Stacking Abilities

How can i tell which abilities stack and which don't. For example if I use a 15% attack speed increase ability that was made by blizzard and gave it to an item. Then I based a new item ability off of that spell and gave it to a different item. If a hero had 1 of each items, would the attack speed increase be 15% or 30%?

I want to know about everything from poison to cold affects.

Well, this is not a full answer to your question, but I know that spells based on the same spell won't stack (the new buff overrides the old), and that item abilities such as attribute bonus do stack. Now, poison, cold and similar orb effects (this also means life-steal attacks and autocast attack-buffs, like searing arrows) do not stack, a hero may only have one active orb ability, if multiple orbs are equiped, the one at the top of the inventory will be the active one. Damage bonuses from the orbs still stack, though.

So, do attack speeds stack? I'm not sure, but if I had to guess, I would say they do... but that's just a guess.

Pretty sure that attack speeds do stack.

Now, poison seems to be able to work both ways, actually. There are options and buffs for both stacking and non-stacking poison, but to be honest, I don't know what has which.

well, it does stack, to a certain extent....E.G. you have two critical strikes, one is 15%, the other one is 25%......
if the multiplier and the damage bonuses are the same, then you would theoreticaly have 40% chance to do a critical, altho its 15% to do critical1 and 25% to do critical2 on every attack...
same for evasion and bash....
the attack speeds and move speeds all stack, but i didn't know the orbs don't stack...
as for poison (the stacking and non-stacking), i'll have to check on that, just to be apsolutly sure on what i'm talking about, and as for cold, there isn't much to understand there, is it ?:)

Poison, well, there's the spell poison (shadow strike&others) that doesn't stack with itself (however, shadow strike does, of course, stack with parasite or any other different spell), and then there's the weapon poison (orb of venom, dryad, vyvern). Orb of venom's poison does not stack with itself (if you hit a target multiple times), I don't know about the dryad and vyvern poison, but I do remember fighting some opponents in some hero siege type custom game that did have a stackable poison, and after a few hits, your HP was going down really fast.

Some of those previous replies are wrong. I have done a decent amount of testing to find out which abilities stack in my AoS. This list should be pretty accurate though there might be a few errors around. I haven't bothered multiplying with 100 when calculating percentages since it should be quite obvious. Besides, that's the way it's listed in WE.

Orbs effects (including searing arrows, dark arrows, slow poison, frost, % chance to cast spell etc.) does not stack. With several orbs in your inventory the one in slot 1 (hotkey 7) will have first priority. I do not know which priority buffs and passive skills have here.

Life steal does not stack. Highest value is used (this applies to all such skills).

Evasion does not stack

Feedback does not stack

Auras of the same kind do not stack. If you made two auras based off unholy aura, one with 30% move and 0% regen and one with 0% move and 30% regen, I do not know how they would stack. I imagine it would pick the most beneficial effect of either and apply it though it might pick the aura which is valued as overall best and use that one for both purposes.

Movement speed (set amount) does not stack.

Magic Damage reduction (by percentage) does not stack when using the same skill and I assume not at all.

Spell Shield does not stack when using the same skill.

Different slow poison or frost effects do not stack.

---
Critical hit does stack. If you have 6 of the exact same item with 20% chance of doing 2 times damage they would all be able to fire and sometimes at the same time thus multiplying your damage further. 6 items would give 0,20^6=0,000064 % chance of doing 2^6=64 times damage.

% chance to use normal skill (bash, pulverize etc.) does stack. Keep in mind that 6 items with 15% bash would not give 90% effective bash since they might overlap and stun at the same time. 6 items like that would give 1-0,85^6=0,62285 % chance of stunning your opponent.

Spell Shield does stack when using different skills. In my map it is possible to combine a 75 cooldown spell shield item with a 60 cooldown spell shield item and a 35/25/15 cooldown hero skill for a nice tripple spell shield effect.

Slow from poison, frost, auras, debuffs (slow, cripple etc.) does stack though most maps have a hard minimum movement speed of 100-150 which is 40-70% reduction of a basic hero movement speed. This can look deceptive when you've slowed it for a total of 97% and it's speed won't go below 50% of normal.

Pretty much everything else stacks. +damage, armor, attack speed, regeneration (not from 2 fountains/healing wards though as that counts as two similar auras) and other basic effects. This does not include some of the more obscure abilities which are available but rarely used. If you want to know whether two different upgrades which both lower the amount of damage spilled when huntress shots bounce will stack, then you'll have to find out yourself.

EDIT: Oh and I think that different poisons doing damage (such as 3 units with envenomed spears or the same damaging slow poison) will stack though the slow effect doesn't.

I tried making two Endurance Auras a while ago (since the buff editor came out). One increases attack speed and the other reduces movement speed. They seemed to stack in effect, though not in visual appearance. It did show both buffs independently in the status card.

Today I tested magic damage reduction and got some surprising results:
Magic damage reduction does not stack. The surprising part was that when I had a 40% reduction as a hero skill and a 33% reduction as an item (slot 5 though I doubt it matters), it would actually use the 33% reduction.

Tides of Blood have however managed to make such items stack though I got bored with that map back in october or so and wouldn't know if it's been hard-coded in a patch since.

I suspect that you might enable non-stacking skills to stack if you assign them different cooldown groups. If you look at evasion then it has a cooldown group whereas critical hit doesn't. The problem is that you can't assign "none" as a cooldown group so though you might assign different evasion skills different cooldown groups you would never make two items of the same kind stack. That is of course assuming that cooldown groups actually have that effect.

I think the cooldown groups' main purpose is, as the name implies, to apply cooldown to all items of the same group, for example, you drink a healing potion, and then, for some time, you can't drink any other potion, because they share a cooldown group with the healing one.
That, of course, doesn't mean that the cooldown groups DON'T have anything to do with stacking of abilities, altrough I doubt it...

BTW, good post, CrazyGamer.

I know the purpose of the cooldown groups but since certain passive skills such as evasion has a cooldown group while other passive skills such as critical strike does not then it might have something to do with it. I'm quite busy at the moment though so I won't be the one testing it.