## Mean Difference - Mann Whitney

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## Mean Difference - Mann Whitney

Hello,

i am wondering how the mean difference is calculated for Mann Whitney U test.
It is not the same as the difference between means, which is okay, since the comparison is not based on means. it is also not the difference between medians (which is also okay, since, strictly speaking, Mann Whitney does not compare medians but mean ranks). However, the number is not even the difference between mean ranks. I have no clue what I am seeing.

Can anyone help me with this? Am I missing something obvious?
Emese
hallgatoemese

Posts: 6
Joined: Tue Nov 17, 2020 7:33 pm

## Re: Mean Difference - Mann Whitney

hi emese,

we use the wilcox.test() function from R to calculate this value, but i'm not sure what it represents. here's the R documentation:

https://stat.ethz.ch/R-manual/R-devel/library/stats/html/wilcox.test.html

although it doesn't seem to shed much light either.

jonathon

jonathon

Posts: 1729
Joined: Fri Jan 27, 2017 10:04 am

## Re: Mean Difference - Mann Whitney

Are you sure about it NOT being the difference of the medians...?
Bobafett

Posts: 16
Joined: Thu Jul 18, 2019 11:33 am

## Re: Mean Difference - Mann Whitney

Sometimes it does seem like it is the difference of the medians. But it didn't took me long to find an example where it is not the case.

(i attached the screenshot)

Emese
Attachments
mannwhitney-example.png (14.49 KiB) Viewed 397 times
hallgatoemese

Posts: 6
Joined: Tue Nov 17, 2020 7:33 pm

## Re: Mean Difference - Mann Whitney

Out of interest - are there any tied ranks in the data? I wonder if they might be affecting these descriptives?
Bobafett

Posts: 16
Joined: Thu Jul 18, 2019 11:33 am