# Is there a phenomenon like this?

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.

• I think you mean Simpson’s paradox.
– Dave
Commented Jun 25, 2021 at 19:09
• @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. Commented Jun 25, 2021 at 19:22
• 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.
– Dave
Commented Jun 25, 2021 at 19:32
• Commented Jun 25, 2021 at 19:44

## 1 Answer

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

• 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. Commented Jun 25, 2021 at 19:22
• @user900476, you need to read through some of the threads we have on the topic. I have a related answer here. Commented Jun 25, 2021 at 20:51