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# Help with Biology coursework (AS) watch

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1. I need desperate help with my biology analysis, does N e 1 know how to do the Mann-Whitney U test?, Please, pretty please?!
2. (Original post by Prancer)
I need desperate help with my biology analysis, does N e 1 know how to do the Mann-Whitney U test?, Please, pretty please?!
Yup

Basically, it's done like this:

1) Rank all your results (for all the variables as one rank) giving rank 1 to the lowest score.

Eg

Variable 1
2
5
6

Control
6
3
8

Rank (variable 1 and control combined)
1 2
2 3
3 5
4.5 6
4.5 6
6 8

As you can see, if there are two numbers the same, take the average of the ranks they cover (in this case, 4+5 /2 to give 4.5 as the rank).

2)After ranking the scores, add the total ranks for each condition. (using the example above)

Variable 1
1 (2)
3 (5)
4.5 (6)

Control
4.5 (6)
2 (3)
6 (8)

Total: Variable 1 rank = 2+5+6 = 13
Control rank = 6+3+8 = 17

3) Calculate U using the following formula:

U = n1n2 + [n1(n1+1) /2] - T1

(n1 = number of subjects in condition 1, n2 = number of subjects in condition 2, t1 = rank total for condition 1)

In the example above:

U = 3x3 + [3(4)/2] - 13
-->U = 9 + [12/2] - 13
-->U = 2

4) Put the values of U, n1 and n2 into the following formula:

U1 = n1n2 - U

U1 = 3x3 - 2 = 7

If U1 is smaller than U, then use U1. If U is smaller than U1, then use U. In this example, U1 = 7 but U = 2 so use 2.

5) Look up the significance in the table. Take note of the use of n1 and n2 in the tables (they're the same as in the formula we just used) and make sure you use the relevant level of significance (usually 0.05 for biology experiments)

Hope that's been of some help

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Updated: January 23, 2004
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