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The Gammon Factor
So far we have looked at doubling decisions without any real discussion of one of the key factors and that is the number of gammons won by the player who doubles.
This week’s position is a typical blitz. White has been aggressively attacking black who now finds himself on the bar facing a four-point home board. White doubles. Should black accept the double?
Let’s try to evaluate it using our four basic criteria:
The Race: – black trails by 13 pips in a long race. Certainly black has racing chances.
Position: - White has the better home board but black has only one checker back to white’s two. Black’s position is slightly inflexible and having a third checker out of play on his 3-pt certainly doesn’t help. I would estimate the position element as equal.
Threat: - White’s big threat is to close black out and then black won’t move again until the bear-off. On the other hand black threatens to escape his back checker when it will be white who has to extricate his checkers. White’s threat is immediate and obvious so advantage to white here.
Opponent: - In this sort of position it helps to know your opponent. Will he play the blitz aggressively? Does he understand the blitz? If he is a cautious player you have more leeway to take.
Overall then, advantage to white and certainly a double as the position is highly volatile. Can black take?
The answer is no and the reason is that when black loses, over 40% of the time that he does so, he will lose a gammon and four points. He will win more than the 25% of time but he doesn’t win often enough to offset the gammon losses.
Is there a rule of thumb for these gammonish positions? Yes, there is:
Divide your expected gammon loss by 2 and add the figure to the “normal” take percentage of 25%. Thus if you expect to lose a gammon 20% of the time (i.e. when you lose the game one fifth of those losses will be gammons) then add 10% to the 25% to give 35%. If you can win the game 35% of the time then you can take otherwise you must drop.
As you can see from the rollout data below black can win from this position about 30% of the time but he loses a gammon about 43% of the time (31.9 expressed as a percentage of 70.3). Thus we can see that Black is nowhere near a take and he must pass the double. Many would be deceived by the closeness of the race and black’s sound structure and take but that would be to overlook the key to the position – the gammon rate!
This article only scratches the very basics of the gammon factor - we will return time and time again to consider how gammons influence doubling decisions.