Forum Archive : Terminology


From:   Gary Wong
Address:   gary@cs.arizona.edu
Date:   25 December 1998
Subject:   Re: Please Explain Equity
Forum:   rec.games.backgammon
Google:   wthfuk3sgd.fsf@brigantine.CS.Arizona.EDU

Craig Yannuzzi writes:
> Can someone please explain how to interpret Equity numbers for various
> moves.

There is a precise definition of equity and some discussion on Backgammon
Galore at:


Another good description is this one by Robertie (_Advanced Backgammon_
vol 2, position 357):

  "_Equity_ is a fundamental concept of backgammon, indeed of any game
   which is a mixture of skill and chance.  The equity of a position is
   the average profit (or loss) that one would net, per game, by playing
   the position to conclusion an infinite number of times.  It is the
   `value' of the position."

Equity may appear somewhat obscure at first, but it really is a very
natural way of measuring the strength of a position.  The unit of measure
is "points per game" (ppg) which has a very intuitive interpretation:
suppose the cubeless equity in a particular position is estimated at
-0.247.  That means we expect to lose 0.247 points per game -- if we played
1000 games for $1 a point with no cube starting from that position, we
would expect to lose $247.

> Why are the numbers negative...

We all try very hard _not_ to make the numbers negative :-)  Since one
player loses exactly as much as the other player wins (backgammon is a
"zero sum" game), if the equity is positive for one player then it will
be negative for the other.  If the equity is positive then you're winning;
if it's negative then you're losing.

> ...with the highest number best?

The higher number is best simply because you expect to win more!  Sometimes
I've seen posts arguing "a move isn't better because the equity is greater;
the equity is greater because the move is better", but I think it's
pointless trying to impose any cause and effect here.  That would be like
saying "You're not taller than me because your height is greater; your
height is greater because you're taller than me."  They are exactly
equivalent; you can't have one without the other; and if you try to
separate the two then you're not doing justice to either.  As Robertie

  "...You are not, of course, mentally comparing two numbers; you are
  choosing whichever position looks better.  The process of your backgammon
  education is properly linking `looking better' with `higher equity'."

        Gary Wong, Department of Computer Science, University of Arizona
            gary@cs.arizona.edu     http://www.cs.arizona.edu/~gary/
Did you find the information in this article useful?          

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