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Home * Search * Alpha-Beta * Fail-High

Paul Klee - Red Balloon, 1922 [1]

There are two related semantics, if talking of a Fail-High - inside the search tree or if using aspiration windows at the root.

Inside the Tree

A Fail-High is associated with a Beta-Cutoff in the alpha-beta algorithm, which appears at so called Cut-Nodes, also called Fail-High nodes. The score returned is a lower bound on the exact score of the node.

Quote by Bruce Moreland [2]:

A fail-high indicates that the search found something that was "too good".  What this means is that the opponent has some way, already found by the search, of avoiding this position, so you have to assume that they'll do this. If they can avoid this position, there is no longer any need to search successors, since this position won't happen. 

Root with Aspiration

Another meaning of a Fail-High, is associated with Aspiration Windows at the Root, where one needs to re-search with wider windows to get the true score rather than a lower bound.

Quote by Bruce again:

You can also talk about failing high and failing low from the root position, if you use an aspiration window. 

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