How does armor work in Shogun 2: Total War?

Question

I've played Shogun 2 for a while now, and something I still don't understand is armor. The unit with the most notable armor is the Naginata Samurai with 9. Compare that to a Yari Ashigaru that has 2 armor, and it seems like there's a huge difference. I can see the number value of unit armor, but I don't understand is how that factors into combat. Does armor reduce the damage done by arrows? Does armor just give an increased dodge chance?

I think these might be questions of how combat is calculated by the game engine and not just of armor, but armor is my real concern. I can immediately see the effects of increased attack or morale, but not as much by increased armor.

How does armor affect the survivability of a unit?

Answer 1

I found this forum post, but I'm not sure if it is true.

Melee damage is calculated as:

(Attack power+Charge bonus if applicable) - (melee defense+armor+special ability if applicable+ any other bonus if applicable).

Ranged damage is calculated as:

(Ranged attack power+accuracy+reloading skill) - (armor+formation if applicable)

Melee makes sense, but ranged is still unclear to me. If you look on the stats sheet for a ranged unit like Bow Warrior Monks, there is no attribute for "Ranged attack power" (range is how far the unit can shoot). Can anyone confirm the validity of these formulas?

Armor subtracts from attack power and other bonuses when damage is calculated.