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Advantageously |
| 2021-07-13 |
Advantageously
I read somewhere that giving the advantage on a d20 was similar to giving a +5. I ran the numbers, thanks to AnyDice and here are the results, arranged in a table:
| target | 1d20 | adv | ~ | dis | ~ |
|---|---|---|---|---|---|
| 2 | 95% | 99.75% | +1 | 90.25% | -1 |
| 3 | 90% | 99.00% | +2 | 81.00% | -2 |
| 4 | 85% | 97.75% | +3 | 72.25% | -3 |
| 5 | 80% | 96.00% | +3 | 64.00% | -3 |
| 6 | 75% | 93.75% | +4 | 56.25% | -4 |
| 7 | 70% | 91.00% | +4 | 49.00% | -4 |
| 8 | 65% | 87.75% | +5 | 42.25% | -5 |
| 9 | 60% | 84.00% | +5 | 36.00% | -5 |
| 10 | 55% | 79.75% | +5 | 30.25% | -5 |
| 11 | 50% | 75.00% | +5 | 25.00% | -5 |
| 12 | 45% | 69.75% | +5 | 20.25% | -5 |
| 13 | 40% | 64.00% | +5 | 16.00% | -5 |
| 14 | 35% | 57.75% | +5 | 12.25% | -5 |
| 15 | 30% | 51.00% | +4 | 9.00% | -4 |
| 16 | 25% | 43.75% | +4 | 6.25% | -4 |
| 17 | 20% | 36.00% | +3 | 4.00% | -3 |
| 18 | 15% | 27.75% | +3 | 2.25% | -3 |
| 19 | 10% | 19.00% | +2 | 1.00% | -2 |
| 20 | 5% | 9.75% | +1 | 0.25% | -1 |
Thus a +1 attack against an AC of 16, targets 15. A regular roll has 30% chances of success. With the advantage, it becomes 51%, the delta of 21% is equivalent to a +4.
The above mentioned +5 is for the targets ranging from 8 to 14 included.
In Stars Without Number and friends, skill check resolution is done with 2d6. Yes, bell curve.
Some foci give an advantage. I can't remember if there is a disadvantage possibility, let's compute it anyway.
| target | 2d6 | adv | ~ | dis | ~ |
|---|---|---|---|---|---|
| 2 | 100.00% | 100.00% | +0 | 100.00% | 0 |
| 3 | 97.22% | 99.54% | +1 | 92.59% | -1 |
| 4 | 91.67% | 98.15% | +1 | 80.09% | -1 |
| 5 | 83.33% | 94.91% | +2 | 64.35% | -2 |
| 6 | 72.22% | 89.35% | +2 | 47.69% | -2 |
| 7 | 58.33% | 80.56% | +2 | 31.94% | -2 |
| 8 | 41.67% | 68.06% | +2 | 19.44% | -2 |
| 9 | 27.78% | 52.31% | +2 | 10.65% | -2 |
| 10 | 16.67% | 35.65% | +2 | 5.09% | -2 |
| 11 | 8.33% | 19.91% | +1 | 1.85% | -1 |
| 12 | 2.78% | 7.41% | +1 | 0.46% | 0 |
A skill check targetting 8 at 2d6+1 becomes a 2d6 targetting 7. A regular roll has 58.33% chance of success. Giving it the advantage (highest 2 of 3d6) turns it into 80.56% chance of success. That is roughly equivalent to a +2.
Turning an advantage to a modifier for a d20 is easy, 5% per increment. But for a 2d6, I went with comparing the advantage percent with a regular percent. Hence, the example above advantages at 80.56% which is close to a target 5 roll, two rows above, thus the +2.
