What is the effect on game play of replacing d20 rolls with 3d6 rolls in a D&D 4e campaign?
I started thinking about this after reading this question where the example was given of playing a d20 system with 3d8 (actually 3z7, but I digress). I like math and probability and it got me curious...
After some searching, I found out that in D&D3.5's Unearthed Arcana, there was a rule variant which replaced every d20 roll with 3d6. Several things had to change (for example, a critical threat range of 19-20 needs to be shifted down because you can't roll a 19), but since the average result was the same (10.5), the math and the mechanics didn't need to change much. What changed was hard things got very hard and easy things got very easy. Beating a DC 17 under d20 happens 20% of the time. Under 3d6, it happens < 2% of the time. Likewise a DC 6 goes from 75% success to a 95% success rate.
From reading what I could find, this variant supposedly makes for a 'grittier' game. The average happens most of the time and that favors the stronger side, which is usually the players. But great rolls don't happen very often.
Here are my questions:
- Are there any mechanics / stats changes that are needed to keep things balanced?
- How does it affect tactics (both for combat and skill challenges)?
- Does rolling average results most of the time turn it into a 'grinding' game that takes forever?
- Does it ever fall apart?
- Is it still fun?
Off the bat, I would guess that Aid Another becomes an action that characters would take on a frequent basis. If the fighter needed a 15 to hit the big guy, under d20, two people aiding her would increase her chances from 25% to 45%, maybe not worth it. Under 3d6, their aiding would change it from 9% to 50% and adding a third aider would up that to 75%.
Here's a table of rolling N using 3d6 for reference:
N =N <=N >=N
3 0.46% 0.46% 100.00% 4 1.39% 1.85% 99.54% 5 2.78% 4.63% 98.15% 6 4.63% 9.26% 95.37% 7 6.94% 16.20% 90.74% 8 9.72% 25.93% 83.80% 9 11.57% 37.50% 74.07% 10 12.50% 50.00% 62.50% 11 12.50% 62.50% 50.00% 12 11.57% 74.07% 37.50% 13 9.72% 83.80% 25.93% 14 6.94% 90.74% 16.20% 15 4.63% 95.37% 9.26% 16 2.78% 98.15% 4.63% 17 1.39% 99.54% 1.85% 18 0.46% 100.00% 0.46%
(Produced using this calculator.)