Swiss Pairing Problem

@odoaker2015 True, however Swiss tournaments do allow players to late-join and drop out early. Since I can't know whether players dropped out or not, I can't know how many rounds there should have been.

I am curious whether OTB Swiss pairings allow rematch pairings to occur; I haven't studies Swiss pairings as extensively as I studied other topics (like ratings) so I'd have to research to understand details like these.

Indeed, this is the difference between otb and online swiss tourneys. In otb tourneys it is not allowed to late join. Of course, you can make exceptions, but that really should only be exceptions. For example, you can only allow late joiners until the second round. But not after that. This is also done in otb swiss tournaments. But this is rarely the case in otb tournaments. As far as I've seen. And the fact that there are dropped outs can not be prevented. @Toadofsky

#22 We got on this topic because there was a concern about a pairing problem (which I referenced in #21):
"Since I can't know whether players dropped out or not, I can't know how many rounds there should have been."

If you want I can file a feature request (to disable late-joins) but the maintainers are just going to reject it... we're getting off of the original topic.

@odoaker2015 thank you man.

#21 OTB Swiss doesn't allow rematch unless the first match was not played. Other rules:
- the same colour only two times in a row
- color difference at the end of the tournament not greater than 1
- only one bye per player
This can make it impossible to pair the last round(s) if the number of players is to low. To make Swiss a replacement for round robin, I think it would be necessary to disobey some of the rules.

The tournament software I use for OTB (Swiss Chess from Germany) uses an algorithm for colour compensation. With that its possible to play 7 rounds with 10 players.

#25 Thanks. That said, now I have to return to the original topic:

"One argument for not implementing round robin on Lichess was the claim that the Swiss system over here is well suited as a replacement for round robin. Unfortunately this is not always the case."

I forget which Lichess staff member (if any) claimed this in an unqualified manner; you are correct that Swiss and round robin function differently.

And back to the first original topic... yes, it might be useful to have round robin pairings, but it might not. lichess.org/swiss explains:

"What about Round Robin?
We'd like to add it, but unfortunately Round Robin doesn't work online.
The reason is that it has no fair way of dealing with people leaving the tournament early. We cannot expect that all players will play all their games in an online event. It just won't happen, and as a result most Round Robin tournaments would be flawed and unfair, which defeats their very reason to exist.
The closest you can get to Round Robin online is to play a Swiss tournament with a very high number of rounds. Then all possible pairings will be played before the tournament ends."

"We cannot expect that all players will play all their games in an online event. " That's true at least for Arena tournaments. If in Swiss to many people leave it could be impossible to go on with the tournament. Round robin would go on (and could indeed be unfair). I think in team events the chance of people leaving early is rather low. OTB this sometimes happens too, but there is always a way to handle it.

I absolutely understand Lichess reasoning. I just don't totally agree.

But we can finish the thread.

Just repeating the last sentence of my first post:

"Don't get me wrong: I appreciate all the hard work and time that went into building this great platform and I am glad to have a place where we can keep on playing chess in times of Corona."

So many thanks to you and the Lichess staff.

Thanks and sorry if I don't acknowledge praise; often people use praise to try to motivate a discussion. It's fine to not agree, but I don't understand where this accusation (#1) comes from since I don't think any Lichess staff said this, although it's possible a confused staff member could have?
"One argument for not implementing round robin on Lichess was the claim that the Swiss system over here is well suited as a replacement for round robin. Unfortunately this is not always the case."

The Lichess team discussed round robin at length and the risk/reward of implementing round robin simply isn't worth it because of the drop-out risk (this has been mentioned in other forum topics several times already, but it's been about a month or more and I wouldn't expect anyone to remember this; I can't find them). And as you point out, OTB this sometimes happens and OTB there are ways to remedy the situation - for example, by blacklisting players, organizers, etc. Online with pseudonymous players and low stakes there's much more incentive to drop out; again, this has been said before in forgotten topics and likely will be said again when people forget this misnamed topic - "Swiss pairing problem" - which is really more of a feature request for round robin than it is statement of an actual problem, and even the feature request is defective...

... to split hairs on this, during private correspondence with Lichess staff I suggested that round robin could make sense for high-stakes invitational events on Lichess, but even then people could use lichess.org/api and create their own 3rd-party pairing system unless we are frequently running high-stakes invitational events. Further discussion pointed out that without a TD accreditation system, people will blame Lichess instead of blaming TDs when players drop out; and given the risk/reward Lichess isn't interested in starting a TD accreditation system at this time; again, these are points I have stated before and likely will have to state again the next time someone makes the same arguments about, "Wouldn't it be great to have round robin pairings?" It doesn't quite fit in the Q&A format that I linked to, but there it is...

Writing in a foreign language for me usually is much harder than writing in my native language. So I didn't mean to accuse anybody. Somtimes it is difficult to find the right words.
And for the "praise": I'm Frisian (not the horses or cows ;-). We tend to say what we think and mean what we say. I know that, for example Americans, a very careful about what to say and how to say it (got American relatives). So even in the western world there are cultural differences.

@Toadofsky You don't need dropouts to get stuck in a Swiss system, it can happen naturally.

As for the "claim" that you can play long Swiss systems as RRs. Yes, that was said early on when the developer wasn't aware that this does not work. I mentioned that to him so the text got changed. But maybe you saw that back then and that's where the confusion is coming from.

Right now there are no RRs on lichess. There has been some work to develop an external application that handles round robin for users but I am not sure what the current state of that application is. I think you may want to ask on discord where some person mentioned they're working on that.