Wednesday, April 20, 2011

The Advantage of Playing Short-Stacked

You'll notice that one of the Decision Modules in the above diagram is called ExpertShortStackedAnalyzer. That's my always-inventive name for a particular class which attempts to model a game of poker in which the stack sizes have been artificially shortened.

I've mentioned a couple times the advantages of creating a short-stacked bot but I don't think I've been clear enough about what those advantages are, and why they're so helpful. First of all, when I say a short-stacked bot, I really mean a short-stacked v1.0 bot. The idea should be to get something basic up and running first, before tackling deep-stacked no-limit Hold'em.

Now, let's define what a short-stacked bot is:

A short-stack bot is a bot which buys in for between 10 and 40 times the big blind, and which leaves the table when it wins much more than that.

Most online poker sites set the minimum allowed buy-in at 20 times the big blind (20xBB), or squarely in the "short-stacked sweet spot." In No Limit Hold'em: Theory and Practice, David Sklansky explains:

Playing a Short Stack

Since most of the money gets bet early in the hand, and betting all-in necessitates a showdown, short stacks should concentrate on playing premium hands and generally avoid bluffing. Play tightly preflop and try to get your money in when you likely have the edge against the hands your opponents might have.

Indeed, a totally algorithmic strategy of playing only premium preflop hands (say AA-77, AK and AQ) will beat almost any no limit game when you have a short stack of approximately twenty times the big blind. Ed suggests just such a strategy to beginners, in more detail, in his book, Getting Started in Hold'em.

That's not to say that such a strategy is optimal. For expert players, playing short routinely is a sure way to slash win rates. But it is remarkable that a robot playing a short stack can beat even relatively tough no limit games. Don't underestimate the intrinsic advantages of playing a short stack.

He goes on to give two reasons why the short-stack has an intrinsic advantage:

It's often correct for deep stacks to play loosely against other deep stacks, but incorrect for them to do so against short stacks. So the short stack is often up against weaker-than-average hands versus his range.

Once the short stack is all-in, opponents will keep betting, forcing players to fold. These folds benefit the short stack mathematically, the moreso because his additional risk is zero.

To which I'd add:

The average player is 100% clueless about how to play, and how to play against, a short stack - whether in a cash game or tourney situation.

Another reason the short-stacked approach is so powerful is that commitment decisions become much easier as the stack sizes dwindle. In Professional No-Limit Hold'em, Matt Flynn, Sunny Mehta, and Ed Miller write:

When the effective stack sizes are 50BB [or less], you can easily create favorable SPRs for top pair and overpair hands with straightforward raising tactics.

If you're not familiar with the terminology, an SPR (stack-to-pot ratio) is simply the ratio of your stack (or technically, the smaller stack) to the total size of the pot on the flop. It's a numeric expression of risk-vs.-reward at the nexus of the hand which, among other things, tells you when you welcome aggression and are willing or even eager to get all of your money in. The beauty of playing short-stacked is that the mix hands which are correct to play when you're short-stacked always produce favorable SPRs for getting your money all-in.

From a programming perspective, that often (not always, but more often than you'd think) reduces the flop decision to a binary one:

- YES, I'm somehow going to get my money all-in here, probably with a straightforward bet or raise.
- NO, I'm not willing to commit. I fold.

That's huge, in terms of cutting into that complexity I was talking about. But it doesn't end there. Playing short-stacked desaturates the game in a number of ways that simplify your job as the programmer:

- A short-stacked bot rarely has to consider any actions beyond the flop. This has the effect of turning a 4-street game (preflop, flop, turn, river) into a 2-street game (preflop, flop).
- Preflop play, while subtle at times, is fairly easy to abstract into a set of rules. This is somewhat true even in deep-stacked play, but it's especially true in short-stacked play.
- Flop play, the crux of the hand, can be simplified through the application of SPRs and some basic commitment mathematics (as I explained above).
- Any mistakes or weaknesses in the bot can only be punished in small increments.

As Sklansky notes, it's usually incorrect for a short stack to bluff. That means you don't have to implement game-theoretic bluffing, and you don't have to implement opportunistic bluffing. In fact, beyond an occasional continuation bet, which will put you all-in anyway, you don't have to think about bluffing at all.

Short stacks don't care or worry about implied odds. Estimating implied odds is one of those fuzzy areas in which humans prosper and robots perish.

Buying in short is appropriate when the other players at the table are better than you, which will often be the case with a home-grown bot.

So while I don't suggest that you rule out the idea of creating a bot that can play deep-stacked, I do think the first step on the road to that goal is creating a bot which is capable of playing short-stacked.

source-- How I Built a Working Online Poker Bot, Part 3

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