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In most RPG systems, most non-trivial actions require dice rolls. Most RPGs roll a fixed number of dice, add a number to the die roll based on the character's attributes and skills, and compare the resulting number with a difficulty rating. However, in some systems the character's attributes and skills determine the number of dice to be rolled.
Dice pool systems generally use a single type of die, the most common being six- or ten-sided dice (d6s or d10s), though in some games a character's Attributes or Skills may determine the size of the dice in the pool, as well as their number (Deadlands is an example). While such games may require different sized dice for different rolls, the dice in a given pool are usually all of the same size.
The results on each die may be added together and compared to a target number (as in Over the Edge), or the player may count the number of dice which roll higher than a specified target number, and compare that to a required number of "successes" (as in early editions of Shadowrun or the Storyteller System). In systems using the latter method, the target number required for a success may be fixed (the same for every roll) or variable (assigned depending on the difficulty of a task); the number of successes required may indicate the degree of success, or a minimum number of successes may be required as another means of determining difficulty. Another variation is that a number of dice are rolled, but only some are added together (as in the "Roll and Keep" system used by Legend of the Five Rings and 7th Sea).
Modifying the dice pool
In dice pool systems it is common to add or subtract dice from the pool to represent different circumstances.
Penalties may temporarily reduce the dice pool for one or more skills (for example, a leg wound may reduce the dice pool for actions such as running, climbing, and jumping), and are usually fixed numbers (i.e. the leg wound may reduce a pool by two dice).
Bonuses may temporarily increase dice pools, and usually represent beneficial circumstances (e.g. a character may have a powerful computer to aid her in a database search) or some special effort on the character's part (an effort of will, a strong desire to succeed, or even a supernatural power). Circumstantial bonuses are also usually fixed numbers - the computer above might grant two additional dice, for example - while character traits which grant bonuses are usually an expendable resource, representing special effort. This may take the form of "points" (e.g. "Willpower points" in the Storytelling System), or an extra pool of dice which may be allocated to other pools to augment rolls (e.g. the Combat and Karma pools of earlier edition Shadowrun).
Other complications may be used to simulate luck, superhuman ability or other conditions; a common one is to allow high (or low) rolling dice to be rolled again, the second roll counting as if it were an additional die.
Advantages and Problems
Dice pool systems allow a greater number of variables to affect a roll, which can be an advantage or a problem depending on the complexity desired by designers and players. One major drawback is that games using dice pools require a large number of dice, meaning a larger outlay for players on equipment, and the need for more room when making a roll. For this reason some games have rules placing a cap on the number of dice that can be rolled, usually granting other bonuses to the roll for each extra dice that would otherwise be granted. Such rules are often optional.
Famously, early versions of the Storyteller System sometimes made rolling botches (critical failures) more likely the higher your skill or attribute was, since a critical failure would occur if any of the dice came up as a "1"; the probability that at least one "1" will be rolled increases the more dice are rolled, and so highly skilled characters would botch surprisingly frequently, whereas poorly skilled characters could frequently get away scot-free. This problem was eliminated in the Revised version of the system and later derivatives by stating that a botch only occurred if no normal successes were scored, and one of the dice came up "1".
The first widely successful game to feature dice pools was Greg Costikyan's Star Wars role-playing game (1987), developing a system pioneered the year before in the Ghostbusters role-playing game by Greg Stafford, Lynn Willis, and Sandy Petersen. (Both games were published by West End Games; Costikyan consulted on Ghostbusters.)
Shadowrun (1989), designed by Bob Charrette, Paul Hulme, and Tom Dowd, was probably the first game to use the "success" mechanic rather than adding the dice together. Vampire: The Masquerade (1991) and Over the Edge (1992) followed, which were written by Ars Magica designers Mark Rein-Hagen and Jonathan Tweet respectively, the pair having been impressed by the potential of the dice pool mechanic and each having decided to make their own game based on dice pools. The majority of White Wolf Publishing's subsequent games use variations on Vampire's Storyteller System, and so also make use of the dice pool mechanic.