[R] Calculate monthly means

From: Carl Nim <cjnim4_at_yahoo.com>
Date: Tue, 15 Mar 2011 07:24:48 -0700 (PDT)

I am trying to calculate monthly means by year of phosphates and nitrates from a multi year data set. Can anybody help me out with the most effective way to do this?

My data looks like this:

Collection_Date                   Test.Name              Value
2000-01-24 17:00:00            Phosphate               0.108
2000-01-24 17:00:00            Nitrate                     0.037
2001-11-12 08:45:00            Phosphate               0.45 ...

Thanks and sorry for the blatantly "newbie" question.

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