Saturday, December 4, 2010

Percentile Based Ability Score Skill System With Exponential Progression

Some of the recent blogosphere discussion of SPI's Universe sci-fi RPG, and it's use of squaring numbers in skill progressions reminded me of an idea I had for a % based ability score skill system a while back, a skill system that you could bolt onto oldchool D&D and variants, and the like, with minimum mechanical impact, bookkeeping, or fuss.

This skill system is pretty lightweight and loose, and is inspired somewhat by both how 3E skills can be classified by associated ability scores and also the use of Castle & Crusade's ability score based SIEGE mechanics as a skill system.

So the basic idea is that when a character attempts to "use a skill," or "attempt a skill-ful stunt," you determine the relevant ability score. Intelligence for deciphering archaic glyphs; wisdom for winning a meditation competition; dexterity for playing darts; charisma for getting in someone's pants.

The % chance of pulling it off, modified by whatever modifiers the DM thinks if fit, is determined by first calculating the square of the relevant ability score (multiplying it by itself; i.e. if the ability score is 10, you would multiply 10 by 10 to arrive at 100); and than dividing the resultant sum by four (in the previous example, one-quarter of 100 would result in 25, so someone with a strength of 10 would have a 25% chance of passing a push-up challenge in order to join a fraternity).

Now I'm basing the skill % logic along the lines of Runequest & Call of Cthulhu beginning character skill levels, aside for the abyssal low ability score %s and the generous high ability score %s. It's a system that allows for PCs to range from Elmer Fudd like levels of incompetence to movie Legeolas-esque near godhood at "skill checks" dependent upon their ability scores.

Myself, I could see using this method for complicated/hard skill uses, and use the roll under the ability score method for simpler, easier tasks such as climbing a tree or catching a football. This system is for performing calculus; landing crashing spaceships; tracking an alien hunter through a jungle; and catching a rope while plummeting kind of deal.

Now if you want to be a nice guy DM, you could add +1% to +5% to skill %s per level, dependent upon class and/or background relevance and the like. You could even incorporate roll-over skill advancement system of Runequest or 3rd Edition style skill points, but my intent was for the system to be bookkeeping free.

Here's the a table with the calculations already performed for you; I'm not going to be fancy like the Tao guy, etc., and make it look like something out of an AD&D manual.

Ability Score and Associated Skill %

3 - 2% (ha ha!)
4 - 4%
5 - 6%
6 - 9%
7 - 11%
8 - 16%
9 - 20%
10 - 25%
11 - 30%
12 - 36%
13 - 42%
14 - 49%
15 - 56%
16 - 64%
17 - 72%
18 - 81%


  1. Interesting. You could always just port-over the % for Thief Skills from AD&D...but that opens worm-cans and needs tweaking. Your solution seems decent, will have to try it out one of these days.

  2. My "System" (and I feel pretentious just even calling it that) is this: REASONABLE action: Automatic success unless there are mitigating factors. DIFFICULT action: Roll under ability on a d20. REALLY DIFFICULT action: Roll under ability on a d30 (I bought the d30 as a semi-joke, but in actual play it's turned out to be really useful). WHAT THE HELL ARE YOU THINKING? action: Roll under ability on d%.

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