Perfect / Imperfect Information

Not to be confused with Complete Information game.

In an imperfect-information game, an agent does not fully know the state of the world or the agents act simultaneously. In a game like poker, the agent can only make partial observations.

We use Information Set.

A game of imperfect information is one in which you lack knowledge of any of the following:

  1. The state of the game (e.g. current market prices).
  2. The rewards you will receive from various states (i.e. utility and cost functions).

Difference with POMDP?

In partially observable process (specifically, a POMDP), the requirement is that you must not know which state you are in.

Imperfect information games are hard because an optimal strategy for a subgame cannot be determined from that subgame alone.

Our entire policy is common knowledge, but the outcome of random processes are not common knowledge.


Perfect Information Games (each player can see all the pieces on the board at all time)

  • Chess
  • Tic-tac-toe
  • Reversi
  • Checkers
  • Go
  • backgammon (has chance events but still perfect information)
  • Monopoly (has chance events but still perfect information)

Imperfect Information Games

  • Ridge
  • Poker
  • Battleship
  • Negotiation
  • Multi-stage auctions
  • Sequential Auctions of multiple items
  • Political Campaigns (TV Spending)
  • Military (allocating troops; spending on space vs. ocean)
  • Medical Treatment