I've chosen to base the likelihood of success of an action in Cairn Hill on simulated rolling dice. I like the RPG flavour of it, plus, it forces me to keep the system relatively simple, and this way the player can follow and understand what's happening. The standard "roll" in the game combines two statistics, generally from different categories (e.g., an Ability + a Skill), to determine the character's proficiency at whatever he is attempting to do. Not all effects are determined by a roll; a roll is simply used whenever an element of randomness is wished for, such as when casting spells or in combat, or trying to manipulate a situation socially.
The two required stats are added to give a rating out of 20. Seven 100-sided "dice" are rolled. An average is taken from two of those dice; the dice chosen for this average are determined by the character's rating, as follows:
The average is taken, and to it is added the character's rating; this becomes the score. Mostly, the score at this point will be the final result, but there is a chance, specifically, a 1/5 chance, that the character will succeed, or fail, in a particularly spectacular manner. Again, a 100-sided die is rolled. If the result is equal to or lower than the character's rating, the result is a critical success. If the result is greater than a character's rating, but less than 21, the result is a critical failure.
If a critical success or failure is made, a 100-sided die is rolled again. If the result is less than 34, the score is raised or lowered by the character's rating; if the result is 34-66, the score is lowered by the character's rating times two; if the result is 67-99, the score is lowered by the character's rating times three; on the off-chance the result is 100, an absolutely spectacular success or failure results: the character's score is raised or lowered by ten times his rating, allowing total scores well in excess of 250.
Let's take an example to clarify. Black Lizzy is putting some poor schmuck in his place for mocking her sexual proclivities. She's none too bad with a knife (Agility 5, Melee 7; total rating of 12), and what's more, a knife attack succeeds with a final score of only 45, so odds are she'll be able to cut this guy up. She thrusts.
First, the seven 100-sided dice are rolled. In this example, let's say the results are as follows:
87 · 78 · 75 · 62 · 37 · 36 · 7
A little high, but not entirely atypical results (and yes, they have already been ordered by magnitude).
Because Lizzy's got a rating of 12, you take her middle and third-best dice (75 and 62), as per the chart above, and average them out, with a result of 68. Add to this Lizzy's rating, 12, to get a total score of 80; not a bad score, and more than sufficient. However, let's say that Lizzy rolls for critical success/failure, and scores an unlucky 13. Because this is higher than her rating, but lower than 21, Lizzy makes a critical error; but all is not lost. Roll again for the modifier; let's say the result of this roll is 51; this makes her critical failure modifier 2. Her final score is lowered by 2 * 12 (her rating), or 24, lowering it to 56; she hits, with a little room to spare.
Yes, it is realized that the critical failure concept means that in certain circumstances a higher rating will give you a lower final score, but sometimes, the more you know, the more there is to screw up. I am unrepentent; which is to say, I don't care.
That's it for dice.