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It's like the mean of every part increases, but the overall mean decrease; or the mean of a group is larger in every part but is smaller in all samples; or something like this.

I remember there is a name for this phenomenon like this (it may not be accurate) but my impression is very vague and I cannot remember it clearly.

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  • $\begingroup$ I think you mean Simpson’s paradox. $\endgroup$
    – Dave
    Commented Jun 25, 2021 at 19:09
  • $\begingroup$ @Dave Thank you very much, I still have a simple question. If group A has a larger mean in every part than group B but has a smaller mean overall, then which group is better? Assuming the larger the better. $\endgroup$
    – user900476
    Commented Jun 25, 2021 at 19:22
  • $\begingroup$ I suggest posting that as a separate question that includes either a drawing or a small data set (can be made up), because I do not totally follow what you mean. $\endgroup$
    – Dave
    Commented Jun 25, 2021 at 19:32
  • $\begingroup$ It’s like everything’s amazing now and nobody’s happy $\endgroup$
    – Aksakal
    Commented Jun 25, 2021 at 19:44

1 Answer 1

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You are thinking of (one version of) Simpson's paradox. You can find many threads about it on the site.

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  • $\begingroup$ Thank you very much, I still have a simple question. If group A has a larger mean in every part than group B but has a smaller mean overall, then which group is better? Assuming the larger the better. $\endgroup$
    – user900476
    Commented Jun 25, 2021 at 19:22
  • $\begingroup$ @user900476, you need to read through some of the threads we have on the topic. I have a related answer here. $\endgroup$
    – gung - Reinstate Monica
    Commented Jun 25, 2021 at 20:51

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