-1D is closer to a penalty of +2 Ob, if I recall correctly. People that are good at math can probably explain why, but I know it’s come up several times. There are also some side-effects of losing dice, like having less opportunity to spend Fate, and becoming unable to roll skills that are reduced to 0 dice.
I don’t understand why this is true. The side effects, I 100% understand, but I don’t see how 1 dice is equal to 2 ob. Let me put forth some sample numbers to try to illustrate my question.
Let’s say you have an OB 2 challenge and a B4 skill. I’ve done the math for 4 dice - you have a 68.75% chance of success against an OB 2 challenge.
Add 1 OB and your odds of success drop to 31.25%.
Subtract 1 dice and your odds of success drop to 50%
Let me illustrate this math real fast. With 4 dice, each dice has a 50% chance of success (4-6 success, 1-3 failure). Each combination (with F for failure and S for success) has a 6.25% chance of occurring (.5^4 is the math for that). So, 4 failures is only one combination - FFFF. That’s a 6.25% chance of occurrence. 3 failures has 4 possible combinations - FFFS, FFSF, FSFF, SFFF. That’s 4*6.25, or 25% chance.
With B4, you need at least 2 successes - you succeed on 2 successes, 3 successes, and 4 successes. So 37.5 + 25 + 6.25, or 68.75% chance of success. With 3 dice, you succeed with 2 or 3 successes, or 37.5+12.5, or 50%. Those are against OB2. Against OB 3, those odds worsen dramatically - to 31.25%.
With this array (and other arrays), it seems outright fact that 1 OB is worth approximately 2 dice, as a rule.
I simply can’t understand how it could be otherwise.