Rollouts

 Tips for doing rollouts

 From: Douglas Zare Address: zare@math.columbia.edu Date: 26 June 2002 Subject: Re: rollout tips please Forum: rec.games.backgammon Google: 3D1A3598.A4AED0C1@math.columbia.edu

```The errors of a rollout come from four things:

First, the bot may misplay unequally from the different sides.
Second, if the rollout is truncated, the evaluations used at the point of
truncation may be wrong.
Third, there is still luck involved in the rollout, since variance
reduction is not perfect.
Fourth, the results may be misinterpreted.

If you believe that the position is easy to play, 2-ply may even be
overkill. On the other hand, some positions are tricky to play from one
side and easy to play from the other. You need a very high level of play to
get an accurate rollout in a position of one-sided errors. 2-ply might be
sufficient, and it might not, but it should be reasonable if your own play
is comparable to 2-ply's play.

Some positions are easy to evaluate, and some are strange enough to warrant
rollouts. If you don't trust the evaluations of positions with early
primes, and are doing rollouts, truncation  might be accurate if you trust
the evaluations of the likely positions after a few rolls: Prime versus
prime, crashed positions, position with several primed checkers, etc. Would
you trust the evaluations then? I'm not sure, so I would tend to avoid
using truncated rollouts then. On the other hand, in some stages of a
holding game, you may know that either a shot has been hit or the game has
turned into a race after a few moves. Then you may be able to trust
truncated rollouts.

Some versions of gnu have had inaccurate error estimates. Snowie apparently
does in match play and when the cube is higher than 1. However, these bugs
aside, you should look at the error estimates and compare them with the
difference between the best action and the second best action. You should
also compare the error estimates with the size of what you consider an
acceptable mistake. For example, I really don't mind if I make a 0.050
take/pass error. If the best plays are too close to distinguish, then if
the confidence interval is wide you should do more rollouts, but if the
confidence interval is small you should accept that there is not a big
difference between the plays, and move on.

Finally, the results of the rollout may be interpreted by applying
something like Janowski's formula. This might give an accurate assessment
of cubeful equity given the cubeless bg/g/w breakdown, but it might be
systematically inaccurate when you have a position with unusually good or
bad opportunities to recube efficiently. In those situations, cubeful
rollouts might be best, assuming that you trust the cube actions during the
rollout... but there are simple positions in which Snowie rollouts reach
the wrong conclusion no matter what settings are used. Also, I am highly
sceptical of Janowski's formula when the position may be too good to
double.

Anyway, remember that rollouts tell you how the bot plays against itself,
not how you play against your opponent, and not how perfect play does
against itself.

Douglas Zare
```

### Rollouts

Cautionary tale  (Kit Woolsey, Sept 1995)
Combining rollouts  (Gregg Cattanach+, Dec 2003)
Confidence intervals  (Bob Koca, Nov 2010)
Confidence intervals  (Timothy Chow, May 2010)
Confidence intervals  (Gerry Tesauro, Feb 1994)
Cubeless vs centered-cube rollouts  (Ron Karr, Dec 1997)
Duplicate dice  (David Montgomery, June 1998)
How reliable are rollouts?  (David Montgomery, Aug 1999)
Level-5 versus level-6 rollouts  (Michael J. Zehr, June 1998)
Level-5 versus level-6 rollouts  (Chuck Bower, Aug 1997)
Positions with inaccurate rollouts  (Douglas Zare, Oct 2002)
Reporting results of rollouts  (David Montgomery, June 1995)
Rollout settings  (Lokicol+, Apr 2010)
Settlement limit  (Michael J. Zehr, Apr 1998)
Settlement limit  (Kit Woolsey, Dec 1997)
Settlement limit in races  (Alexander Nitschke, Dec 1997)
Some guidelines  (Kit Woolsey, Apr 1996)
Standard error and JSD  (rambiz+, Feb 2011)
Standard error and JSD  (Stick+, Oct 2007)
Systematic error  (Chuck Bower, Oct 1996)
Tips for doing rollouts  (Douglas Zare, June 2002)
Truncated rollouts  (Gregg Cattanach, Oct 2002)
Truncated rollouts: pros and cons  (Jason Lee+, Jan 2006)
What is a rollout?  (Gregg Cattanach, Dec 1999)