two-part dice pool mechanic [closed]

I'm working on a dice mechanic/resolution system based on the Ghost/Echo (hereafter shortened to G/E) tabletop RPG. Specifically, since G/E can be a little harsh dealing with consequences and failure, I was hoping to soften the system and add a little more player control, as well as offer the chance to evolve player characters into something unique, right from creation. So, here's the mechanic: Players roll 2 separate d12 against each of the two statistics for their character (each is a number from 2-11, and may be rolled above or below depending on the nature of the action attempted, rolling your stat exactly always fails). Depending on the success for that roll, they add dice to the pool rolled for a modified G/E style action.

The acting player gets two dice anyhow, and I am debating offering a bonus die for each success, or a single bonus die for succeeding on both of the statistic-compared rolls.

Once the size of the dice pool is set, the entire pool is rolled, and the players are allowed to assign the rolled dice to a goal and a danger, one to each. Assigned results are judged as follows:

1-4 means the attempted goal fails, or the danger comes true.

5-8 is a partial success at the goal, or partially avoiding the danger.

9-12 means the goal is achieved, or the danger avoided.

My concerns are twofold:

Firstly, is the two-stage action too complicated, with two rolls to judge separately before anything can happen?

Secondly, are the statistics involved going too far in softening the game? I've run some basic simulations, and the approximate statistics follow:

                 2 dice     (up to) 3 dice      (up to) 4 dice
failure          ~33%            ~25%               ~20%

partial          ~33%            ~35%               ~35%

success          ~33%            ~40%               ~45%


I'd appreciate any advice that addresses my concerns or offers to refine my simulation (right now the first roll is statistically modeled as sign(1d12-1d12), where >0 is a success).

• So what's the question exactly? "Do you have any advice for me?" isn't really answerable, so this would be closed as not a real question here, and you'd be better off asking a forum like GameDev.net. – doppelgreener Sep 21 '12 at 1:25
• I actually asked for advice pertaining to the concerns I listed in my original question. I have re-phrased my concerns so the text's tone follows it's closed-ended intent. – bythenumbers Sep 28 '12 at 21:42

You mention 2d12, this would imply rolling 2 dice that have 12 sides each, I believe you really mean 2d6 which would be rolling 2 dice with 6 sides each, this would give you a value between 2 and 12 fitting into your rule setting better.

You also assume that rolling two dice will give you 33% chance of getting each number between 1 to 12, in fact you can't even get the number 1 (since neither dice can roll a 0).

To calculate the odds of something (the long way) you can draw the results onto a paper in a tree structure. Start with a dot in the middle of one of the sides of a paper, then draw 6 lines out from that, write the numbers 1 to 6 at the end of these lines. Now draw 6 lines from each number and write 1 to 6 at the end of these lines. You will now have a tree with a root to one side, and 36 leaves on the other side. Go from the root up to the leaves and add the numbers as you pass them by, this will give you 36 sums of all the possible rolls two dice can do. Now count the number of sums that are below 4 and write this down. Do the same for 5-8 and 9-12, the end result of this will be the following table:

Failure   6 16.7%
Partial  20 55.6%
Success  10 27.8%


Should you continue this tree for another dice roll you would get 216 sums and the following table:

Failure   4  1.9%
Partial  52 24.1%
Success 160 74.1%


As you can see, since your now rolling 3 dice you can only get 4 failures, this is because the lowest any of them can roll is 1 which sums up to 3, this only gives 2 failing rolls, all 1's or two 1's and a single 2. And since the highest values, added together, winds up being 18, which is 50% higher than your maximum for two rolls, you get a lot more successes compared to two rolls.

Needless to say, with 4 dice you can only fail by rolling four 1's (1 roll out of 1 296, or 0.07% chance).

I would suggest only using two dice rolls and instead add a fixed number to the roll, this way it won't cascade out of control.

Failure   2-5   27.8%
Partial   6-8   44.4%
Success   9-14  27.8%


By adding 1 to the dice rolls for each success from your stat system you would get the following chances:

          Dice    Dice+1  Dice+2
Failure   27.8%   16.7%    8.3%
Partial   44.4%   41.7%   33.3%
Success   27.8%   41.7%   58.3%


My suggestion is to map out your rolls very carefully using these probability trees so you know how your changes affect the outcome.

• Probability trees sound like a great idea!! Also, I think my original question was a bit unclear, since some of your analysis is not -quite- accurate in regards to my situation, so I've re-worded my question. Thanks for your help, though! – bythenumbers Sep 28 '12 at 21:39