Instead of a PSA, a beginning character picks two skills to be trained in, and one more to be his favored skill. All other skills are untrained. Learn by doing skills do not have separate levels, instead your raw percentage based on your ability score and experience is your skill level. So you might have Pistol skill 45% and Computers skill 80%.

Using a skill during an adventure grants the character an improvement roll for that skill. If the improvement roll succeeds, the skill rises – 1d5% for untrained skills and 2d5% for trained and favored skills. Favored skills rise by 1% even if the improvement roll fails. You can improve every single skill you use once each adventure.

The improvement roll is based on your chance of skill failure. So you have a 55% chance to improve a skill of 45%. Improvement rolls may also be made for ability scores in the same fashion. An ability rises by 1d5% when improved.

As you can see, the basic rule is like the BRP/Call of Cthulu skill system. But Learn by Doing adds levels of training into the mix, allowing you to simulate skills that need specific training as well as a character who's a natural with a skill.

I'd love comments on how easy it is to understand as well as general opinions. Particularly if you allow improvement each game session, it's a system with almost no bookkeeping. It also has the advantage of not making you save up for skills, the improvement roll either succeeds or fails.