The mathematical measure of luck gained on a roll of the dice is your equity after the roll minus your equity before the roll, i.e., luck is the equity you gain through the roll of the dice. It is the equity of your best possible play minus your equity before the roll, or your equity after your opponent's best play minus your equity before the roll. (For an introduction to equity, see Gary Wong's article on equity in the rec.games.backgammon archive.)

Note that there is no luck involved in using the doubling cube, though it might affect which subsequent rolls are lucky and by how much. Further, there is no luck involved in how one plays a roll. When you make imperfect plays or imperfect cube decisions, you lose equity, but that is not due to luck. If your equity is +0.3 and you roll well so that the best play has an equity of +0.5 but you blunder, only getting +0.4, then your luck was +0.2 and you erred by 0.1.

We have a reasonable definition of the luck on a roll, but it is hard to compute—that takes some understanding of backgammon. Programs such as Snowie, Jellyfish, MonteCarlo, Motif, and Gnu have guesses about what the equity is in each position, but they disagree about the equities, and even about what the correct plays are, so they can't all be correct. Nevertheless, they are better than nothing, and their guesses are extremely useful for the purpose of variance reduction. Most beginning and intermediate players would do well to assume that Snowie 3 is correct. Snowie's analysis can reassure you that your opponent was very lucky, or point out that you blundered away your equity rather than losing it with the dice.

We generally don't care that much about the luck on a particular roll, but rather the whole game or whole match. If one adds up the luck from each roll, it turns out that this is much more interesting.

Theorem: (folklore?) The outcome of a match or money game equals the net luck plus the net skill difference.

We would like to estimate the sum of the changes in the equities after making the best possible plays (whether or not those were made). What we can see is the sum of the changes in equity, since we know what the equity is at the start and at the end. At each step, the equity lost or gained through your or your opponent's technically incorrect plays is the difference between the equity of the best possible play and the equity of the actual play. So the difference between the total luck and the outcome is the sum of the net errors.

Example: Suppose you are playing a money game. If you give up 0.2 due to incorrect play and your opponent gives back 0.5, then if you win you have been lucky by an amount of +0.7 and if you lose your total luck has been -1.3. If you win a gammon with the cube on 4, your luck has been +7.7.

It doesn't matter how you won or lost the match, what its score was, or what your strategy was: If you know the outcome and the relative skill displayed, you can determine the amount of net luck for each player.

1. The luck involved in any match won is about the same as the luck involved in any other, barring serious errors.

   If neither side makes errors, then if you lose a 5 point match 5-4 by losing a close race at double match point you have had exactly as much net bad luck as if you had lost the match 24-0 by being backgammoned with the cube at 8 or if you lost 5 single games in a row (within the match). Your luck was -1 in each case. In the first case the last few rolls had tremendous quantities of luck, and it is easy to underestimate their importance.

   If someone exclaims about how much luck you had after you win, they are rudely asserting that they played much better than you did. Of course, it is much more obnoxious if you lost.

2. From any non-gin position in a match, you need good luck to win and bad luck to lose.

   Example: Suppose you are down 10-0 in an 11 point match. You have about a 3% chance of winning the match if you and your opponent play perfectly. Your opponent still needs +3% good luck in order to win: if you come back to tie the match 10-10 and then lose, then you have either had bad luck or made errors. Though the score looks more respectable than the "median" result of losing 11-1, if neither side makes errors you have had just as much net bad luck as you would by losing 11-0. To go from 10-0 to 10-10 with perfect play requires +47% luck, and losing at 10-10 requires -50% (bad) luck, for a total of -3%.

3. Strong players are lucky more often than weak players.

   Strong players still have to be lucky to win, just less so. Because they don't need to be as lucky, their good luck is spread out to win many matches and their bad luck is concentrated in a few losses. Someone who bears in and bears off efficiently doesn't roll larger numbers than someone who aims for the ace point. He/she just uses the pips received more efficiently. The same thing happens with luck.

   A strong player throwing away .2 less equity than I do will still have to be lucky by +.8 to beat me (in a match). Of course, this will happen 60% of the time.

4. Forced rolls might be good luck and they might be bad, but you play them perfectly.

   Some measures of a player's errors average the errors over the number of unforced moves. Having a proper measure of luck allows one to see that the total errors of the two players are comparable, rather than the errors averaged over unforced moves.

   Suppose you have a choice between making an error of size .03, and having your next three moves forced rather than no error this play but errors of size .02 for the next three moves due to your unfamiliarity with the type of position. The first way has a higher error per unforced move (.03 rather than .015) but afterwards it requires .03 less luck to win.

5. Someone who is manipulating the dice to their advantage will show up as "luckier."

   A cheater will not necessarily have a higher average number of pips rolled, and there will not necessarily be any particular pattern (e.g., more 4-4's than normal) but their luck will inevitably be better, assuming all of their moves are legal (and that they are not cheating through getting unfair advice). Over the long run, they will probably appear unreasonably lucky.

   A stronger player will probably not show up as unreasonably lucky over the long run. To distinguish between the two, one needs to measure luck carefully, more carefully than most human players can do by hand. Many people claim that the free versions of Jellyfish cheat despite documented methods for checking that they do not, but unbiased estimates indicate that Jellyfish is not particularly lucky.

How much of backgammon is luck? Almost all, if you and your opponent play it very well, and play relatively short matches. Both players are vying to be luckier than the other. If not, luck could be relatively insignificant.

That's why I don't say "good luck" at the start of my matches.