In some role-playing game (RPG) systems, rather than rolling a single die to determine the success or failure of an action, the player rolls a number of dice simultaneously, known as a dice pool. The number and type of dice to be rolled are defined by the mechanics of the game.
Definition
In his 2022 paper "Icepool: Efficient Computation of Dice Pool Probabilities", Albert Julius Liu defined the term "dice pool":<blockquote>While some dice mechanics determine the result from a roll of a single die, others have a player or players rolling a "pool" of multiple dice. For most such mechanics, all of the dice are thrown simultaneously and without order, with the dice being treated as indistinguishable other than the number they show. In other words, the roll of a pool is fully described by a multiset. The in-game consequences of the roll are then evaluated as a function of the multiset according to the rules of the game.</blockquote>
The use of dice pools
Jenny Ford of the British game publisher Man o' Kent Games points out that for a game designer, using a dice pool rather than a single die has the advantage of probability control: "Game designers want to give players a certain experience, and to give that experience they need to have some chance of predicting what will happen to their players. That means that totally random results [from rolling a single die] are difficult to use consistently to produce a desired player experience ... Rolling lots of dice at once ... will tend towards the mean." In essence, if a lot of dice are rolled, the average of all of the dice rolled will approach the mean of the die used. For example, rolling 10 six-sided dice should result in about half of the dice being 4 or more. The game designer can then express success as the number of dice with a given value — say 4 or more — allowing the designer to more accurately control the probable outcome of the desired action. The first widely successful game to feature dice pools was Greg Costikyan's Star Wars: The Roleplaying Game (1987), developing the system pioneered the year before in Ghostbusters.
Shadowrun (1989), designed by Bob Charrette, Paul Hume, and Tom Dowd, used a comparative dice pool, in which players roll a set of six-sided dice and each die rolled was compared to a target number to determine if that die was a success or a failure, with the number of successes determining the outcome of the action taken.