"Can you believe it? I made 18, he drew a 20, and the dealer, of course, got in between us with 19." Sound familiar? It certainly does to me. I have heard that phrase in what seems like thousands of tournament bad beat stories. Those dealer 'tweeners' are a critical part of just about every bad beat. It also seems that no bad beat tale is complete without a mention of just how unlikely the outcome was. More often than not, the ensuing discussion includes a mention of the fabled "12%".

The origin of the oft-quoted 12% swing goes back to Stanford Wong's book Casino Tournament Strategy. Among much useful information Wong imparts in that staple of tournament literature is a table showing how two basic strategy players fare against each other. In particular, Wong's Table 4: Probability of Win, Push, Lose includes the following line:

Many players thus assume that the probability of a "swing", where one player in a hand wins while the other loses is 12%. That is true, as long as you are very careful in defining what constitutes a swing. As usual, things get complicated in a hurry.

The most common mistake players make with this 12% number is using it in situations where it does not apply. In fact, it is actually uncommon to be in a situation where you must win while the other player loses. More commonly, a push by one player or the other is an important factor.

For example, consider an example final hand. The chip leader Bob is playing last, with a total bankroll of $500. His opponent Adam bets $100 from a bankroll of $490, and Bob matches his $100 bet. Does Adam now have only a 12% chance of winning the table? No.

In this case, either player's push is possibly valuable. If one player pushes while the other loses, that is good enough to win the table. Even if we eliminate Adam's ability to double or split in an effort to make Wong's table apply, we need to add in another 7% for the "Adam wins/Bob pushes" and "Adam pushes/Bob loses" outcomes. That gets us to 19%, and that still understates Adam's chances. In reality, he can double, split, or deviate from basic strategy to increase his chances. That "bad beat" is getting more likely by the minute.