Misc: Generic Probabilities

"There will be times in which the rules do not cover a specific action that a player will attempt. In such situations, instead of being forced to make a decision, take the option to allow the dice to control the situation. This can be done by assigning reasonable probability to an event and then letting the player dice to see if he or she can make that percentage. You can weigh the dice in any way so as to give the advantage to either the player or the non-player character, whichever seems more correct and logical to you while being fair to both sides."

The little excerpt above was quoted from the 1st Edition AD&D Dungeon Masters Guide, page 110.
This basic chart takes that idea and removes the guess work to give DMs an easy-reference table for different difficulties and probabilities. This especially useful for Secondary Skills, but can and is intended for any situation that doesn't already have an officially described method of handling.

Generic Probability Chart

Additional Modifiers:

 • +5% for being eligible for the Prime Requisite Experience Bonus

 • +5% for each circumstantial benefit; ie top dollar equipment, safe or beneficial location, etc

 • -10% for each circumstantial hazard