Dodge runes are the awesomeness early game : Final Answer

Dodge and armor have completely different mathematical properties.

Armor is a completely linear form of scaling. 100 armor basically doubles your effective health against physical damage. 200 armor triples it.

Dodge has an exponential scaling (an attenuated exponential scaling in reality because of dodge %s being multiplicative instead of additive). 50% dodge doubles your effective health. 75% dodge quadruples your health. 90% dodge multiplies your effective health by 10!

Think of it this way. If the game had an item with 50% dodge, and you bought two of them, the second would greatly increase your survivability versus the first (first would double your original health, adding the second quadruples your original health). If you bought two 100 armor items, they would have an identical impact on your survivability.

In reality though, since items with dodge have well under 50%, the ability for dodge's exponential properties to really kick in is rarely realized. So, in fact, in most normal circumstances, dodge and armor will be equal in efficiency in terms of gold per time to live, with a mix being stronger than stacking either alone.

This is the most concise, simplest, and overall best explanation of Effective Hit Points I have ever read. I bow to your mastery of mechanics and analogy. Dubbed: JJ's Prism of EHP, or JJ's Prism for short.

One thing to note about this explanation is that it holds true for the percent values, not the numerical values. 50% damage reduction from armor = 50% evasion = Having hp twice as large as the damage they deal. Adding 5% more damage reduction is just as effective as adding 5% more evasion. However, the actual values you need in game to accomplish these feats vary. Also, the items needed to get these values vary in efficiency and cost-effectiveness. That is where all the trouble comes from.