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Perfect and Imperfect Information
Such games have perfect information: Each player, when deciding his move, must have complete information about the current position of the board (I include in "position" qualities that may be physically undetectable, such as whether a player may castle), or equivalently, about the original position of the board and all moves made so far. Examples of perfect information games would include Chess and Backgammon; games like Stratego, Kriegspiel, or the recent Stealth Chess are not perfect information games.—J. Mark Thompson, "Defining the Abstract"
Parlett describes card games as games of "imperfect information," due to the fact that the two-sided nature of cards permits them to hide their informational value. In the quote above, mathematician and game aficionado J. Mark Thompson[1] points out the opposite kind of system: games of "perfect information." Perfect and imperfect information refer to the relationship a player has to the information contained in the formal system of the game.
Perfect information exists in a game when all players have complete knowledge about every element in the game at all times. Thompson includes Chess and Backgammon in this category. In games of imperfect information, some of the information may be hidden from players during the game. Thompson names Stratego, a game in which the value of each piece on the gameboard is kept hidden from each player's opponent, as a game of imperfect information. Card games are also good examples of games of imperfect information. Although it would be possible to design a card game in which all of the cards are visible for the entire game, almost every card game does feature imperfect information in some form. Games of perfect and imperfect information can both provide meaningful play, but they do so in different ways. Games of perfect information tend to be more analytically competitive games, in which players are pitted directly against each other, each player's moves and strategies available for the other to see. Games of imperfect information add an element of mystery and uncertainty to a game. Imperfect information invites treachery, trickery, and deception, and can be used as a design element in games meant to inspire mistrust among players. Although imperfect information can heighten a feeling of uncertainty in a game, actual randomness is not intrinsically tied to either perfect or imperfect information. A game can have chance mechanisms as part of the game and still possess either perfect or imperfect information. As Thompson points out, both Chess and Backgammon are games of perfect information, even though Backgammon makes use of die rolls every turn. An imperfect information game such as Stratego does not use any kind of chance mechanism, whereas Poker does, in the form of a shuffled deck. Chance is not related to our definitions of perfect and imperfect information. But whether or not a game has a random element does affect the kind of information that exists in a game. In The Interactive Book, designer and scholar Celia Pearce presents a different typology for understanding the ways games manifest information. She proposes four scenarios:
Information known to all players: In Chess, this would consist of the rules of the game, board layout, and piece movement parameters.
Information known to only one player: In Gin, this would be the cards in your hand.
Information known to the game only: In Gin, this would be unused cards in deck. In Space Invaders, this would be the paths and frequency of alien space ships.
Randomly generated information: In Backgammon, this would be the roll of the dice.[2]
Pearce's categories offer another way of describing the informational component of a game. She differentiates between two kinds of hidden information, one in which each player possesses private information and another in which the game system itself hides information from all of the players. As she illustrates through her example of Gin, any card game with a shuffled deck and private hands for each player contains both kinds of hidden information. Pearce also designates randomly generated information within its own informational category. One disagreement we have with Pearce's typology regards the inclusion of the rules of a game in her information model. Pearce states that the rules of Chess should be considered information known to all players. Although this is arguably the case, we don't feel that including rules as part of the perfect information of a game is particularly useful, since in all games, some or all of the game rules are known to all players. There may be hidden strategic relationships in a game that are gradually uncovered (like the strengths and weaknesses of particular unit combinations) but these are not rules. Rules are the formal foundation of a game that allows players to manipulate information. Rules generally do not constitute the information being manipulated during play.
There are other slightly ambiguous aspects of her model as well. For example, although a deck of cards in Gin might contain information "known to the game only," one could imagine a deck of cards that is reshuffled every turn, functioning in exactly the same way as a random die roll—Pearce's "randomly generated information".Perhaps the subtle difference between these two types of information is whether or not the exact makeup of the remaining deck is completely known to all players. If it is, then the deck would function like a random die roll, in which the precise chance to roll a particular number (or draw a particular card) is known to all players. On the other hand, when players draw cards in Gin, the face-down deck contains information known only to the game. Despite our critique of Pearce's model, the four categories she proposes are in fact quite useful. For example, information is not always either public or private, and often moves between categories. As a deck of face-down cards are exposed to players, there is movement from once-private information to informa-tion that is public and shared. Similarly, in Battleship, both play- ers try to uncover information that is hidden from them, but known to their opponent. As Battleship proceeds, the positions of the players' ships gradually become public knowledge.[1]<http://www.flash.net/~markthom/html/game_thoughtsl.
>[2]Celia Pearce, The Interactive Book (New York: Macmillan Technical Pub-lishing,1997), p. 422–423.
