I just noticed that the Double D20 System on p. 24 of the GPC is not doing what I think it is supposed to. The problem is that the probability for the GM’s roll is constant – before rolling any of the two dice, he has always a chance of 52.5 % to succeed (including critical success). In other words, the „undefined [skill] value“ mentioned by said rule is always the same, just slightly lower than 11.

Or is this the intent of the rule? The effect is about equal to flipping a coin.

If this is not the intent, I suggest changing this rule: instruct the GM to simply assign a value to whatever danger the PC is facing, analogue to a skill value, and then roll for that.

(I checked with two of my friends to be sure – for two dice with n sides, the probability for the second die to roll higher than or equal to the first die is (n²/2)+(n/2) = (n+1)/(2n). Assuming a D20, that's (20+1)/(2*20) = 21/40 = 0.525 = 52.5 %.)

Each test works out to be as you say when viewed atomically across both rolls, however although the rules speak of rolling two dice simultaneously I think the intent when handling these tests (at least it's the way I play it) is to take a narrative beat to describe the threat level indicated by the first roll before moving on to assess how that threat level resolves itself via the second roll.

Thus while the stats don't change, the specific instance of a 'first roll' of 4 resolves/narrates differently to an instance when the 'first roll' is a 14.

Note also, that whilst the stats are approximately the same as flipping a coin this only accounts for the 'opposition' half of the test - you still have the player's roll to add to the mix before you know how things turned out.

Your alternative rule is a valid approach - but if you are subbing in the GM's judgement for the first roll (or for that matter some kind of generalised threat level, as used in Heroquest for example), then you are moving away from the idea that the threat of the test is 'undefined', since the first thing the GM has to do is to define it.

Regards
Luke

It’s the simultaneity of the roll and the way the example reads that made me wonder. The example describes the situation and establishes the threat before the roll; and the description of the roll is limited to a simple one word statement.



The ship lurches crazily and the warhorse has to step as nimbly, as when dodging a spear. The Gamemaster rolls a red d20 and a blue d20, saying “red is the skill, blue is the roll.”

The red die comes up 12, and the blue die 8. “Success,” says the Gamemaster. “What did you roll?”


Greg, could you clear up my doubts?

It is as Silburni says