Having PCs roll "Defenses" rather than the DM rolling "Attacks"

[Nellisir]

Well, I tried this in our last game session. Our former GURPS players all loved it, although one of my two hardcore DnD guys flat out told me he didn't like it at all. So I rolled as normal for him, let the other guys roll their defenses, and everyone seemed happy.

Did he say why?

 

[Wik]

I wasn't really paying attention, but the actual "rule" from Unearthed Arcana, for d20/3e, is +11, not +12. Here's the link Players Roll All The Dice :: d20srd.org

Oh, I'm familiar with the rule - I used it for the last year or so of my 3e days. It's a great rule.

Someone mentioned that +12 is the way to go, though.... and it's been shown multiple times, in examples. Why, though, is 12 the new baseline? That's what I'm curious about.

 

[keterys]

Someone mentioned that +12 is the way to go, though.... and it's been shown multiple times, in examples. Why, though, is 12 the new baseline? That's what I'm curious about.

The reason why has been shown mathematically in this thread. At which point do you need help?

 

[Wik]

Then I'm missing something. I've seen you USE the number 12, but you haven't explained why.

Basically, you use 12 + Monster attack bonuse for the flat modifier, and subtract 10 from the PC's defence to get their defence roll.

I'm wondering about the logic behind this (yeah, the math works) as compared to using 11 for both monster attack and PC defence.

Hope that makes sense.

 

[keterys]

If the math demonstrably works, why do you need the explanation?

http://www.enworld.org/forum/4e-fan...roll-defense-against-npc-monster-attacks.html was linked off this thread and shows it a little bit more, but basically...

The average of 1d20 is 10.5 - you need to pay attention to where that .5 lands. Core rules only give defense +10, thereby giving the attacker a .5 bonus equivalent.
When you say you have +10 and need to roll AC 20, you need to roll a 10. That's because you win ties you don't need to 'beat' the AC of 20 by getting a 21. When you reverse the equation, you need to account for the fact that the _attacker_ 'wins on ties'

So, you had +10 vs. AC 20 is 55% chance to hit (need a 10)

When you say you have AC +10 and you need beat an attack roll that would have be the equivalent chance, you need to bump up the DC for both the roundings involved (.5 on die, .5 on formula) and ties (+1)

Attack = 20 (+10) would be a 45% chance to be hit, clearly off by 10% (or 2)
Attack = 21 (+11) would be a 50% chance to be hit, clearly off by 5% (or 1)
Attack = 22 (+12) is just right

 

[FireLance]

Then I'm missing something. I've seen you USE the number 12, but you haven't explained why.

Basically, you use 12 + Monster attack bonuse for the flat modifier, and subtract 10 from the PC's defence to get their defence roll.

I'm wondering about the logic behind this (yeah, the math works) as compared to using 11 for both monster attack and PC defence.

Hope that makes sense.

The crux of the issue is: who wins ties. The standard assumption here is that the roller wins ties. So, if the monster is rolling attack, it hits an AC of 20 if his d20 roll + modifiers is 20 or higher (assuming a +10 modifier, he hits 55% of the time). If the PC is rolling defense, he avoids a static attack of 20 if his d20 roll + modifiers is 20 or higher (assuming a +10 modifier, he avoids the attack 55% of the time). The shift from the monster winning the tie if it rolls attacks to the PC winning the tie if he rolls defence accounts for the two-point shift (in order for the monster to hit 55% of the time, the PC must avoid the attack 45% of the time, and must therefore roll against a static attack of 12 + attack modifier instead of 10 + attack modifier).

 

[Mort_Q]

The crux of the issue is: who wins ties. The standard assumption here is that the roller wins ties. So, if the monster is rolling attack, it hits an AC of 20 if his d20 roll + modifiers is 20 or higher (assuming a +10 modifier, he hits 55% of the time). If the PC is rolling defense, he avoids a static attack of 20 if his d20 roll + modifiers is 20 or higher (assuming a +10 modifier, he avoids the attack 55% of the time). The shift from the monster winning the tie if it rolls attacks to the PC winning the tie if he rolls defence accounts for the two-point shift (in order for the monster to hit 55% of the time, the PC must avoid the attack 45% of the time, and must therefore roll against a static attack of 12 + attack modifier instead of 10 + attack modifier).

Why not just say "tie goes to the attacker" ?

 

[Garthanos]

Why not just say "tie goes to the attacker" ?

So you would rather change the die roll to act different
than all other rolls? you have to meet or exceed a target
number.... rather than increase the target number from 11+ to 12+?

It is a precomputed number we are talking about here.

 

[keterys]

You already have to change it from +10 to +11 to even make 'switch who wins ties' work... you may as well go the next step to +12 and just make it work right with the rest of the system.