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I think fun is highly dependent on whether you won or not, not the quality of the game.
When you get the survey, your opponent always does, too. We divide the responses by the game result, because as you guessed, winners tend to be happy more frequently.
Have you folks learned anything useful from it? Like the kinds of games that tend to make both players happy?
Absolutely. We also especially pay attention to cases where winners come away more "sad" than usual or losers are especially "happy." (Just recently, the Mastermind workshop fell into both categories, which is an interesting case.)