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Ranking in a Mann-Whitney U test

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I am doing my IA in psychology and I am doing my statistical test by using the Mann-Whitney U test but I am so lost in how to do it.

My results are

Group A - 6 , 8, 8, 8, 8, 8, 9, 9, 9, 9, 10, 10, 10

Group B - 6, 7, 7, 8, 8, 8, 9, 10, 10, 11, 11, 12, 14

So how do I rank these as needed for the Mann-Whitney U test?

/Moa

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I am doing my IA in psychology and I am doing my statistical test by using the Mann-Whitney U test but I am so lost in how to do it.

My results are

Group A - 6 , 8, 8, 8, 8, 8, 9, 9, 9, 9, 10, 10, 10

Group B - 6, 7, 7, 8, 8, 8, 9, 10, 10, 11, 11, 12, 14

So how do I rank these as needed for the Mann-Whitney U test?

/Moa

Which IA topic did you choose? Are you sure Mann Whitney is the appropriate test? If you are working with ordinal data and it is an independent measures design then read on :).

You treat it as one large data set and then put them in order. You have many tied ranks which is tricky but I'll try and walk you through it.

Combine the data set from both groups so it looks like this 6, 6, 7, 7, 8, 8, 8, 8, 8, 8, 8, 8 etc

Then you assign ranks to the each number, if the ranks are tied you add up the rank and divide it by how many numbers are in that rank. This sounds confusing I know, let me try and explain it.

The lowest number gets the first rank, so it should be 1, then 2, but because there are two sixes you add the ranks (1 and 2) and divide by the number of that rank (2, there are two sixes). So the 6s get rank 1.5 each. If you had one 6 and then one 7 in this set, 6 would get rank 1 and 7 rank 2. Following?

So because 1 and 2 as ranks have been used up, when you get to 7 you start with rank 3. Since there are two 7s you need to use 3 and 4 as ranks. Again add ranks (3 and 4) and divide by number of 7s. So 3+4/2 = 3.5.

Since you have used up 3 and 4 you use the next rank which is 5. You have eight 8s, so the ranks it will take up are 5, 6, 7, 8, 9, 10, 11, 12. Again add up the ranks and divide by number of 8s 68/8= 8.5. All the 8s get rank 8.5.

Hope you can follow this, let me know.

Alternatively you can use software to do it :P.

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