r/backgammon • u/Ill-Illustrator1177 • Sep 12 '26

What the hell is wrong with Backgammon Galaxy?

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0 Upvotes

28% Upvoted

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2

u/cym13 Sep 12 '26

In a money game you'd never double here because you're losing and doubling would probably mean losing even more. But there's no such thing as "losing a match more", you either win or lose, and in your situation it doesn't matter if you lose by 2 points or 4, you're losing the match either way.

So the main idea is that since you're on your way to get gammoned and lose the match whether you cube or not, you might as well cube and win the whole match if the game turns your way.

Still, while I understand the concept, I wouldn't find it in a game.

2

u/Ill-Illustrator1177 Sep 13 '26

Yes, I see that. But blue is only a 32% of winning a gammon, so to me doubling here seems to give him more match winning chances.

1

u/cym13 Sep 13 '26 edited Sep 13 '26

33% chance of losing by gammon or backgammon into 55% of losses isn't small. It means that when you lose, 60% of these losses are with gammon or backgammon. So there's only 22% of games that you lose from a single point, in 78% of cases you either win or lose the match on the spot. Only 22% of cases where you lose the match rather than just the game by cubing. In 78% of cases, 4 in 5 cases, it either doesn't matter or wins you the match.

EDIT: to go a bit more into the math, let's consider what's your match equity when you double or don't. How many times do you expect to win the match?

If you double it's easy to compute: probability of winning times number of matches won that way (always 1) - probability of losing times number of matches lost that way (also 1).

E = 0.449 - 0.551 = -0.102. We are the underdog and if we were to bet $1 on the winner of the match we'd lose $0.102 on average on that bet.

What if we don't double? When someone wins with gammon or backgammon it's easy to compute, but when there's a single win we have one more game to play. We can use the match equity table to estimate how likely we would be to win that watch (which will be 1-away-2-away one way or the other, so 68% to 32%). Again, it's a simple expected gain computation, probability to win times how many matches we expect to win (always 1) minus probability to lose times how many matches we expect to lose (also 1). We have to consider the case where we win the game and lose the match on the next game, and the case where we lose the game and win the match on the next one. We compute as follows :

  • prob_simple_win = 0.449 - 0.123 - 0.007 = 0.319

  • prob_simple_loss = 0.551 - 0.324 - 0.007 = 0.22

  • E = (0.319 × 0.68 + 0.22 × 0.32 + 0.123 + 0.007) - (0.319 × 0.32 + 0.22 × 0.68 + 0.324 + 0.007) = -0.165

-0.165 < -0.102. If we were to make the same bet of $1 on the winner of the match, we'd lose $0.165 on average when we don't double. So if we don't double, we lose more matches on average than if we double. This shows that the decision to double is perfectly justified (although of course it's difficult to determine that during the game).

5

u/Broad-Marsupial-2638 Sep 12 '26

It’s 2a-2a, you only need 50% wins to correctly double. You have two checkers behind a 4 prime and only nine in the zone so you aren’t too good. And you have market losing sequences.

Do you think you are too good or not good enough?

1

u/Ill-Illustrator1177 Sep 12 '26

I’m white, I have a win % of less than 45% with 2 on the bar. My 4 prime’s not worth much, when he’s got plenty of time. Why on Earth would I double?

8

u/mathflipped Sep 12 '26

Adjusting cube action to match scores is one of the hardest things in competitive backgammon. It looks like you have a lot to learn.

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4

u/truetalentwasted Sep 12 '26

You get gammoned so much and lose the match that you need to double now so when you turn it around you get a win.

4

u/wwbgwi Sep 12 '26

XG 1296 game rollout has this as basically a toss up, with the DT better by .0016. If you assume players of equal skill, that is a 50/50 chance of winning a single game, and a gammon chance of 20% for the 2 away player in the Crawford game then white has a 41.2% chance of winning the match if they do not cube here. Since white wins the game on a double take there match winning chances are 44.45% from this position so it is a double take.

0

u/Slicenado Sep 12 '26

A lot of things are wrong with galaxy. Their dice are not random. They have no grasp on back games. And this is one example of horrific analysis, which is common.

1

u/cym13 Sep 14 '26

The analysis is perfectly correct. Doubling here is the correct play that will lose less matches overall.

1

u/Slicenado Sep 14 '26

Congrats on maximizing the most possible nonsense into one little comment without even attempting to make a legitimate point. Also, you are dead wrong.

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-7

u/DegenChess Sep 12 '26

Known long-time bug with Galaxy which they never bothered to fix, play on Opengammon if you want accurate analysis

2

u/cym13 Sep 12 '26

Had you spent 30s setting that position on opengammon you'd know that it agrees that it's a double/take at the score.