From the Game's Introduction:
As an old Call of Cthulhu RPG Keeper, I had problems with a percentile dice system. While players could easily determine if they were successful, *how* successful they were often broke the momentum of the game (especially if a player swiped up his dice before I asked!). Dice modifiers were arbitrary (do you divide or have a straight penalty?), as were automatic successes (make sure two players had a 20% to drive a car!)
Here's a quick-and-dirty way of using the Steve Jackson Games Cthulhu Dice die for a storytelling RPG, with variable degrees of success and failure. Note that the rules lean towards the characters succeeding in their skill rolls, necessary in CoC games where missing a clue often stalls the game. The Keeper is also given leeway to advance the plot.