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Disadvantages of Seed 1

Posted By: Taper_Mike
Date: Monday, 20 February 2012, at 8:08 a.m.

In Response To: Disadvantages of Seed 0 (Maik Stiebler)

Maik: I don't know a thing about how Extreme Gammon creates its random numbers, but I would think that that is not a problem with good random number generators. Different seed means overlapping is ridiculously unlikely, unless our typical number of trials expands to 10^30 or something, or the seeding is done in a particularly unfortunate way.

Right. Long period RNGs mean that the chance of overlap is finite, but small. It is not zero.

Mike: Some suggest that when both rollouts use the seem seed, variance is lessened. I am not so certain myself.

Maik: I am not certain either. I believe that the effect is negligible if variance reduction is used.

Another reason to be skeptical is that having the same dice rolls in two different positions is irrelevant. We have seen this in duplicate backgammon, where games being played in two different rooms are fed the same dice rolls. As soon as play diverges in the two games, the fact of duplicate dice rolls is irrelevant. Double sixes in one game, for instance, might be a winning throw, while in the other, it could be the worst.

Maik: Also, what Xavier said here means that seeds will only be the same if the displayed seeds match AND the positions are the same.

Thanks for the link to Xavier’s post. Here’s what Xavier says:

Note that the final seed for the dice in the rollout is a combination of

  • the user entered seed
  • a hash of the initial position (before moving)

This is has always been the same since XG was released.

According to Xavier then, even if you do believe that always using the same RNG seed somehow lessens variance, you can never make that happen in XG. Providing identical seeds in two different positions does not generate the same random number sequence for both.

You can change XG’s seed options by opening the Options and Setting dialog box. Click Options->Settings, and then click the Rollouts tab. At the bottom of the tab, put a check mark in the check box labeled “Generate a new seed randomly each time the program starts.” I like Nack’s idea: This setting should be the factory default. I also agree that the seed should always be displayed, even when its value is 1.

Mike

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