEN.**