Anybody good at statistics 'round here?

[St Fan]

Hello,

I was wondering if anybody knowledgeable in maths and statistics could answer this question (my own skills are way too rusty).

It's about the Deadly Precision (http://www.d20srd.org/srd/psionic/psionicFeats.htm#deadlyPrecision) feat. It allows to reroll any result of "1" on the damage dice for a sneak attack.

What I'd like to know is exactly how valuable it would be for a rogue. I know the average result on a d6 is 3.5; how much would this feat change the result on average?

[noce]

You have equal chances to roll from 1 to 6.
The average is 3.5 as you say.

When you roll 1, you get to reroll and must keep the result, so you change 1 in the average roll, i.e. 3.5.

So you get equal chances at rolling one of the following:
3.5 2 3 4 5 6

The average of the new roll is 3.92.
Since without the feat you deal 3.5 damage, the feat gives you an increase of 0.42 damage per die.

[weckar]

Your average result gets raised to 4. It's not a major buff really. It's a stronger buff if you could somehow decrease the size of your dice (relatively speaking) but afaik there is no way in the game to do that.

EDIT: this is assuming a 1 on the reroll can be rerolled again.
EDIT2: Re-reading the feat, you don't. So the above poster is correct, I am wrong.

[Necroticplague]

Average of a normal d6:(1/6)(1+2+3+4+5+6)=3.5
Average of 1d6, rerolling 1: (1/6)(2+3+4+5+6)+(1/6)(3.5)=3.916.
So, compared to the normal die of sneak attack damage, this is a +.416 improvement.

[Mordaedil]

Which you time by the number of dice you roll, so it's actually quite a nice boost.

[weckar]

For the record, if infinite rerolls were allowed the formula would be to roll Xd(Y-1)+X, where X is the number of dice and Y the usual die size.

[Necroticplague]

Which you time by the number of dice you roll, so it's actually quite a nice boost.

Eh, not really. SA dice typically advance at .5/level rate, so this feat has a return of roughly .21 damage/level, barely over one-fifth the next comparable feat (Craven). Even if you manage to have SA dice=Level, this still has less than half the returns of Craven. At low levels, .42 damage is insignificant. At higher level 20, 4.3/attack is not particularly impressive. And the worst part is, since the improved damage comes from a die based source, instead of a flat modifier, it's not multiplied on a crit or similar. Craven is a much better pick over this.

For the next comparable feat (Martial Stance (Assassin's Stance)), you'd need 16 Sneak Attack die for this to be better.

So, in summary, the damage bonus is so small that there's no reason to pick it, instead of one of the above mentioned, which perform the same task (improving SA damage), while being much more effective at it, and having more utility beside (Craven can be multiplied, while Assassin's Stance grants more sneak attack, so it can do things like fuel Ambush feats).

[weckar]

There is no reason you have to pick only one feat to buff your SA, though...

[Mordaedil]

Right, it's the difference between 35 damage and 39 damage. Not significant, a straight up +1 bonus would be much better, but it's hardly the worst thing you can do to your sneak attacks.

[noce]

Do not take the feat, buy Bracers of Murder instead.
They're from Drow of the Underdark.

They make you reroll 1s on sneak attacks AND sudden strikes, and they do not specify you keep the second roll, so it's assumed you keep re-rolling 1s.
In addition, you get a to-hit and damage bonus against flat footed enemies.
In addition, if you have Death Attack you get a bonus to its DC.

And they only cost 8.000.

[St Fan]

Well, yeah, I did expect it to be a lesser advantage than Craven or Assassin's Stance, for sure. Just wanted to know where to put it exactly.

It's an option only if you've already picked the above. And not surprisingly, it's another feat that's easily topped by a magic item.

[Zanos]

Which you time by the number of dice you roll, so it's actually quite a nice boost.
+4 damage per sneak attack at 10d6 sneak attack is pretty bad. You'd probably get more mileage out of weapon specialization over the course of a game.