Mann Whitney U Test: Determining "First" Population

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The question relate to the Mann Whitney U test with two populations.

For the test statistic formula, there is a R1 term, which is the sum of the ranks of the "first" population. My question is, given any two populations, how do i decide which one is the "first" population?

 

[HallsofIvy]

The Mann Whitney U test essentially tests the absolute value of the difference in means of two populations. It doesn't matter which is the "first" and which is the "second" population.