Information set
The group of situations a player cannot tell apart from one another when it is their turn to move.
Often misread asDrawing an information set across nodes where the player has different moves available, or a different number of them.
How to see it
The nodes you cannot tell apart
An information set gathers the points in the tree that a player cannot distinguish when it is their turn. They know they are somewhere in the set; they do not know where. Because of that, they must choose the same action at every node in it.
The checkIf the menus differ, the player could tell the nodes apart just by looking at their own options, so they cannot be in one set.
Try itDraw any tree, put two nodes in one information set, and try to write a strategy that plays left at one and right at the other. The fact that you cannot say how the player would know which node they are at is the whole point.
Games that show it
Know these first
Leads on to
Easily confused with
Sources
- Kuhn, H. (1953). In Kuhn, H. & Tucker, A. (eds), Contributions to the Theory of Games, Volume II, Annals of Mathematics Studies 28, Princeton University Press, 193-216.
- Osborne, M. & Rubinstein, A. (1994). MIT Press.