[R] grouping data together

[Swanton0822]

Hi,
i have group all the data, but now if i would want to sum all the rank in
each group, how can i do it?
ie. i want to sum the rank in every group, not total. so there will be a sum
of rank for month Jan,Feb,.....Dec, therefore there will be total of 12
vaule of ranking sum.
many thanks,
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