Talk:List of runes/@comment-192.223.163.6-20120328205832/@comment-4896865-20120806150125

I have no idea what the anonymous contributor above there is babbling about effective HP and all that and how AP adds damage every hit and MrP doesn't.

The fact is that both AP and MrP add damage to your spells every time you cast a spell.

Any time you cast a spell, it deals its base damage and added damage from your AP (or mana, or hp, or AD, or whatever it happens to scale on) depending on the ratio of the particular spell. This damage is then reduced or increased based on the MR of the target (increased if negative MR, decreased if positive MR). MrP reduces the MR of the target and it's calculated every time you cast a spell.

For the details on how it goes check out this.

Two examples, both using a spell with 300 base damage and 1:1 AP ratio. In both examples your target has 30 MR.

Example A: You have bought an Amplifying Tome and have 20 AP. The spell does (300 + 20 [from your extra AP] ) * 100/130 = 250,8 damage.

Example B: You have bought Sorceror's Shoes and have 20 MrP. The spell will now do 300 * 100/110 [reduced by your extra MR] = 272,7 damage.

If the base damage was only 100 instead of 300, the damage would be 92,3 in example A and 90,9 in example B. As your damage goes up, the effect of MR goes up as well as the amount of damage reduced is always a percentage of the damage dealt. This is why MrP is generally speaking more effective late game and AP in early game.