The Keeper determines the difficulty level for skill rolls. There are three levels of difficulty in Call of Cthulhu: Regular, Hard, and Extreme.
An average example of what would challenge a competent person. The player needs to roll equal to or below their full skill or characteristic in order to succeed. This should be used in the vast majority of cases.
This task would challenge a professional. The player needs to roll equal to or below a half of their skill or characteristic in order to succeed. This should only be used occasionally
This task would challenge an expert; it is on the border of what is humanly possible. The
player needs to roll equal to or below a fifth of their skill or characteristic in order to succeed. This should be reserved for extreme situations.
Success or Failure
There are two possible outcomes of a roll: success or failure. The various divisions of skill (fifth, half, full) do not equate with graduations of success. The player states a goal and then rolls the dice. If the dice roll is equal to or below the required number, the goal is fully achieved. If the dice roll is below a half or a fifth of the specified skill, the goal will not necessarily be performed to a higher standard. Any further interpretation of the roll is at the discretion of the Keeper
Pushing a skill roll provides the player with a second and final attempt to achieve a goal. A pushed roll is only allowed if it can be justified, and it is up to the player to do this. Whenever a player asks, “Can I push the roll?” the Keeper should always ask, “What action are you taking to push the situation?” It is not for the Keeper to simply say yes or no; it is for the player to describe the extra effort or time taken to justify the pushed roll. If the player is stuck, other players or the Keeper might offer suggestions. By making a pushed roll, the player is upping the ante and giving the Keeper permission to bring dire consequences should the roll be failed a second time. Pushing a roll means
pushing the situation to the limit:
A fumble means something worse than a straight fail has occured, the nature of which is at the keeper's discretion.
A roll of 01 means that something beneficial occurs beyond simply achieving the goal. The nature of the good fortune is at the Keeper’s discretion, although the players may make
suggestions. In combat, for example, a critical success means that the attacker has hit a vulnerable spot and causes maximum damage.