Rollout (backgammon)
Encyclopedia
A rollout is an analysis technique for backgammon
positions and moves. A rollout consists in playing the same position many times (with different dice rolls) and recording the results. The balance of wins and losses is used to evaluate the equity
of the position. This used to be done by hand, but it is now done mostly by computer programs. Even then, a complex rollout can take days to finish.
In order to compare two or more ways to move, rollouts can be performed from the positions after each move. Better choices will yield a more favorable position, and thus will win more times (and lose more rarely) in the end.
Computer programs usually play rollouts where the number of games is a multiple of 36, and ensure that the first dice roll is uniformly distributed. That is, 1/36th of the played games will start with a roll of 1-1, another 36th will start with 1-2, and so on. This improves the accuracy of the technique.
Rollouts depend on the availability of a good evaluator. If the computer makes mistakes in particular scenarios, the rollout results may be invalid. For example, if a computer AI's backgame strategy was weak, rollouts starting in a backgame position will skew the equity against the player who chose that strategy. When comparing moves, a weak backgame AI may favor less aggressive style. It is therefore not uncommon to see slightly different outcomes from rollouts done with different programs.
Nevertheless, rollouts whose results are consistently nonintuitive occur, and their results are usually accepted by most backgammon players. In fact, modern backgammon opening theory
is mostly based on rollouts.
Backgammon
Backgammon is one of the oldest board games for two players. The playing pieces are moved according to the roll of dice, and players win by removing all of their pieces from the board. There are many variants of backgammon, most of which share common traits...
positions and moves. A rollout consists in playing the same position many times (with different dice rolls) and recording the results. The balance of wins and losses is used to evaluate the equity
Equity theory
Equity theory is a theory that attempts to explain relational satisfaction in terms of perceptions of fair/unfair distributions of resources within interpersonal relationships...
of the position. This used to be done by hand, but it is now done mostly by computer programs. Even then, a complex rollout can take days to finish.
In order to compare two or more ways to move, rollouts can be performed from the positions after each move. Better choices will yield a more favorable position, and thus will win more times (and lose more rarely) in the end.
Computer programs usually play rollouts where the number of games is a multiple of 36, and ensure that the first dice roll is uniformly distributed. That is, 1/36th of the played games will start with a roll of 1-1, another 36th will start with 1-2, and so on. This improves the accuracy of the technique.
Rollouts depend on the availability of a good evaluator. If the computer makes mistakes in particular scenarios, the rollout results may be invalid. For example, if a computer AI's backgame strategy was weak, rollouts starting in a backgame position will skew the equity against the player who chose that strategy. When comparing moves, a weak backgame AI may favor less aggressive style. It is therefore not uncommon to see slightly different outcomes from rollouts done with different programs.
Nevertheless, rollouts whose results are consistently nonintuitive occur, and their results are usually accepted by most backgammon players. In fact, modern backgammon opening theory
Backgammon opening theory
The first moves of a backgammon game are the opening moves, collectively referred to as the opening, and studied in the backgammon opening theory. Backgammon opening theory is not developed in as much detail as opening theory in chess, which has been widely studied...
is mostly based on rollouts.