ELO dropping after Victories

[SHKätzchen]

I played a couple games this afternoon on Introductory as a two player with an average player. To my shock, after each game my ELO decreased by a few points despite the fact that we won each game.

I get that as a player with a high ELO I can't expect much if any of an ELO bump from winning easy games, but surely I shouldn't be punished for doing so either? I didn't put this as a bug because I'm not sure it is one (and I wasn't sure what bug category to put it under), but I'm quite confused.

[shermanator822]

At the end of 2023, there was a set of site-wide changes to ELO that didn't make sense in many ways. This outcome may be a consequence of those changes, which in any case were not calibrated for co-op games.

There should still be a thread on the official announcements forum about it, being kept evergreen by a few dedicated souls.

[shermanator822]

https://boardgamearena.com/forum/viewtopic.php?t=33785

link to other thread i hope

[ufm]

This has nothing to do with the last year's ELO update.
That update only hid the current rating value from the game pages and did nothing to the calculation itself.

The problem lies in the game's rating calculation system - artificial ELO simulation.
This method compares the game's difficulty against the players' average rating value.
If the game difficulty is sufficiently low, players can lose rating even if they win.
This type of co-op rating system is now out of date and is not used anymore in new BGA co-op implementations.
So, this is not a bug per se, but it might be considered a room for improvement: https://boardgamearena.com/bug?id=53840