Or a Series of Successes
I've started modeling skills and a thing I'm calling "skill chains". Basically, they're a series of skills each only attempted if the previous skill in the chain was successful.
As a quick example, a zombie's attack is currently a skill chain that looks like
scratch -> grab -> bite. Each skill's attempt is based on the skill's level
(and adjusted by the character's rating for that skill's associated attribute).
Characters level up skills by gaining experience points for their use, and each skill's award can be tailored (or "balanced" in "game dev" parlance) with a success and a failure amount of experience points. Level requirements are loosely based on a fibonacci series, for a quick and dirty natural curve.
Here's a screenshot of combat where the characters have some skill chain based attacks:
The humans here have no experience at all in their weapons, but (and this is why I like this skill chain thing) Waldo Collins successfully landed a second tier attack with his katana with his first attack! I drilled down a bit to show you the unlikelihood of that happening… but it did! With 0 levels across the board, (and no attribute bonus) a second tier attack has a 0.25% chance of success (yes, a quarter of a percent). Go Waldo!