I am playing with implementing some statistics in Excel, with no macros, only formulas.

I have implemented, among other things, Mann-Whitney test following the procedure described in this very useful site (http://faculty.vassar.edu/lowry/webtext.html).

In essence, given two samples, the implementation I made in Excel calculates:

a) the U value

b) the z-score

c) the significance (p) (two-tailed) (applying the function: norm.distrib.st(z-score value)*2.

Cross-checking the test's results with the ones from other statistical programs, I noted that sometimes p is not correct.

That is, I noted that if norm.distrib.st(z-score value) is > 0,5, to get the right p I have to use (1-distrib.norm.st(z-score))*2.

Can anyone help me to understand this issue? It seems to me that I am missing something in there.

Thanks

Regards

Gm