Fail-Low

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

=Inside the Tree= A Fail-low appears at so called All-Nodes inside the alpha-beta algorithm, also called Fail-Low nodes. The score returned is a upper bound on the exact score of the node.

Quote by Bruce Moreland : A fail-low indicates that this position was not good enough for us. We will not reach this position, because we have some other means of reaching a position that is better. We will not make the move that allowed the opponent to put us in this position.

=Root with Aspiration= Another meaning of a Fail-Low, is associated with aspiration windows at the root, where one needs to re-search with wider windows to get the true score rather than an upper bound.

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

=See also=
 * Fail-High
 * Upper Bound
 * Lower Bound
 * Exact Score
 * Move Ordering
 * Null Window

=Forum Posts=

1995 ...

 * Failing low at the root by Guido Schimmels, CCC, August 03, 1998
 * Question: Fail High then Low at Root by William Bryant, CCC, December 28, 1999 » Fail-High, Root, Search Instability

2000 ...

 * Question: Fail low at root and time management by William Bryant, CCC, February 08, 2000 » Root, Time Management
 * Fail-Low ? by Nobuhiro Yoshimura, CCC, April 05, 2000
 * Researching after a fail-low by J. Wesley Cleveland, CCC, August 17, 2000
 * Fail highs..which subsequently fail low by Tom King, CCC, August 27, 2000 » Fail-High, Search Instability
 * Researching after a deep fail low by José Carlos, CCC, September 24, 2001
 * MTD, IID, fail-low, root-research by Juergen Wolf, CCC, August 14, 2003 » MTD(f), Internal Iterative Deepening, Root
 * Search behavior in a case of root fail high/low by Sergei S. Markoff, CCC, March 10, 2004 » Fail-High, MTD(f)

2005 ...

 * Fail-low pruning by Tommi Rimpiläinen, CCC, January 17, 2006

2010 ...

 * Fail soft vs fail hard by Sergei S. Markoff, CCC, February 15, 2014 » Fail-Soft, Fail-Hard, Internal Iterative Deepening
 * Solving a fail low situation at the root by Alberto Sanjuan, CCC, November 03, 2014 » Aspiration Windows
 * What does "fail low at the root" mean? by watersky33, OpenChess Forum, December 24, 2014

2015 ...

 * Fail low after fail high by J. Wesley Cleveland, CCC, April 04, 2015 » Fail-High, Search Instability
 * Restarting iterative deepening by Harm Geert Muller, CCC, December 09, 2015 » Aspiration Windows, Iterative Deepening
 * (I)ID and PV dropout by Harm Geert Muller, CCC, June 17, 2017 » Internal Iterative Deepening, Iterative Deepening

=External Links=
 * Programming - Glossary - Fail Low, Fail High from Bruce Moreland's Programming Topics Site
 * The Beatles - I'm Down (1966), YouTube Video


 * Stevie Ray Vaughan with Jeff Beck and Terry Bozzio - I'm Going Down (1989), YouTube Video

=References=

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