Monday, March 16, 2009

More on D6 dice pools...

So, I've been toying with just using 1d6 and manipulating the TN. That is, if you're "trained," the odds of success are 4/6 (66%), if you're "untrained," the odds are 2/6 (33%). You can move in intervals of 16%, and can generally give up to +/- 1 or 2 in modifiers.

How do the odds work out if you just start with a fixed TN of 5 and then add dice to a pool, taking just the best result? A "success" requires at least one die showing a 5 or higher.

TN 5
1d6: 33%
2d6: 55.56%
3d6: 70%
4d6: 80%
5d6: ~86%
6d6: ~91%

TN 6
1d6: 17%
2d6: 31%

This does not let you modify the odds of success down much below 33% -- although you could make a rule that says that if you're down to 1 die in your pool, additional penalties jack the TN up to 6 and then its impossible.

The first +1 modifier (adding a die to the pool) gives a +22% boost. More than a +1 on the linear D6 roll (+16%), but not by much. The second +1 modifier (getting to the third die) adds a +15% boost. Again, close to our other odds. You then start to get diminishing returns. This is nice because it allows you to add many more positive modifiers. You can give more than +2 out as a bonus without having to worry about auto-success.

Another advantage is that you can use this for degree of success pretty easily. For example, a character using the Athletics skill can leap a number of squares equal to their dice pool roll. Or, you can count successes to see if they can run a race faster than someone else.

Dice Pool Odds stolen from here:

1 comment:

Nazim said...

Solomoriah, in his TSRPG, assigns target numbers above 6. A target number above 6 requires at least one additional die in the pool beside the one that rolls a 6, and that die must be equal to or greater than the target number minus 6.

In other words, if the target number is 7, rolling a 6 and having at least two dice in your pool will get you a success (sinc the other die will roll at least a 1, and 1+6=7). If the target number were, say, 9, you must at least have two dice in your pool, one must roll a 6, and the other at least a 3.

It sounds more complicated than it really is. This allows a DM to set more difficult thresholds.