There is some misunderstanding here, I guess. 
Mann-Whitney requires at a minimum all 
the individual ranks. It can't work on just
input of averages, or even medians. 
Nick 
[email protected] 
[email protected]
> I have to run the mann-whitney test to check the difference 
> in the trend
> between two treatments in a public good game experiment.
> This test has to be run on average contributions of groups. 
> Does it mean
> that I should have in my dataset a single observation for 
> each group that
> represents the average contribution of the group over all the periods?
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