Statistical Tables for Dice Rolling

Many roleplaying games depend on die rolls for conflict resolution, but relatively few roleplayers can actually tell how good (or bad) their character’s chances are to succeed at the task at hand. How easy is it to score 13 or less on a 3d6 roll? how hard is it to roll more than 17 of a 1d20 roll? is it worth while spending your hard earned experience points to improve your skill roll from 12- to 13-, or is it negligible?
The table below will help you answer all these questions.