eXtreme Gammon writes:
Variance reduction

Variance reduction is a process that will adjust the result of each game
played during a rollout by the luck of each roll. On the long run this is a
0 effect on the final equity but it allows rollouts to converge much
faster. This reduces greatly the amount of trials you need to get to get an
accurate result.
For instance in your position rolling out 23/13 with VR (3ply 2592 games)
with Variance reduction your confidence interval is 0.012 (meaning that
there 95% confidence that a rollout with infinite number of trial will be
within 0.012 equity of the result reported). Without variance reduction the
confidence interval is 0.087 and it will take 136,242 trial to get the same
confidence (Confidence interval decrease as the square root of the number
of game: i.e. rolling 4 time more game will decrease the confidence by a
factor 2).
This setting should always be on. I am considering to remove it as an
option for rollout.
Truncation

Truncation means that after N dice roll the rollout will stop and the
position evaluated using the truncation level. This result in much less
variance, but it can create problem if the positions after this N move is
not evaluated very accurately.
I do not advice to use short (anything less than 7) truncation.
Roll for both No double and Double

Applies only to checker play and if the opponent can double. If this option
is checked the program will roll the position twice one if the opponent
doesn't double, once if the opponent doubles. It will report the best
decision. This can be an useful settings if you suspect the computer will
get the very next cube decision wrong. It has a cost in term of speed:
rollout using this setting are about 25% slower.
Level

The stronger the level the more trustworthy a rollout is. But this come as
a cost: TIME. Increasing from Nply to (N+1)ply will make the rollout
about 20 times slower. Ultimately it depends on how much time you want to
spend, what computer you have etc
Conclusion

Rollout settings are a difficult subject: explaining what is the best
setting is extremely complex as it depends largely on what the user is
looking for, what time they are willing to spend and what hardware they
have.
