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2 ply vs 3 ply vs 4 ply vs rollout

So here is a position from a 1 pt match. I rolled 6-4 and played it 24-18 and hit with the 4. Gnubg 3 ply said it was a blunder (.099) but 2 ply and 4 ply said .055 and the rollout (0 ply) was even lower error rate. So how can 2 ply to 3 ply to 4 ply jump around like that double the error rate? Since 2 4 and rollout all agree it is less than .050 I have to assume 3 ply must be wrong no? So 2 ply can sometimes make a better move than 3 ply?
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User avatar
admin
Site Admin
Posts: 343
Joined: Sun Oct 24, 2010 8:32 pm
Years playing backgammon: 20+ Learned to play on fibs.com playing bots and review with gnubg analysis FIBS rating 1750-1850
Location: USA

Re: 2 ply vs 3 ply vs 4 ply vs rollout

On Monday, July 27, 2015 at 12:32:44 AM UTC-4, greedygammon wrote:
> So how can 2 ply to 3 ply to 4 ply jump around like that double the error
> rate? Since 2 4 and rollout all agree it is less than .050 I have to assume
> 3 ply must be wrong no? So 2 ply can sometimes make a better move than 3 ply?

Our friend Tim Chow from rec.games.backgammon says...


Well, anything can *sometimes* occur---the nets aren't perfect.

Regarding GNU 1.0 2-ply versus 3-ply, there is some conflicting evidence. The Depreli study suggests that 3-ply is usually slightly stronger than 2-ply:

http://www.extremegammon.com/studies.aspx

On the other hand, recently on the GNUBG mailing list, Lucas reported that 2-ply was significantly better than 3-ply in a head-to-head competition. See the following post and its followups:

http://lists.gnu.org/archive/html/bug-g ... 00006.html

The idiosyncrasies of how GNUBG was trained has led to what people have called an "odd/even effect," meaning that 3-ply isn't as much stronger than 2-ply as you might expect it to be. That was version 0.x and version 1.x has supposedly mitigated the odd/even effect, but as I said, the evidence is not conclusive.


> I've been doing 3 ply analysis which takes about twice as long as 2 ply.
> It is worth it?


Hard to say. I personally wouldn't bother. However, I think that a truncated rollout is generally worth it for complicated decisions. See here for some decent truncated rollout settings:

http://www.bgonline.org/forums/webbbs_c ... =134413%FD

---
Tim Chow
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