UA talk:Players Roll All the Dice
Why 11?
Does anyone know why this rule adds 11 instead of, say 10? My group might implement this rule and we're curious about where 11 comes from. My only answer is Wizard's found something in there tests, but this topic of probability is beyond me. --windandfire 16:54, 30 July 2008 (MDT)
- The average roll on a d20 is 10.5. On a .5, you usually round up. Hence, 11. --Daniel Draco 22:26, 30 July 2008 (MDT)
- I would have thought the average on a d20 is 10. That's interesting, thanks.--windandfire 23:02, 30 July 2008 (MDT)
- I'm surprised they didn't use the standard D&D convention of "always round down". --Aarnott 11:08, 31 July 2008 (MDT)
- Isn't the math wrong on this rule? All else being equal, attacks hit 55% of the time (1d20 vs. 10 succeeds on 10-20). Defenses should therefore stop an attack 45% of the time, meaning that the number should be 12, not 11 (1d20 vs. 12 succeeds on 12-20). --80.187.107.225 21:49, 12 February 2010 (UTC)
SCENARIO 1:The math is wrong: Example. PC with AC 15. Monster with a +5 to hit. On a regular role, the monster must roll between 10-20 (inclusive) to hit the player. That is 11 different numbers (10-20) that result in a successful hit for the monster.
SCENARIO 2:To use the UA rules, with 11, you would have the player rolling a d20+5 (AC 15) against a DC of 16 (11+5 attack bonus). The player SUCCEEDS (monster misses) on an 11 or higher. That means that numbers 10-1 result in a HIT against the player. 10-1 is only 10 different numbers. For the monster to have the same chance of success, you must roll against 12+attack bonus. This would result in the player rolling against DC 17. 11-1 results in failure which is a hit on the character. 11-1 is 11 different numbers. This is exactly the same as SCENARIO 1 above (11 different numbers that result in a successful hit for the monster).
ANSWER:
First remember that against a DC, in case of draw, the one who throws the dice wins. So when you change the dice thrower you have to take this into account too.
The 11 is right if you think that is still the attacker (the monster) the one who wins a draw. The 12 is right if you think that the dice thrower (the player) wins a draw. Since this is the normal way to calculate success against DC, i suggest to use 12 instead of 11.
I think the developers missed this detail.