Double in a cash game, but not in matchplay, why?

[josephmcg]

Hi folks,

In a match to 7, black (to play) leads 2-1.
The cube is on two; what's the action?


I've simmed this with gnubg & nearly driven myself demented trying to understand the results.

gnubg says that black is poised to win 68% of the time, including getting gammon 23% of the time; and yet it clearly favours 'no double, take'.

In a cash game, gnubg clearly favours a double or a redouble, that result seems intuitive.

It's the matchplay sim result which I'm having trouble wrapping my head around.

I'm new(ish) to simming, so I'd also welcome tips about how to sim effectively, or feedback as to how to post a position here.

[dorbel]

A nice instructive position Joseph, not least because for match or money Red has a correct take.
Why should Black keep the cube leading 2-1 to 7? It's because of overage. If he redoubles to 4 he can use all the points from winning a plain game, but he can only use one of the four extra points if he wins a gammon. Worse still, if Red redoubles to 8 later on, then the whole match is on the line for either side, with gammons not counting at all. Clearly this hurts Black much more than Red, as much more of his equity comes from his gammon wins and with the cube on 8 will waste three of the points that he can win with a plain game, while Red wastes only two.

The simplest way to see this is to imagine it as a money game played to a bizarre set of rules, where if Black redoubles to 4 and wins a gammon, he will only be paid 5 points instead of 8. If Red then recubes to 8, gammons don't count at all and Black will only get five points for a win while Red will get six. The cube will also die when it gets to 8, so it will be of no value to Black. In this match, Black should play on for a gammon until his chances of that fall significantly, when he will hope to either cash or correctly redouble Red in. Note too that owning the cube will keep Black in the game until the end if things go Red's way.
Practically speaking, I think Black could expect a lot of wrong passes here though. A one in five chance of a drop would justify a cube, so it might be a reasonable over the board play.

Why is it a take? Partly because Black is still behind a five prime and still hasn't completed his six prime. One of his spares is badly placed on the 3pt while Red's spares have a lot of play. Note too that if Red dances, it doesn't hurt her too much, because she keeps all her points, whereas Black has to keep playing and may well go bust before he can escape.

Nice position, very instructive, thanks Joseph.

[josephmcg]

Thanks for the explanation, Dorbel -
Would you say this type of situation occurs commonly or rarely?

I notice that my link to the image is broken when i view this on my mobile.  
Here's the original photo of the position just in case (but brown and white replace red and black)



You can see from the cube [octahedron actually] that I did redouble, and in fact my opponent dropped - so it worked out well for me, though I was blissfully unaware of the subtleties of the position.  The presence of the recently drained pint of Guinness on my opponent's side of the board could factor into the analysis - but I couldn't find the option to represent that in gnubg.

[dorbel]

Yes, a very common situation. Understanding these (as far as we are able) is key to defending a lead or of course overcoming a deficit. Getting a pass was an excellent result and yes, you would be entitled to factor in the pint of Guinness! Keep in mind that doubling in positions that are technically not a double does offer your opponent the opportunity to make a mistake. Not only might his take/pass decision be wrong, but also he (and only he) may make doubling errors later, sometimes a string of them!

This is a very good position from which to learn. Playing this from here for practice and analysing how well you play it will teach you a lot. 50 times with the cube on your side and 50 times redoubling will teach you more than any amount of reading books. When you find something similar in future you will have a store of confidence and knowledge to draw on.

[stog]

re image not showing on the mobile - it could be that you are not logged in on the mobile (image attachments are only seen by registered users)
also as your image was uploaded to a different board with different permissions -- there could be an issue there.

the images are showing up well here though as is the interesting position and dorbel's answer